1This is gdb.info, produced by makeinfo version 4.13 from ./gdb.texinfo. 2 3INFO-DIR-SECTION Software development 4START-INFO-DIR-ENTRY 5* Gdb: (gdb). The GNU debugger. 6END-INFO-DIR-ENTRY 7 8 Copyright (C) 1988-2013 Free Software Foundation, Inc. 9 10 Permission is granted to copy, distribute and/or modify this document 11under the terms of the GNU Free Documentation License, Version 1.3 or 12any later version published by the Free Software Foundation; with the 13Invariant Sections being "Free Software" and "Free Software Needs Free 14Documentation", with the Front-Cover Texts being "A GNU Manual," and 15with the Back-Cover Texts as in (a) below. 16 17 (a) The FSF's Back-Cover Text is: "You are free to copy and modify 18this GNU Manual. Buying copies from GNU Press supports the FSF in 19developing GNU and promoting software freedom." 20 21 This file documents the GNU debugger GDB. 22 23 This is the Tenth Edition, of `Debugging with GDB: the GNU 24Source-Level Debugger' for GDB (GDB) Version 7.6.1. 25 26 Copyright (C) 1988-2013 Free Software Foundation, Inc. 27 28 Permission is granted to copy, distribute and/or modify this document 29under the terms of the GNU Free Documentation License, Version 1.3 or 30any later version published by the Free Software Foundation; with the 31Invariant Sections being "Free Software" and "Free Software Needs Free 32Documentation", with the Front-Cover Texts being "A GNU Manual," and 33with the Back-Cover Texts as in (a) below. 34 35 (a) The FSF's Back-Cover Text is: "You are free to copy and modify 36this GNU Manual. Buying copies from GNU Press supports the FSF in 37developing GNU and promoting software freedom." 38 39 40File: gdb.info, Node: Top, Next: Summary, Prev: (dir), Up: (dir) 41 42Debugging with GDB 43****************** 44 45This file describes GDB, the GNU symbolic debugger. 46 47 This is the Tenth Edition, for GDB (GDB) Version 7.6.1. 48 49 Copyright (C) 1988-2013 Free Software Foundation, Inc. 50 51 This edition of the GDB manual is dedicated to the memory of Fred 52Fish. Fred was a long-standing contributor to GDB and to Free software 53in general. We will miss him. 54 55* Menu: 56 57* Summary:: Summary of GDB 58* Sample Session:: A sample GDB session 59 60* Invocation:: Getting in and out of GDB 61* Commands:: GDB commands 62* Running:: Running programs under GDB 63* Stopping:: Stopping and continuing 64* Reverse Execution:: Running programs backward 65* Process Record and Replay:: Recording inferior's execution and replaying it 66* Stack:: Examining the stack 67* Source:: Examining source files 68* Data:: Examining data 69* Optimized Code:: Debugging optimized code 70* Macros:: Preprocessor Macros 71* Tracepoints:: Debugging remote targets non-intrusively 72* Overlays:: Debugging programs that use overlays 73 74* Languages:: Using GDB with different languages 75 76* Symbols:: Examining the symbol table 77* Altering:: Altering execution 78* GDB Files:: GDB files 79* Targets:: Specifying a debugging target 80* Remote Debugging:: Debugging remote programs 81* Configurations:: Configuration-specific information 82* Controlling GDB:: Controlling GDB 83* Extending GDB:: Extending GDB 84* Interpreters:: Command Interpreters 85* TUI:: GDB Text User Interface 86* Emacs:: Using GDB under GNU Emacs 87* GDB/MI:: GDB's Machine Interface. 88* Annotations:: GDB's annotation interface. 89* JIT Interface:: Using the JIT debugging interface. 90* In-Process Agent:: In-Process Agent 91 92* GDB Bugs:: Reporting bugs in GDB 93 94 95* Command Line Editing:: Command Line Editing 96* Using History Interactively:: Using History Interactively 97* In Memoriam:: In Memoriam 98* Formatting Documentation:: How to format and print GDB documentation 99* Installing GDB:: Installing GDB 100* Maintenance Commands:: Maintenance Commands 101* Remote Protocol:: GDB Remote Serial Protocol 102* Agent Expressions:: The GDB Agent Expression Mechanism 103* Target Descriptions:: How targets can describe themselves to 104 GDB 105* Operating System Information:: Getting additional information from 106 the operating system 107* Trace File Format:: GDB trace file format 108* Index Section Format:: .gdb_index section format 109* Copying:: GNU General Public License says 110 how you can copy and share GDB 111* GNU Free Documentation License:: The license for this documentation 112* Concept Index:: Index of GDB concepts 113* Command and Variable Index:: Index of GDB commands, variables, 114 functions, and Python data types 115 116 117File: gdb.info, Node: Summary, Next: Sample Session, Prev: Top, Up: Top 118 119Summary of GDB 120************** 121 122The purpose of a debugger such as GDB is to allow you to see what is 123going on "inside" another program while it executes--or what another 124program was doing at the moment it crashed. 125 126 GDB can do four main kinds of things (plus other things in support of 127these) to help you catch bugs in the act: 128 129 * Start your program, specifying anything that might affect its 130 behavior. 131 132 * Make your program stop on specified conditions. 133 134 * Examine what has happened, when your program has stopped. 135 136 * Change things in your program, so you can experiment with 137 correcting the effects of one bug and go on to learn about another. 138 139 You can use GDB to debug programs written in C and C++. For more 140information, see *note Supported Languages: Supported Languages. For 141more information, see *note C and C++: C. 142 143 Support for D is partial. For information on D, see *note D: D. 144 145 Support for Modula-2 is partial. For information on Modula-2, see 146*note Modula-2: Modula-2. 147 148 Support for OpenCL C is partial. For information on OpenCL C, see 149*note OpenCL C: OpenCL C. 150 151 Debugging Pascal programs which use sets, subranges, file variables, 152or nested functions does not currently work. GDB does not support 153entering expressions, printing values, or similar features using Pascal 154syntax. 155 156 GDB can be used to debug programs written in Fortran, although it 157may be necessary to refer to some variables with a trailing underscore. 158 159 GDB can be used to debug programs written in Objective-C, using 160either the Apple/NeXT or the GNU Objective-C runtime. 161 162* Menu: 163 164* Free Software:: Freely redistributable software 165* Free Documentation:: Free Software Needs Free Documentation 166* Contributors:: Contributors to GDB 167 168 169File: gdb.info, Node: Free Software, Next: Free Documentation, Up: Summary 170 171Free Software 172============= 173 174GDB is "free software", protected by the GNU General Public License 175(GPL). The GPL gives you the freedom to copy or adapt a licensed 176program--but every person getting a copy also gets with it the freedom 177to modify that copy (which means that they must get access to the 178source code), and the freedom to distribute further copies. Typical 179software companies use copyrights to limit your freedoms; the Free 180Software Foundation uses the GPL to preserve these freedoms. 181 182 Fundamentally, the General Public License is a license which says 183that you have these freedoms and that you cannot take these freedoms 184away from anyone else. 185 186 187File: gdb.info, Node: Free Documentation, Next: Contributors, Prev: Free Software, Up: Summary 188 189Free Software Needs Free Documentation 190====================================== 191 192The biggest deficiency in the free software community today is not in 193the software--it is the lack of good free documentation that we can 194include with the free software. Many of our most important programs do 195not come with free reference manuals and free introductory texts. 196Documentation is an essential part of any software package; when an 197important free software package does not come with a free manual and a 198free tutorial, that is a major gap. We have many such gaps today. 199 200 Consider Perl, for instance. The tutorial manuals that people 201normally use are non-free. How did this come about? Because the 202authors of those manuals published them with restrictive terms--no 203copying, no modification, source files not available--which exclude 204them from the free software world. 205 206 That wasn't the first time this sort of thing happened, and it was 207far from the last. Many times we have heard a GNU user eagerly 208describe a manual that he is writing, his intended contribution to the 209community, only to learn that he had ruined everything by signing a 210publication contract to make it non-free. 211 212 Free documentation, like free software, is a matter of freedom, not 213price. The problem with the non-free manual is not that publishers 214charge a price for printed copies--that in itself is fine. (The Free 215Software Foundation sells printed copies of manuals, too.) The problem 216is the restrictions on the use of the manual. Free manuals are 217available in source code form, and give you permission to copy and 218modify. Non-free manuals do not allow this. 219 220 The criteria of freedom for a free manual are roughly the same as for 221free software. Redistribution (including the normal kinds of 222commercial redistribution) must be permitted, so that the manual can 223accompany every copy of the program, both on-line and on paper. 224 225 Permission for modification of the technical content is crucial too. 226When people modify the software, adding or changing features, if they 227are conscientious they will change the manual too--so they can provide 228accurate and clear documentation for the modified program. A manual 229that leaves you no choice but to write a new manual to document a 230changed version of the program is not really available to our community. 231 232 Some kinds of limits on the way modification is handled are 233acceptable. For example, requirements to preserve the original 234author's copyright notice, the distribution terms, or the list of 235authors, are ok. It is also no problem to require modified versions to 236include notice that they were modified. Even entire sections that may 237not be deleted or changed are acceptable, as long as they deal with 238nontechnical topics (like this one). These kinds of restrictions are 239acceptable because they don't obstruct the community's normal use of 240the manual. 241 242 However, it must be possible to modify all the _technical_ content 243of the manual, and then distribute the result in all the usual media, 244through all the usual channels. Otherwise, the restrictions obstruct 245the use of the manual, it is not free, and we need another manual to 246replace it. 247 248 Please spread the word about this issue. Our community continues to 249lose manuals to proprietary publishing. If we spread the word that 250free software needs free reference manuals and free tutorials, perhaps 251the next person who wants to contribute by writing documentation will 252realize, before it is too late, that only free manuals contribute to 253the free software community. 254 255 If you are writing documentation, please insist on publishing it 256under the GNU Free Documentation License or another free documentation 257license. Remember that this decision requires your approval--you don't 258have to let the publisher decide. Some commercial publishers will use 259a free license if you insist, but they will not propose the option; it 260is up to you to raise the issue and say firmly that this is what you 261want. If the publisher you are dealing with refuses, please try other 262publishers. If you're not sure whether a proposed license is free, 263write to <licensing@gnu.org>. 264 265 You can encourage commercial publishers to sell more free, copylefted 266manuals and tutorials by buying them, and particularly by buying copies 267from the publishers that paid for their writing or for major 268improvements. Meanwhile, try to avoid buying non-free documentation at 269all. Check the distribution terms of a manual before you buy it, and 270insist that whoever seeks your business must respect your freedom. 271Check the history of the book, and try to reward the publishers that 272have paid or pay the authors to work on it. 273 274 The Free Software Foundation maintains a list of free documentation 275published by other publishers, at 276`http://www.fsf.org/doc/other-free-books.html'. 277 278 279File: gdb.info, Node: Contributors, Prev: Free Documentation, Up: Summary 280 281Contributors to GDB 282=================== 283 284Richard Stallman was the original author of GDB, and of many other GNU 285programs. Many others have contributed to its development. This 286section attempts to credit major contributors. One of the virtues of 287free software is that everyone is free to contribute to it; with 288regret, we cannot actually acknowledge everyone here. The file 289`ChangeLog' in the GDB distribution approximates a blow-by-blow account. 290 291 Changes much prior to version 2.0 are lost in the mists of time. 292 293 _Plea:_ Additions to this section are particularly welcome. If you 294 or your friends (or enemies, to be evenhanded) have been unfairly 295 omitted from this list, we would like to add your names! 296 297 So that they may not regard their many labors as thankless, we 298particularly thank those who shepherded GDB through major releases: 299Andrew Cagney (releases 6.3, 6.2, 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); Jim 300Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs 301(release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, 302and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, 303and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim 304Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3053.1, and 3.0). 306 307 Richard Stallman, assisted at various times by Peter TerMaat, Chris 308Hanson, and Richard Mlynarik, handled releases through 2.8. 309 310 Michael Tiemann is the author of most of the GNU C++ support in GDB, 311with significant additional contributions from Per Bothner and Daniel 312Berlin. James Clark wrote the GNU C++ demangler. Early work on C++ 313was by Peter TerMaat (who also did much general update work leading to 314release 3.0). 315 316 GDB uses the BFD subroutine library to examine multiple object-file 317formats; BFD was a joint project of David V. Henkel-Wallace, Rich 318Pixley, Steve Chamberlain, and John Gilmore. 319 320 David Johnson wrote the original COFF support; Pace Willison did the 321original support for encapsulated COFF. 322 323 Brent Benson of Harris Computer Systems contributed DWARF 2 support. 324 325 Adam de Boor and Bradley Davis contributed the ISI Optimum V support. 326Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS 327support. Jean-Daniel Fekete contributed Sun 386i support. Chris 328Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki 329Hasei contributed Sony/News OS 3 support. David Johnson contributed 330Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. 331Jeff Law contributed HP PA and SOM support. Keith Packard contributed 332NS32K support. Doug Rabson contributed Acorn Risc Machine support. 333Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith 334contributed Convex support (and Fortran debugging). Jonathan Stone 335contributed Pyramid support. Michael Tiemann contributed SPARC support. 336Tim Tucker contributed support for the Gould NP1 and Gould Powernode. 337Pace Willison contributed Intel 386 support. Jay Vosburgh contributed 338Symmetry support. Marko Mlinar contributed OpenRISC 1000 support. 339 340 Andreas Schwab contributed M68K GNU/Linux support. 341 342 Rich Schaefer and Peter Schauer helped with support of SunOS shared 343libraries. 344 345 Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about 346several machine instruction sets. 347 348 Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped 349develop remote debugging. Intel Corporation, Wind River Systems, AMD, 350and ARM contributed remote debugging modules for the i960, VxWorks, 351A29K UDI, and RDI targets, respectively. 352 353 Brian Fox is the author of the readline libraries providing 354command-line editing and command history. 355 356 Andrew Beers of SUNY Buffalo wrote the language-switching code, the 357Modula-2 support, and contributed the Languages chapter of this manual. 358 359 Fred Fish wrote most of the support for Unix System Vr4. He also 360enhanced the command-completion support to cover C++ overloaded symbols. 361 362 Hitachi America (now Renesas America), Ltd. sponsored the support for 363H8/300, H8/500, and Super-H processors. 364 365 NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx 366processors. 367 368 Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and 369M32R/D processors. 370 371 Toshiba sponsored the support for the TX39 Mips processor. 372 373 Matsushita sponsored the support for the MN10200 and MN10300 374processors. 375 376 Fujitsu sponsored the support for SPARClite and FR30 processors. 377 378 Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware 379watchpoints. 380 381 Michael Snyder added support for tracepoints. 382 383 Stu Grossman wrote gdbserver. 384 385 Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly 386innumerable bug fixes and cleanups throughout GDB. 387 388 The following people at the Hewlett-Packard Company contributed 389support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 390(narrow mode), HP's implementation of kernel threads, HP's aC++ 391compiler, and the Text User Interface (nee Terminal User Interface): 392Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, 393Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase 394provided HP-specific information in this manual. 395 396 DJ Delorie ported GDB to MS-DOS, for the DJGPP project. Robert 397Hoehne made significant contributions to the DJGPP port. 398 399 Cygnus Solutions has sponsored GDB maintenance and much of its 400development since 1991. Cygnus engineers who have worked on GDB 401fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin 402Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim 403Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler, 404Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek 405Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In 406addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, 407JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug 408Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff 409Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, 410Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin 411Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela 412Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David 413Zuhn have made contributions both large and small. 414 415 Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for 416Cygnus Solutions, implemented the original GDB/MI interface. 417 418 Jim Blandy added support for preprocessor macros, while working for 419Red Hat. 420 421 Andrew Cagney designed GDB's architecture vector. Many people 422including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, 423Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, 424Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna 425Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration 426of old architectures to this new framework. 427 428 Andrew Cagney completely re-designed and re-implemented GDB's 429unwinder framework, this consisting of a fresh new design featuring 430frame IDs, independent frame sniffers, and the sentinel frame. Mark 431Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind 432unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad 433unwinders. The architecture-specific changes, each involving a 434complete rewrite of the architecture's frame code, were carried out by 435Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane 436Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel 437Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei 438Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich 439Weigand. 440 441 Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from 442Tensilica, Inc. contributed support for Xtensa processors. Others who 443have worked on the Xtensa port of GDB in the past include Steve Tjiang, 444John Newlin, and Scott Foehner. 445 446 Michael Eager and staff of Xilinx, Inc., contributed support for the 447Xilinx MicroBlaze architecture. 448 449 450File: gdb.info, Node: Sample Session, Next: Invocation, Prev: Summary, Up: Top 451 4521 A Sample GDB Session 453********************** 454 455You can use this manual at your leisure to read all about GDB. 456However, a handful of commands are enough to get started using the 457debugger. This chapter illustrates those commands. 458 459 One of the preliminary versions of GNU `m4' (a generic macro 460processor) exhibits the following bug: sometimes, when we change its 461quote strings from the default, the commands used to capture one macro 462definition within another stop working. In the following short `m4' 463session, we define a macro `foo' which expands to `0000'; we then use 464the `m4' built-in `defn' to define `bar' as the same thing. However, 465when we change the open quote string to `<QUOTE>' and the close quote 466string to `<UNQUOTE>', the same procedure fails to define a new synonym 467`baz': 468 469 $ cd gnu/m4 470 $ ./m4 471 define(foo,0000) 472 473 foo 474 0000 475 define(bar,defn(`foo')) 476 477 bar 478 0000 479 changequote(<QUOTE>,<UNQUOTE>) 480 481 define(baz,defn(<QUOTE>foo<UNQUOTE>)) 482 baz 483 Ctrl-d 484 m4: End of input: 0: fatal error: EOF in string 485 486Let us use GDB to try to see what is going on. 487 488 $ gdb m4 489 GDB is free software and you are welcome to distribute copies 490 of it under certain conditions; type "show copying" to see 491 the conditions. 492 There is absolutely no warranty for GDB; type "show warranty" 493 for details. 494 495 GDB 7.6.1, Copyright 1999 Free Software Foundation, Inc... 496 (gdb) 497 498GDB reads only enough symbol data to know where to find the rest when 499needed; as a result, the first prompt comes up very quickly. We now 500tell GDB to use a narrower display width than usual, so that examples 501fit in this manual. 502 503 (gdb) set width 70 504 505We need to see how the `m4' built-in `changequote' works. Having 506looked at the source, we know the relevant subroutine is 507`m4_changequote', so we set a breakpoint there with the GDB `break' 508command. 509 510 (gdb) break m4_changequote 511 Breakpoint 1 at 0x62f4: file builtin.c, line 879. 512 513Using the `run' command, we start `m4' running under GDB control; as 514long as control does not reach the `m4_changequote' subroutine, the 515program runs as usual: 516 517 (gdb) run 518 Starting program: /work/Editorial/gdb/gnu/m4/m4 519 define(foo,0000) 520 521 foo 522 0000 523 524To trigger the breakpoint, we call `changequote'. GDB suspends 525execution of `m4', displaying information about the context where it 526stops. 527 528 changequote(<QUOTE>,<UNQUOTE>) 529 530 Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) 531 at builtin.c:879 532 879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3)) 533 534Now we use the command `n' (`next') to advance execution to the next 535line of the current function. 536 537 (gdb) n 538 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ 539 : nil, 540 541`set_quotes' looks like a promising subroutine. We can go into it by 542using the command `s' (`step') instead of `next'. `step' goes to the 543next line to be executed in _any_ subroutine, so it steps into 544`set_quotes'. 545 546 (gdb) s 547 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") 548 at input.c:530 549 530 if (lquote != def_lquote) 550 551The display that shows the subroutine where `m4' is now suspended (and 552its arguments) is called a stack frame display. It shows a summary of 553the stack. We can use the `backtrace' command (which can also be 554spelled `bt'), to see where we are in the stack as a whole: the 555`backtrace' command displays a stack frame for each active subroutine. 556 557 (gdb) bt 558 #0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") 559 at input.c:530 560 #1 0x6344 in m4_changequote (argc=3, argv=0x33c70) 561 at builtin.c:882 562 #2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 563 #3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) 564 at macro.c:71 565 #4 0x79dc in expand_input () at macro.c:40 566 #5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195 567 568We step through a few more lines to see what happens. The first two 569times, we can use `s'; the next two times we use `n' to avoid falling 570into the `xstrdup' subroutine. 571 572 (gdb) s 573 0x3b5c 532 if (rquote != def_rquote) 574 (gdb) s 575 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ 576 def_lquote : xstrdup(lq); 577 (gdb) n 578 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ 579 : xstrdup(rq); 580 (gdb) n 581 538 len_lquote = strlen(rquote); 582 583The last line displayed looks a little odd; we can examine the variables 584`lquote' and `rquote' to see if they are in fact the new left and right 585quotes we specified. We use the command `p' (`print') to see their 586values. 587 588 (gdb) p lquote 589 $1 = 0x35d40 "<QUOTE>" 590 (gdb) p rquote 591 $2 = 0x35d50 "<UNQUOTE>" 592 593`lquote' and `rquote' are indeed the new left and right quotes. To 594look at some context, we can display ten lines of source surrounding 595the current line with the `l' (`list') command. 596 597 (gdb) l 598 533 xfree(rquote); 599 534 600 535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ 601 : xstrdup (lq); 602 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ 603 : xstrdup (rq); 604 537 605 538 len_lquote = strlen(rquote); 606 539 len_rquote = strlen(lquote); 607 540 } 608 541 609 542 void 610 611Let us step past the two lines that set `len_lquote' and `len_rquote', 612and then examine the values of those variables. 613 614 (gdb) n 615 539 len_rquote = strlen(lquote); 616 (gdb) n 617 540 } 618 (gdb) p len_lquote 619 $3 = 9 620 (gdb) p len_rquote 621 $4 = 7 622 623That certainly looks wrong, assuming `len_lquote' and `len_rquote' are 624meant to be the lengths of `lquote' and `rquote' respectively. We can 625set them to better values using the `p' command, since it can print the 626value of any expression--and that expression can include subroutine 627calls and assignments. 628 629 (gdb) p len_lquote=strlen(lquote) 630 $5 = 7 631 (gdb) p len_rquote=strlen(rquote) 632 $6 = 9 633 634Is that enough to fix the problem of using the new quotes with the `m4' 635built-in `defn'? We can allow `m4' to continue executing with the `c' 636(`continue') command, and then try the example that caused trouble 637initially: 638 639 (gdb) c 640 Continuing. 641 642 define(baz,defn(<QUOTE>foo<UNQUOTE>)) 643 644 baz 645 0000 646 647Success! The new quotes now work just as well as the default ones. The 648problem seems to have been just the two typos defining the wrong 649lengths. We allow `m4' exit by giving it an EOF as input: 650 651 Ctrl-d 652 Program exited normally. 653 654The message `Program exited normally.' is from GDB; it indicates `m4' 655has finished executing. We can end our GDB session with the GDB `quit' 656command. 657 658 (gdb) quit 659 660 661File: gdb.info, Node: Invocation, Next: Commands, Prev: Sample Session, Up: Top 662 6632 Getting In and Out of GDB 664*************************** 665 666This chapter discusses how to start GDB, and how to get out of it. The 667essentials are: 668 * type `gdb' to start GDB. 669 670 * type `quit' or `Ctrl-d' to exit. 671 672* Menu: 673 674* Invoking GDB:: How to start GDB 675* Quitting GDB:: How to quit GDB 676* Shell Commands:: How to use shell commands inside GDB 677* Logging Output:: How to log GDB's output to a file 678 679 680File: gdb.info, Node: Invoking GDB, Next: Quitting GDB, Up: Invocation 681 6822.1 Invoking GDB 683================ 684 685Invoke GDB by running the program `gdb'. Once started, GDB reads 686commands from the terminal until you tell it to exit. 687 688 You can also run `gdb' with a variety of arguments and options, to 689specify more of your debugging environment at the outset. 690 691 The command-line options described here are designed to cover a 692variety of situations; in some environments, some of these options may 693effectively be unavailable. 694 695 The most usual way to start GDB is with one argument, specifying an 696executable program: 697 698 gdb PROGRAM 699 700You can also start with both an executable program and a core file 701specified: 702 703 gdb PROGRAM CORE 704 705 You can, instead, specify a process ID as a second argument, if you 706want to debug a running process: 707 708 gdb PROGRAM 1234 709 710would attach GDB to process `1234' (unless you also have a file named 711`1234'; GDB does check for a core file first). 712 713 Taking advantage of the second command-line argument requires a 714fairly complete operating system; when you use GDB as a remote debugger 715attached to a bare board, there may not be any notion of "process", and 716there is often no way to get a core dump. GDB will warn you if it is 717unable to attach or to read core dumps. 718 719 You can optionally have `gdb' pass any arguments after the 720executable file to the inferior using `--args'. This option stops 721option processing. 722 gdb --args gcc -O2 -c foo.c 723 This will cause `gdb' to debug `gcc', and to set `gcc''s 724command-line arguments (*note Arguments::) to `-O2 -c foo.c'. 725 726 You can run `gdb' without printing the front material, which 727describes GDB's non-warranty, by specifying `-silent': 728 729 gdb -silent 730 731You can further control how GDB starts up by using command-line 732options. GDB itself can remind you of the options available. 733 734Type 735 736 gdb -help 737 738to display all available options and briefly describe their use (`gdb 739-h' is a shorter equivalent). 740 741 All options and command line arguments you give are processed in 742sequential order. The order makes a difference when the `-x' option is 743used. 744 745* Menu: 746 747* File Options:: Choosing files 748* Mode Options:: Choosing modes 749* Startup:: What GDB does during startup 750 751 752File: gdb.info, Node: File Options, Next: Mode Options, Up: Invoking GDB 753 7542.1.1 Choosing Files 755-------------------- 756 757When GDB starts, it reads any arguments other than options as 758specifying an executable file and core file (or process ID). This is 759the same as if the arguments were specified by the `-se' and `-c' (or 760`-p') options respectively. (GDB reads the first argument that does 761not have an associated option flag as equivalent to the `-se' option 762followed by that argument; and the second argument that does not have 763an associated option flag, if any, as equivalent to the `-c'/`-p' 764option followed by that argument.) If the second argument begins with 765a decimal digit, GDB will first attempt to attach to it as a process, 766and if that fails, attempt to open it as a corefile. If you have a 767corefile whose name begins with a digit, you can prevent GDB from 768treating it as a pid by prefixing it with `./', e.g. `./12345'. 769 770 If GDB has not been configured to included core file support, such 771as for most embedded targets, then it will complain about a second 772argument and ignore it. 773 774 Many options have both long and short forms; both are shown in the 775following list. GDB also recognizes the long forms if you truncate 776them, so long as enough of the option is present to be unambiguous. 777(If you prefer, you can flag option arguments with `--' rather than 778`-', though we illustrate the more usual convention.) 779 780`-symbols FILE' 781`-s FILE' 782 Read symbol table from file FILE. 783 784`-exec FILE' 785`-e FILE' 786 Use file FILE as the executable file to execute when appropriate, 787 and for examining pure data in conjunction with a core dump. 788 789`-se FILE' 790 Read symbol table from file FILE and use it as the executable file. 791 792`-core FILE' 793`-c FILE' 794 Use file FILE as a core dump to examine. 795 796`-pid NUMBER' 797`-p NUMBER' 798 Connect to process ID NUMBER, as with the `attach' command. 799 800`-command FILE' 801`-x FILE' 802 Execute commands from file FILE. The contents of this file is 803 evaluated exactly as the `source' command would. *Note Command 804 files: Command Files. 805 806`-eval-command COMMAND' 807`-ex COMMAND' 808 Execute a single GDB command. 809 810 This option may be used multiple times to call multiple commands. 811 It may also be interleaved with `-command' as required. 812 813 gdb -ex 'target sim' -ex 'load' \ 814 -x setbreakpoints -ex 'run' a.out 815 816`-init-command FILE' 817`-ix FILE' 818 Execute commands from file FILE before loading the inferior (but 819 after loading gdbinit files). *Note Startup::. 820 821`-init-eval-command COMMAND' 822`-iex COMMAND' 823 Execute a single GDB command before loading the inferior (but 824 after loading gdbinit files). *Note Startup::. 825 826`-directory DIRECTORY' 827`-d DIRECTORY' 828 Add DIRECTORY to the path to search for source and script files. 829 830`-r' 831`-readnow' 832 Read each symbol file's entire symbol table immediately, rather 833 than the default, which is to read it incrementally as it is 834 needed. This makes startup slower, but makes future operations 835 faster. 836 837 838 839File: gdb.info, Node: Mode Options, Next: Startup, Prev: File Options, Up: Invoking GDB 840 8412.1.2 Choosing Modes 842-------------------- 843 844You can run GDB in various alternative modes--for example, in batch 845mode or quiet mode. 846 847`-nx' 848`-n' 849 Do not execute commands found in any initialization file. There 850 are three init files, loaded in the following order: 851 852 ``system.gdbinit'' 853 This is the system-wide init file. Its location is specified 854 with the `--with-system-gdbinit' configure option (*note 855 System-wide configuration::). It is loaded first when GDB 856 starts, before command line options have been processed. 857 858 ``~/.gdbinit'' 859 This is the init file in your home directory. It is loaded 860 next, after `system.gdbinit', and before command options have 861 been processed. 862 863 ``./.gdbinit'' 864 This is the init file in the current directory. It is loaded 865 last, after command line options other than `-x' and `-ex' 866 have been processed. Command line options `-x' and `-ex' are 867 processed last, after `./.gdbinit' has been loaded. 868 869 For further documentation on startup processing, *Note Startup::. 870 For documentation on how to write command files, *Note Command 871 Files: Command Files. 872 873`-nh' 874 Do not execute commands found in `~/.gdbinit', the init file in 875 your home directory. *Note Startup::. 876 877`-quiet' 878`-silent' 879`-q' 880 "Quiet". Do not print the introductory and copyright messages. 881 These messages are also suppressed in batch mode. 882 883`-batch' 884 Run in batch mode. Exit with status `0' after processing all the 885 command files specified with `-x' (and all commands from 886 initialization files, if not inhibited with `-n'). Exit with 887 nonzero status if an error occurs in executing the GDB commands in 888 the command files. Batch mode also disables pagination, sets 889 unlimited terminal width and height *note Screen Size::, and acts 890 as if `set confirm off' were in effect (*note Messages/Warnings::). 891 892 Batch mode may be useful for running GDB as a filter, for example 893 to download and run a program on another computer; in order to 894 make this more useful, the message 895 896 Program exited normally. 897 898 (which is ordinarily issued whenever a program running under GDB 899 control terminates) is not issued when running in batch mode. 900 901`-batch-silent' 902 Run in batch mode exactly like `-batch', but totally silently. All 903 GDB output to `stdout' is prevented (`stderr' is unaffected). 904 This is much quieter than `-silent' and would be useless for an 905 interactive session. 906 907 This is particularly useful when using targets that give `Loading 908 section' messages, for example. 909 910 Note that targets that give their output via GDB, as opposed to 911 writing directly to `stdout', will also be made silent. 912 913`-return-child-result' 914 The return code from GDB will be the return code from the child 915 process (the process being debugged), with the following 916 exceptions: 917 918 * GDB exits abnormally. E.g., due to an incorrect argument or 919 an internal error. In this case the exit code is the same as 920 it would have been without `-return-child-result'. 921 922 * The user quits with an explicit value. E.g., `quit 1'. 923 924 * The child process never runs, or is not allowed to terminate, 925 in which case the exit code will be -1. 926 927 This option is useful in conjunction with `-batch' or 928 `-batch-silent', when GDB is being used as a remote program loader 929 or simulator interface. 930 931`-nowindows' 932`-nw' 933 "No windows". If GDB comes with a graphical user interface (GUI) 934 built in, then this option tells GDB to only use the command-line 935 interface. If no GUI is available, this option has no effect. 936 937`-windows' 938`-w' 939 If GDB includes a GUI, then this option requires it to be used if 940 possible. 941 942`-cd DIRECTORY' 943 Run GDB using DIRECTORY as its working directory, instead of the 944 current directory. 945 946`-data-directory DIRECTORY' 947 Run GDB using DIRECTORY as its data directory. The data directory 948 is where GDB searches for its auxiliary files. *Note Data Files::. 949 950`-fullname' 951`-f' 952 GNU Emacs sets this option when it runs GDB as a subprocess. It 953 tells GDB to output the full file name and line number in a 954 standard, recognizable fashion each time a stack frame is 955 displayed (which includes each time your program stops). This 956 recognizable format looks like two `\032' characters, followed by 957 the file name, line number and character position separated by 958 colons, and a newline. The Emacs-to-GDB interface program uses 959 the two `\032' characters as a signal to display the source code 960 for the frame. 961 962`-annotate LEVEL' 963 This option sets the "annotation level" inside GDB. Its effect is 964 identical to using `set annotate LEVEL' (*note Annotations::). 965 The annotation LEVEL controls how much information GDB prints 966 together with its prompt, values of expressions, source lines, and 967 other types of output. Level 0 is the normal, level 1 is for use 968 when GDB is run as a subprocess of GNU Emacs, level 3 is the 969 maximum annotation suitable for programs that control GDB, and 970 level 2 has been deprecated. 971 972 The annotation mechanism has largely been superseded by GDB/MI 973 (*note GDB/MI::). 974 975`--args' 976 Change interpretation of command line so that arguments following 977 the executable file are passed as command line arguments to the 978 inferior. This option stops option processing. 979 980`-baud BPS' 981`-b BPS' 982 Set the line speed (baud rate or bits per second) of any serial 983 interface used by GDB for remote debugging. 984 985`-l TIMEOUT' 986 Set the timeout (in seconds) of any communication used by GDB for 987 remote debugging. 988 989`-tty DEVICE' 990`-t DEVICE' 991 Run using DEVICE for your program's standard input and output. 992 993`-tui' 994 Activate the "Text User Interface" when starting. The Text User 995 Interface manages several text windows on the terminal, showing 996 source, assembly, registers and GDB command outputs (*note GDB 997 Text User Interface: TUI.). Do not use this option if you run GDB 998 from Emacs (*note Using GDB under GNU Emacs: Emacs.). 999 1000`-interpreter INTERP' 1001 Use the interpreter INTERP for interface with the controlling 1002 program or device. This option is meant to be set by programs 1003 which communicate with GDB using it as a back end. *Note Command 1004 Interpreters: Interpreters. 1005 1006 `--interpreter=mi' (or `--interpreter=mi2') causes GDB to use the 1007 "GDB/MI interface" (*note The GDB/MI Interface: GDB/MI.) included 1008 since GDB version 6.0. The previous GDB/MI interface, included in 1009 GDB version 5.3 and selected with `--interpreter=mi1', is 1010 deprecated. Earlier GDB/MI interfaces are no longer supported. 1011 1012`-write' 1013 Open the executable and core files for both reading and writing. 1014 This is equivalent to the `set write on' command inside GDB (*note 1015 Patching::). 1016 1017`-statistics' 1018 This option causes GDB to print statistics about time and memory 1019 usage after it completes each command and returns to the prompt. 1020 1021`-version' 1022 This option causes GDB to print its version number and no-warranty 1023 blurb, and exit. 1024 1025 1026 1027File: gdb.info, Node: Startup, Prev: Mode Options, Up: Invoking GDB 1028 10292.1.3 What GDB Does During Startup 1030---------------------------------- 1031 1032Here's the description of what GDB does during session startup: 1033 1034 1. Sets up the command interpreter as specified by the command line 1035 (*note interpreter: Mode Options.). 1036 1037 2. Reads the system-wide "init file" (if `--with-system-gdbinit' was 1038 used when building GDB; *note System-wide configuration and 1039 settings: System-wide configuration.) and executes all the 1040 commands in that file. 1041 1042 3. Reads the init file (if any) in your home directory(1) and 1043 executes all the commands in that file. 1044 1045 4. Executes commands and command files specified by the `-iex' and 1046 `-ix' options in their specified order. Usually you should use the 1047 `-ex' and `-x' options instead, but this way you can apply 1048 settings before GDB init files get executed and before inferior 1049 gets loaded. 1050 1051 5. Processes command line options and operands. 1052 1053 6. Reads and executes the commands from init file (if any) in the 1054 current working directory as long as `set auto-load local-gdbinit' 1055 is set to `on' (*note Init File in the Current Directory::). This 1056 is only done if the current directory is different from your home 1057 directory. Thus, you can have more than one init file, one 1058 generic in your home directory, and another, specific to the 1059 program you are debugging, in the directory where you invoke GDB. 1060 1061 7. If the command line specified a program to debug, or a process to 1062 attach to, or a core file, GDB loads any auto-loaded scripts 1063 provided for the program or for its loaded shared libraries. 1064 *Note Auto-loading::. 1065 1066 If you wish to disable the auto-loading during startup, you must 1067 do something like the following: 1068 1069 $ gdb -iex "set auto-load python-scripts off" myprogram 1070 1071 Option `-ex' does not work because the auto-loading is then turned 1072 off too late. 1073 1074 8. Executes commands and command files specified by the `-ex' and 1075 `-x' options in their specified order. *Note Command Files::, for 1076 more details about GDB command files. 1077 1078 9. Reads the command history recorded in the "history file". *Note 1079 Command History::, for more details about the command history and 1080 the files where GDB records it. 1081 1082 Init files use the same syntax as "command files" (*note Command 1083Files::) and are processed by GDB in the same way. The init file in 1084your home directory can set options (such as `set complaints') that 1085affect subsequent processing of command line options and operands. 1086Init files are not executed if you use the `-nx' option (*note Choosing 1087Modes: Mode Options.). 1088 1089 To display the list of init files loaded by gdb at startup, you can 1090use `gdb --help'. 1091 1092 The GDB init files are normally called `.gdbinit'. The DJGPP port 1093of GDB uses the name `gdb.ini', due to the limitations of file names 1094imposed by DOS filesystems. The Windows port of GDB uses the standard 1095name, but if it finds a `gdb.ini' file in your home directory, it warns 1096you about that and suggests to rename the file to the standard name. 1097 1098 ---------- Footnotes ---------- 1099 1100 (1) On DOS/Windows systems, the home directory is the one pointed to 1101by the `HOME' environment variable. 1102 1103 1104File: gdb.info, Node: Quitting GDB, Next: Shell Commands, Prev: Invoking GDB, Up: Invocation 1105 11062.2 Quitting GDB 1107================ 1108 1109`quit [EXPRESSION]' 1110`q' 1111 To exit GDB, use the `quit' command (abbreviated `q'), or type an 1112 end-of-file character (usually `Ctrl-d'). If you do not supply 1113 EXPRESSION, GDB will terminate normally; otherwise it will 1114 terminate using the result of EXPRESSION as the error code. 1115 1116 An interrupt (often `Ctrl-c') does not exit from GDB, but rather 1117terminates the action of any GDB command that is in progress and 1118returns to GDB command level. It is safe to type the interrupt 1119character at any time because GDB does not allow it to take effect 1120until a time when it is safe. 1121 1122 If you have been using GDB to control an attached process or device, 1123you can release it with the `detach' command (*note Debugging an 1124Already-running Process: Attach.). 1125 1126 1127File: gdb.info, Node: Shell Commands, Next: Logging Output, Prev: Quitting GDB, Up: Invocation 1128 11292.3 Shell Commands 1130================== 1131 1132If you need to execute occasional shell commands during your debugging 1133session, there is no need to leave or suspend GDB; you can just use the 1134`shell' command. 1135 1136`shell COMMAND-STRING' 1137`!COMMAND-STRING' 1138 Invoke a standard shell to execute COMMAND-STRING. Note that no 1139 space is needed between `!' and COMMAND-STRING. If it exists, the 1140 environment variable `SHELL' determines which shell to run. 1141 Otherwise GDB uses the default shell (`/bin/sh' on Unix systems, 1142 `COMMAND.COM' on MS-DOS, etc.). 1143 1144 The utility `make' is often needed in development environments. You 1145do not have to use the `shell' command for this purpose in GDB: 1146 1147`make MAKE-ARGS' 1148 Execute the `make' program with the specified arguments. This is 1149 equivalent to `shell make MAKE-ARGS'. 1150 1151 1152File: gdb.info, Node: Logging Output, Prev: Shell Commands, Up: Invocation 1153 11542.4 Logging Output 1155================== 1156 1157You may want to save the output of GDB commands to a file. There are 1158several commands to control GDB's logging. 1159 1160`set logging on' 1161 Enable logging. 1162 1163`set logging off' 1164 Disable logging. 1165 1166`set logging file FILE' 1167 Change the name of the current logfile. The default logfile is 1168 `gdb.txt'. 1169 1170`set logging overwrite [on|off]' 1171 By default, GDB will append to the logfile. Set `overwrite' if 1172 you want `set logging on' to overwrite the logfile instead. 1173 1174`set logging redirect [on|off]' 1175 By default, GDB output will go to both the terminal and the 1176 logfile. Set `redirect' if you want output to go only to the log 1177 file. 1178 1179`show logging' 1180 Show the current values of the logging settings. 1181 1182 1183File: gdb.info, Node: Commands, Next: Running, Prev: Invocation, Up: Top 1184 11853 GDB Commands 1186************** 1187 1188You can abbreviate a GDB command to the first few letters of the command 1189name, if that abbreviation is unambiguous; and you can repeat certain 1190GDB commands by typing just <RET>. You can also use the <TAB> key to 1191get GDB to fill out the rest of a word in a command (or to show you the 1192alternatives available, if there is more than one possibility). 1193 1194* Menu: 1195 1196* Command Syntax:: How to give commands to GDB 1197* Completion:: Command completion 1198* Help:: How to ask GDB for help 1199 1200 1201File: gdb.info, Node: Command Syntax, Next: Completion, Up: Commands 1202 12033.1 Command Syntax 1204================== 1205 1206A GDB command is a single line of input. There is no limit on how long 1207it can be. It starts with a command name, which is followed by 1208arguments whose meaning depends on the command name. For example, the 1209command `step' accepts an argument which is the number of times to 1210step, as in `step 5'. You can also use the `step' command with no 1211arguments. Some commands do not allow any arguments. 1212 1213 GDB command names may always be truncated if that abbreviation is 1214unambiguous. Other possible command abbreviations are listed in the 1215documentation for individual commands. In some cases, even ambiguous 1216abbreviations are allowed; for example, `s' is specially defined as 1217equivalent to `step' even though there are other commands whose names 1218start with `s'. You can test abbreviations by using them as arguments 1219to the `help' command. 1220 1221 A blank line as input to GDB (typing just <RET>) means to repeat the 1222previous command. Certain commands (for example, `run') will not 1223repeat this way; these are commands whose unintentional repetition 1224might cause trouble and which you are unlikely to want to repeat. 1225User-defined commands can disable this feature; see *note dont-repeat: 1226Define. 1227 1228 The `list' and `x' commands, when you repeat them with <RET>, 1229construct new arguments rather than repeating exactly as typed. This 1230permits easy scanning of source or memory. 1231 1232 GDB can also use <RET> in another way: to partition lengthy output, 1233in a way similar to the common utility `more' (*note Screen Size: 1234Screen Size.). Since it is easy to press one <RET> too many in this 1235situation, GDB disables command repetition after any command that 1236generates this sort of display. 1237 1238 Any text from a `#' to the end of the line is a comment; it does 1239nothing. This is useful mainly in command files (*note Command Files: 1240Command Files.). 1241 1242 The `Ctrl-o' binding is useful for repeating a complex sequence of 1243commands. This command accepts the current line, like <RET>, and then 1244fetches the next line relative to the current line from the history for 1245editing. 1246 1247 1248File: gdb.info, Node: Completion, Next: Help, Prev: Command Syntax, Up: Commands 1249 12503.2 Command Completion 1251====================== 1252 1253GDB can fill in the rest of a word in a command for you, if there is 1254only one possibility; it can also show you what the valid possibilities 1255are for the next word in a command, at any time. This works for GDB 1256commands, GDB subcommands, and the names of symbols in your program. 1257 1258 Press the <TAB> key whenever you want GDB to fill out the rest of a 1259word. If there is only one possibility, GDB fills in the word, and 1260waits for you to finish the command (or press <RET> to enter it). For 1261example, if you type 1262 1263 (gdb) info bre <TAB> 1264 1265GDB fills in the rest of the word `breakpoints', since that is the only 1266`info' subcommand beginning with `bre': 1267 1268 (gdb) info breakpoints 1269 1270You can either press <RET> at this point, to run the `info breakpoints' 1271command, or backspace and enter something else, if `breakpoints' does 1272not look like the command you expected. (If you were sure you wanted 1273`info breakpoints' in the first place, you might as well just type 1274<RET> immediately after `info bre', to exploit command abbreviations 1275rather than command completion). 1276 1277 If there is more than one possibility for the next word when you 1278press <TAB>, GDB sounds a bell. You can either supply more characters 1279and try again, or just press <TAB> a second time; GDB displays all the 1280possible completions for that word. For example, you might want to set 1281a breakpoint on a subroutine whose name begins with `make_', but when 1282you type `b make_<TAB>' GDB just sounds the bell. Typing <TAB> again 1283displays all the function names in your program that begin with those 1284characters, for example: 1285 1286 (gdb) b make_ <TAB> 1287GDB sounds bell; press <TAB> again, to see: 1288 make_a_section_from_file make_environ 1289 make_abs_section make_function_type 1290 make_blockvector make_pointer_type 1291 make_cleanup make_reference_type 1292 make_command make_symbol_completion_list 1293 (gdb) b make_ 1294 1295After displaying the available possibilities, GDB copies your partial 1296input (`b make_' in the example) so you can finish the command. 1297 1298 If you just want to see the list of alternatives in the first place, 1299you can press `M-?' rather than pressing <TAB> twice. `M-?' means 1300`<META> ?'. You can type this either by holding down a key designated 1301as the <META> shift on your keyboard (if there is one) while typing 1302`?', or as <ESC> followed by `?'. 1303 1304 Sometimes the string you need, while logically a "word", may contain 1305parentheses or other characters that GDB normally excludes from its 1306notion of a word. To permit word completion to work in this situation, 1307you may enclose words in `'' (single quote marks) in GDB commands. 1308 1309 The most likely situation where you might need this is in typing the 1310name of a C++ function. This is because C++ allows function 1311overloading (multiple definitions of the same function, distinguished 1312by argument type). For example, when you want to set a breakpoint you 1313may need to distinguish whether you mean the version of `name' that 1314takes an `int' parameter, `name(int)', or the version that takes a 1315`float' parameter, `name(float)'. To use the word-completion 1316facilities in this situation, type a single quote `'' at the beginning 1317of the function name. This alerts GDB that it may need to consider 1318more information than usual when you press <TAB> or `M-?' to request 1319word completion: 1320 1321 (gdb) b 'bubble( M-? 1322 bubble(double,double) bubble(int,int) 1323 (gdb) b 'bubble( 1324 1325 In some cases, GDB can tell that completing a name requires using 1326quotes. When this happens, GDB inserts the quote for you (while 1327completing as much as it can) if you do not type the quote in the first 1328place: 1329 1330 (gdb) b bub <TAB> 1331GDB alters your input line to the following, and rings a bell: 1332 (gdb) b 'bubble( 1333 1334In general, GDB can tell that a quote is needed (and inserts it) if you 1335have not yet started typing the argument list when you ask for 1336completion on an overloaded symbol. 1337 1338 For more information about overloaded functions, see *note C++ 1339Expressions: C Plus Plus Expressions. You can use the command `set 1340overload-resolution off' to disable overload resolution; see *note GDB 1341Features for C++: Debugging C Plus Plus. 1342 1343 When completing in an expression which looks up a field in a 1344structure, GDB also tries(1) to limit completions to the field names 1345available in the type of the left-hand-side: 1346 1347 (gdb) p gdb_stdout.M-? 1348 magic to_fputs to_rewind 1349 to_data to_isatty to_write 1350 to_delete to_put to_write_async_safe 1351 to_flush to_read 1352 1353This is because the `gdb_stdout' is a variable of the type `struct 1354ui_file' that is defined in GDB sources as follows: 1355 1356 struct ui_file 1357 { 1358 int *magic; 1359 ui_file_flush_ftype *to_flush; 1360 ui_file_write_ftype *to_write; 1361 ui_file_write_async_safe_ftype *to_write_async_safe; 1362 ui_file_fputs_ftype *to_fputs; 1363 ui_file_read_ftype *to_read; 1364 ui_file_delete_ftype *to_delete; 1365 ui_file_isatty_ftype *to_isatty; 1366 ui_file_rewind_ftype *to_rewind; 1367 ui_file_put_ftype *to_put; 1368 void *to_data; 1369 } 1370 1371 ---------- Footnotes ---------- 1372 1373 (1) The completer can be confused by certain kinds of invalid 1374expressions. Also, it only examines the static type of the expression, 1375not the dynamic type. 1376 1377 1378File: gdb.info, Node: Help, Prev: Completion, Up: Commands 1379 13803.3 Getting Help 1381================ 1382 1383You can always ask GDB itself for information on its commands, using 1384the command `help'. 1385 1386`help' 1387`h' 1388 You can use `help' (abbreviated `h') with no arguments to display 1389 a short list of named classes of commands: 1390 1391 (gdb) help 1392 List of classes of commands: 1393 1394 aliases -- Aliases of other commands 1395 breakpoints -- Making program stop at certain points 1396 data -- Examining data 1397 files -- Specifying and examining files 1398 internals -- Maintenance commands 1399 obscure -- Obscure features 1400 running -- Running the program 1401 stack -- Examining the stack 1402 status -- Status inquiries 1403 support -- Support facilities 1404 tracepoints -- Tracing of program execution without 1405 stopping the program 1406 user-defined -- User-defined commands 1407 1408 Type "help" followed by a class name for a list of 1409 commands in that class. 1410 Type "help" followed by command name for full 1411 documentation. 1412 Command name abbreviations are allowed if unambiguous. 1413 (gdb) 1414 1415`help CLASS' 1416 Using one of the general help classes as an argument, you can get a 1417 list of the individual commands in that class. For example, here 1418 is the help display for the class `status': 1419 1420 (gdb) help status 1421 Status inquiries. 1422 1423 List of commands: 1424 1425 info -- Generic command for showing things 1426 about the program being debugged 1427 show -- Generic command for showing things 1428 about the debugger 1429 1430 Type "help" followed by command name for full 1431 documentation. 1432 Command name abbreviations are allowed if unambiguous. 1433 (gdb) 1434 1435`help COMMAND' 1436 With a command name as `help' argument, GDB displays a short 1437 paragraph on how to use that command. 1438 1439`apropos ARGS' 1440 The `apropos' command searches through all of the GDB commands, 1441 and their documentation, for the regular expression specified in 1442 ARGS. It prints out all matches found. For example: 1443 1444 apropos alias 1445 1446 results in: 1447 1448 alias -- Define a new command that is an alias of an existing command 1449 aliases -- Aliases of other commands 1450 d -- Delete some breakpoints or auto-display expressions 1451 del -- Delete some breakpoints or auto-display expressions 1452 delete -- Delete some breakpoints or auto-display expressions 1453 1454`complete ARGS' 1455 The `complete ARGS' command lists all the possible completions for 1456 the beginning of a command. Use ARGS to specify the beginning of 1457 the command you want completed. For example: 1458 1459 complete i 1460 1461 results in: 1462 1463 if 1464 ignore 1465 info 1466 inspect 1467 1468 This is intended for use by GNU Emacs. 1469 1470 In addition to `help', you can use the GDB commands `info' and 1471`show' to inquire about the state of your program, or the state of GDB 1472itself. Each command supports many topics of inquiry; this manual 1473introduces each of them in the appropriate context. The listings under 1474`info' and under `show' in the Command, Variable, and Function Index 1475point to all the sub-commands. *Note Command and Variable Index::. 1476 1477`info' 1478 This command (abbreviated `i') is for describing the state of your 1479 program. For example, you can show the arguments passed to a 1480 function with `info args', list the registers currently in use 1481 with `info registers', or list the breakpoints you have set with 1482 `info breakpoints'. You can get a complete list of the `info' 1483 sub-commands with `help info'. 1484 1485`set' 1486 You can assign the result of an expression to an environment 1487 variable with `set'. For example, you can set the GDB prompt to a 1488 $-sign with `set prompt $'. 1489 1490`show' 1491 In contrast to `info', `show' is for describing the state of GDB 1492 itself. You can change most of the things you can `show', by 1493 using the related command `set'; for example, you can control what 1494 number system is used for displays with `set radix', or simply 1495 inquire which is currently in use with `show radix'. 1496 1497 To display all the settable parameters and their current values, 1498 you can use `show' with no arguments; you may also use `info set'. 1499 Both commands produce the same display. 1500 1501 Here are three miscellaneous `show' subcommands, all of which are 1502exceptional in lacking corresponding `set' commands: 1503 1504`show version' 1505 Show what version of GDB is running. You should include this 1506 information in GDB bug-reports. If multiple versions of GDB are 1507 in use at your site, you may need to determine which version of 1508 GDB you are running; as GDB evolves, new commands are introduced, 1509 and old ones may wither away. Also, many system vendors ship 1510 variant versions of GDB, and there are variant versions of GDB in 1511 GNU/Linux distributions as well. The version number is the same 1512 as the one announced when you start GDB. 1513 1514`show copying' 1515`info copying' 1516 Display information about permission for copying GDB. 1517 1518`show warranty' 1519`info warranty' 1520 Display the GNU "NO WARRANTY" statement, or a warranty, if your 1521 version of GDB comes with one. 1522 1523 1524 1525File: gdb.info, Node: Running, Next: Stopping, Prev: Commands, Up: Top 1526 15274 Running Programs Under GDB 1528**************************** 1529 1530When you run a program under GDB, you must first generate debugging 1531information when you compile it. 1532 1533 You may start GDB with its arguments, if any, in an environment of 1534your choice. If you are doing native debugging, you may redirect your 1535program's input and output, debug an already running process, or kill a 1536child process. 1537 1538* Menu: 1539 1540* Compilation:: Compiling for debugging 1541* Starting:: Starting your program 1542* Arguments:: Your program's arguments 1543* Environment:: Your program's environment 1544 1545* Working Directory:: Your program's working directory 1546* Input/Output:: Your program's input and output 1547* Attach:: Debugging an already-running process 1548* Kill Process:: Killing the child process 1549 1550* Inferiors and Programs:: Debugging multiple inferiors and programs 1551* Threads:: Debugging programs with multiple threads 1552* Forks:: Debugging forks 1553* Checkpoint/Restart:: Setting a _bookmark_ to return to later 1554 1555 1556File: gdb.info, Node: Compilation, Next: Starting, Up: Running 1557 15584.1 Compiling for Debugging 1559=========================== 1560 1561In order to debug a program effectively, you need to generate debugging 1562information when you compile it. This debugging information is stored 1563in the object file; it describes the data type of each variable or 1564function and the correspondence between source line numbers and 1565addresses in the executable code. 1566 1567 To request debugging information, specify the `-g' option when you 1568run the compiler. 1569 1570 Programs that are to be shipped to your customers are compiled with 1571optimizations, using the `-O' compiler option. However, some compilers 1572are unable to handle the `-g' and `-O' options together. Using those 1573compilers, you cannot generate optimized executables containing 1574debugging information. 1575 1576 GCC, the GNU C/C++ compiler, supports `-g' with or without `-O', 1577making it possible to debug optimized code. We recommend that you 1578_always_ use `-g' whenever you compile a program. You may think your 1579program is correct, but there is no sense in pushing your luck. For 1580more information, see *note Optimized Code::. 1581 1582 Older versions of the GNU C compiler permitted a variant option 1583`-gg' for debugging information. GDB no longer supports this format; 1584if your GNU C compiler has this option, do not use it. 1585 1586 GDB knows about preprocessor macros and can show you their expansion 1587(*note Macros::). Most compilers do not include information about 1588preprocessor macros in the debugging information if you specify the 1589`-g' flag alone. Version 3.1 and later of GCC, the GNU C compiler, 1590provides macro information if you are using the DWARF debugging format, 1591and specify the option `-g3'. 1592 1593 *Note Options for Debugging Your Program or GCC: (gcc.info)Debugging 1594Options, for more information on GCC options affecting debug 1595information. 1596 1597 You will have the best debugging experience if you use the latest 1598version of the DWARF debugging format that your compiler supports. 1599DWARF is currently the most expressive and best supported debugging 1600format in GDB. 1601 1602 1603File: gdb.info, Node: Starting, Next: Arguments, Prev: Compilation, Up: Running 1604 16054.2 Starting your Program 1606========================= 1607 1608`run' 1609`r' 1610 Use the `run' command to start your program under GDB. You must 1611 first specify the program name (except on VxWorks) with an 1612 argument to GDB (*note Getting In and Out of GDB: Invocation.), or 1613 by using the `file' or `exec-file' command (*note Commands to 1614 Specify Files: Files.). 1615 1616 1617 If you are running your program in an execution environment that 1618supports processes, `run' creates an inferior process and makes that 1619process run your program. In some environments without processes, 1620`run' jumps to the start of your program. Other targets, like 1621`remote', are always running. If you get an error message like this 1622one: 1623 1624 The "remote" target does not support "run". 1625 Try "help target" or "continue". 1626 1627then use `continue' to run your program. You may need `load' first 1628(*note load::). 1629 1630 The execution of a program is affected by certain information it 1631receives from its superior. GDB provides ways to specify this 1632information, which you must do _before_ starting your program. (You 1633can change it after starting your program, but such changes only affect 1634your program the next time you start it.) This information may be 1635divided into four categories: 1636 1637The _arguments._ 1638 Specify the arguments to give your program as the arguments of the 1639 `run' command. If a shell is available on your target, the shell 1640 is used to pass the arguments, so that you may use normal 1641 conventions (such as wildcard expansion or variable substitution) 1642 in describing the arguments. In Unix systems, you can control 1643 which shell is used with the `SHELL' environment variable. *Note 1644 Your Program's Arguments: Arguments. 1645 1646The _environment._ 1647 Your program normally inherits its environment from GDB, but you 1648 can use the GDB commands `set environment' and `unset environment' 1649 to change parts of the environment that affect your program. 1650 *Note Your Program's Environment: Environment. 1651 1652The _working directory._ 1653 Your program inherits its working directory from GDB. You can set 1654 the GDB working directory with the `cd' command in GDB. *Note 1655 Your Program's Working Directory: Working Directory. 1656 1657The _standard input and output._ 1658 Your program normally uses the same device for standard input and 1659 standard output as GDB is using. You can redirect input and output 1660 in the `run' command line, or you can use the `tty' command to set 1661 a different device for your program. *Note Your Program's Input 1662 and Output: Input/Output. 1663 1664 _Warning:_ While input and output redirection work, you cannot use 1665 pipes to pass the output of the program you are debugging to 1666 another program; if you attempt this, GDB is likely to wind up 1667 debugging the wrong program. 1668 1669 When you issue the `run' command, your program begins to execute 1670immediately. *Note Stopping and Continuing: Stopping, for discussion 1671of how to arrange for your program to stop. Once your program has 1672stopped, you may call functions in your program, using the `print' or 1673`call' commands. *Note Examining Data: Data. 1674 1675 If the modification time of your symbol file has changed since the 1676last time GDB read its symbols, GDB discards its symbol table, and 1677reads it again. When it does this, GDB tries to retain your current 1678breakpoints. 1679 1680`start' 1681 The name of the main procedure can vary from language to language. 1682 With C or C++, the main procedure name is always `main', but other 1683 languages such as Ada do not require a specific name for their 1684 main procedure. The debugger provides a convenient way to start 1685 the execution of the program and to stop at the beginning of the 1686 main procedure, depending on the language used. 1687 1688 The `start' command does the equivalent of setting a temporary 1689 breakpoint at the beginning of the main procedure and then invoking 1690 the `run' command. 1691 1692 Some programs contain an "elaboration" phase where some startup 1693 code is executed before the main procedure is called. This 1694 depends on the languages used to write your program. In C++, for 1695 instance, constructors for static and global objects are executed 1696 before `main' is called. It is therefore possible that the 1697 debugger stops before reaching the main procedure. However, the 1698 temporary breakpoint will remain to halt execution. 1699 1700 Specify the arguments to give to your program as arguments to the 1701 `start' command. These arguments will be given verbatim to the 1702 underlying `run' command. Note that the same arguments will be 1703 reused if no argument is provided during subsequent calls to 1704 `start' or `run'. 1705 1706 It is sometimes necessary to debug the program during elaboration. 1707 In these cases, using the `start' command would stop the execution 1708 of your program too late, as the program would have already 1709 completed the elaboration phase. Under these circumstances, 1710 insert breakpoints in your elaboration code before running your 1711 program. 1712 1713`set exec-wrapper WRAPPER' 1714`show exec-wrapper' 1715`unset exec-wrapper' 1716 When `exec-wrapper' is set, the specified wrapper is used to 1717 launch programs for debugging. GDB starts your program with a 1718 shell command of the form `exec WRAPPER PROGRAM'. Quoting is 1719 added to PROGRAM and its arguments, but not to WRAPPER, so you 1720 should add quotes if appropriate for your shell. The wrapper runs 1721 until it executes your program, and then GDB takes control. 1722 1723 You can use any program that eventually calls `execve' with its 1724 arguments as a wrapper. Several standard Unix utilities do this, 1725 e.g. `env' and `nohup'. Any Unix shell script ending with `exec 1726 "$@"' will also work. 1727 1728 For example, you can use `env' to pass an environment variable to 1729 the debugged program, without setting the variable in your shell's 1730 environment: 1731 1732 (gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so' 1733 (gdb) run 1734 1735 This command is available when debugging locally on most targets, 1736 excluding DJGPP, Cygwin, MS Windows, and QNX Neutrino. 1737 1738`set disable-randomization' 1739`set disable-randomization on' 1740 This option (enabled by default in GDB) will turn off the native 1741 randomization of the virtual address space of the started program. 1742 This option is useful for multiple debugging sessions to make the 1743 execution better reproducible and memory addresses reusable across 1744 debugging sessions. 1745 1746 This feature is implemented only on certain targets, including 1747 GNU/Linux. On GNU/Linux you can get the same behavior using 1748 1749 (gdb) set exec-wrapper setarch `uname -m` -R 1750 1751`set disable-randomization off' 1752 Leave the behavior of the started executable unchanged. Some bugs 1753 rear their ugly heads only when the program is loaded at certain 1754 addresses. If your bug disappears when you run the program under 1755 GDB, that might be because GDB by default disables the address 1756 randomization on platforms, such as GNU/Linux, which do that for 1757 stand-alone programs. Use `set disable-randomization off' to try 1758 to reproduce such elusive bugs. 1759 1760 On targets where it is available, virtual address space 1761 randomization protects the programs against certain kinds of 1762 security attacks. In these cases the attacker needs to know the 1763 exact location of a concrete executable code. Randomizing its 1764 location makes it impossible to inject jumps misusing a code at 1765 its expected addresses. 1766 1767 Prelinking shared libraries provides a startup performance 1768 advantage but it makes addresses in these libraries predictable 1769 for privileged processes by having just unprivileged access at the 1770 target system. Reading the shared library binary gives enough 1771 information for assembling the malicious code misusing it. Still 1772 even a prelinked shared library can get loaded at a new random 1773 address just requiring the regular relocation process during the 1774 startup. Shared libraries not already prelinked are always loaded 1775 at a randomly chosen address. 1776 1777 Position independent executables (PIE) contain position 1778 independent code similar to the shared libraries and therefore 1779 such executables get loaded at a randomly chosen address upon 1780 startup. PIE executables always load even already prelinked 1781 shared libraries at a random address. You can build such 1782 executable using `gcc -fPIE -pie'. 1783 1784 Heap (malloc storage), stack and custom mmap areas are always 1785 placed randomly (as long as the randomization is enabled). 1786 1787`show disable-randomization' 1788 Show the current setting of the explicit disable of the native 1789 randomization of the virtual address space of the started program. 1790 1791 1792 1793File: gdb.info, Node: Arguments, Next: Environment, Prev: Starting, Up: Running 1794 17954.3 Your Program's Arguments 1796============================ 1797 1798The arguments to your program can be specified by the arguments of the 1799`run' command. They are passed to a shell, which expands wildcard 1800characters and performs redirection of I/O, and thence to your program. 1801Your `SHELL' environment variable (if it exists) specifies what shell 1802GDB uses. If you do not define `SHELL', GDB uses the default shell 1803(`/bin/sh' on Unix). 1804 1805 On non-Unix systems, the program is usually invoked directly by GDB, 1806which emulates I/O redirection via the appropriate system calls, and 1807the wildcard characters are expanded by the startup code of the 1808program, not by the shell. 1809 1810 `run' with no arguments uses the same arguments used by the previous 1811`run', or those set by the `set args' command. 1812 1813`set args' 1814 Specify the arguments to be used the next time your program is 1815 run. If `set args' has no arguments, `run' executes your program 1816 with no arguments. Once you have run your program with arguments, 1817 using `set args' before the next `run' is the only way to run it 1818 again without arguments. 1819 1820`show args' 1821 Show the arguments to give your program when it is started. 1822 1823 1824File: gdb.info, Node: Environment, Next: Working Directory, Prev: Arguments, Up: Running 1825 18264.4 Your Program's Environment 1827============================== 1828 1829The "environment" consists of a set of environment variables and their 1830values. Environment variables conventionally record such things as 1831your user name, your home directory, your terminal type, and your search 1832path for programs to run. Usually you set up environment variables with 1833the shell and they are inherited by all the other programs you run. 1834When debugging, it can be useful to try running your program with a 1835modified environment without having to start GDB over again. 1836 1837`path DIRECTORY' 1838 Add DIRECTORY to the front of the `PATH' environment variable (the 1839 search path for executables) that will be passed to your program. 1840 The value of `PATH' used by GDB does not change. You may specify 1841 several directory names, separated by whitespace or by a 1842 system-dependent separator character (`:' on Unix, `;' on MS-DOS 1843 and MS-Windows). If DIRECTORY is already in the path, it is moved 1844 to the front, so it is searched sooner. 1845 1846 You can use the string `$cwd' to refer to whatever is the current 1847 working directory at the time GDB searches the path. If you use 1848 `.' instead, it refers to the directory where you executed the 1849 `path' command. GDB replaces `.' in the DIRECTORY argument (with 1850 the current path) before adding DIRECTORY to the search path. 1851 1852`show paths' 1853 Display the list of search paths for executables (the `PATH' 1854 environment variable). 1855 1856`show environment [VARNAME]' 1857 Print the value of environment variable VARNAME to be given to 1858 your program when it starts. If you do not supply VARNAME, print 1859 the names and values of all environment variables to be given to 1860 your program. You can abbreviate `environment' as `env'. 1861 1862`set environment VARNAME [=VALUE]' 1863 Set environment variable VARNAME to VALUE. The value changes for 1864 your program only, not for GDB itself. VALUE may be any string; 1865 the values of environment variables are just strings, and any 1866 interpretation is supplied by your program itself. The VALUE 1867 parameter is optional; if it is eliminated, the variable is set to 1868 a null value. 1869 1870 For example, this command: 1871 1872 set env USER = foo 1873 1874 tells the debugged program, when subsequently run, that its user 1875 is named `foo'. (The spaces around `=' are used for clarity here; 1876 they are not actually required.) 1877 1878`unset environment VARNAME' 1879 Remove variable VARNAME from the environment to be passed to your 1880 program. This is different from `set env VARNAME ='; `unset 1881 environment' removes the variable from the environment, rather 1882 than assigning it an empty value. 1883 1884 _Warning:_ On Unix systems, GDB runs your program using the shell 1885indicated by your `SHELL' environment variable if it exists (or 1886`/bin/sh' if not). If your `SHELL' variable names a shell that runs an 1887initialization file--such as `.cshrc' for C-shell, or `.bashrc' for 1888BASH--any variables you set in that file affect your program. You may 1889wish to move setting of environment variables to files that are only 1890run when you sign on, such as `.login' or `.profile'. 1891 1892 1893File: gdb.info, Node: Working Directory, Next: Input/Output, Prev: Environment, Up: Running 1894 18954.5 Your Program's Working Directory 1896==================================== 1897 1898Each time you start your program with `run', it inherits its working 1899directory from the current working directory of GDB. The GDB working 1900directory is initially whatever it inherited from its parent process 1901(typically the shell), but you can specify a new working directory in 1902GDB with the `cd' command. 1903 1904 The GDB working directory also serves as a default for the commands 1905that specify files for GDB to operate on. *Note Commands to Specify 1906Files: Files. 1907 1908`cd [DIRECTORY]' 1909 Set the GDB working directory to DIRECTORY. If not given, 1910 DIRECTORY uses `'~''. 1911 1912`pwd' 1913 Print the GDB working directory. 1914 1915 It is generally impossible to find the current working directory of 1916the process being debugged (since a program can change its directory 1917during its run). If you work on a system where GDB is configured with 1918the `/proc' support, you can use the `info proc' command (*note SVR4 1919Process Information::) to find out the current working directory of the 1920debuggee. 1921 1922 1923File: gdb.info, Node: Input/Output, Next: Attach, Prev: Working Directory, Up: Running 1924 19254.6 Your Program's Input and Output 1926=================================== 1927 1928By default, the program you run under GDB does input and output to the 1929same terminal that GDB uses. GDB switches the terminal to its own 1930terminal modes to interact with you, but it records the terminal modes 1931your program was using and switches back to them when you continue 1932running your program. 1933 1934`info terminal' 1935 Displays information recorded by GDB about the terminal modes your 1936 program is using. 1937 1938 You can redirect your program's input and/or output using shell 1939redirection with the `run' command. For example, 1940 1941 run > outfile 1942 1943starts your program, diverting its output to the file `outfile'. 1944 1945 Another way to specify where your program should do input and output 1946is with the `tty' command. This command accepts a file name as 1947argument, and causes this file to be the default for future `run' 1948commands. It also resets the controlling terminal for the child 1949process, for future `run' commands. For example, 1950 1951 tty /dev/ttyb 1952 1953directs that processes started with subsequent `run' commands default 1954to do input and output on the terminal `/dev/ttyb' and have that as 1955their controlling terminal. 1956 1957 An explicit redirection in `run' overrides the `tty' command's 1958effect on the input/output device, but not its effect on the controlling 1959terminal. 1960 1961 When you use the `tty' command or redirect input in the `run' 1962command, only the input _for your program_ is affected. The input for 1963GDB still comes from your terminal. `tty' is an alias for `set 1964inferior-tty'. 1965 1966 You can use the `show inferior-tty' command to tell GDB to display 1967the name of the terminal that will be used for future runs of your 1968program. 1969 1970`set inferior-tty /dev/ttyb' 1971 Set the tty for the program being debugged to /dev/ttyb. 1972 1973`show inferior-tty' 1974 Show the current tty for the program being debugged. 1975 1976 1977File: gdb.info, Node: Attach, Next: Kill Process, Prev: Input/Output, Up: Running 1978 19794.7 Debugging an Already-running Process 1980======================================== 1981 1982`attach PROCESS-ID' 1983 This command attaches to a running process--one that was started 1984 outside GDB. (`info files' shows your active targets.) The 1985 command takes as argument a process ID. The usual way to find out 1986 the PROCESS-ID of a Unix process is with the `ps' utility, or with 1987 the `jobs -l' shell command. 1988 1989 `attach' does not repeat if you press <RET> a second time after 1990 executing the command. 1991 1992 To use `attach', your program must be running in an environment 1993which supports processes; for example, `attach' does not work for 1994programs on bare-board targets that lack an operating system. You must 1995also have permission to send the process a signal. 1996 1997 When you use `attach', the debugger finds the program running in the 1998process first by looking in the current working directory, then (if the 1999program is not found) by using the source file search path (*note 2000Specifying Source Directories: Source Path.). You can also use the 2001`file' command to load the program. *Note Commands to Specify Files: 2002Files. 2003 2004 The first thing GDB does after arranging to debug the specified 2005process is to stop it. You can examine and modify an attached process 2006with all the GDB commands that are ordinarily available when you start 2007processes with `run'. You can insert breakpoints; you can step and 2008continue; you can modify storage. If you would rather the process 2009continue running, you may use the `continue' command after attaching 2010GDB to the process. 2011 2012`detach' 2013 When you have finished debugging the attached process, you can use 2014 the `detach' command to release it from GDB control. Detaching 2015 the process continues its execution. After the `detach' command, 2016 that process and GDB become completely independent once more, and 2017 you are ready to `attach' another process or start one with `run'. 2018 `detach' does not repeat if you press <RET> again after executing 2019 the command. 2020 2021 If you exit GDB while you have an attached process, you detach that 2022process. If you use the `run' command, you kill that process. By 2023default, GDB asks for confirmation if you try to do either of these 2024things; you can control whether or not you need to confirm by using the 2025`set confirm' command (*note Optional Warnings and Messages: 2026Messages/Warnings.). 2027 2028 2029File: gdb.info, Node: Kill Process, Next: Inferiors and Programs, Prev: Attach, Up: Running 2030 20314.8 Killing the Child Process 2032============================= 2033 2034`kill' 2035 Kill the child process in which your program is running under GDB. 2036 2037 This command is useful if you wish to debug a core dump instead of a 2038running process. GDB ignores any core dump file while your program is 2039running. 2040 2041 On some operating systems, a program cannot be executed outside GDB 2042while you have breakpoints set on it inside GDB. You can use the 2043`kill' command in this situation to permit running your program outside 2044the debugger. 2045 2046 The `kill' command is also useful if you wish to recompile and 2047relink your program, since on many systems it is impossible to modify an 2048executable file while it is running in a process. In this case, when 2049you next type `run', GDB notices that the file has changed, and reads 2050the symbol table again (while trying to preserve your current 2051breakpoint settings). 2052 2053 2054File: gdb.info, Node: Inferiors and Programs, Next: Threads, Prev: Kill Process, Up: Running 2055 20564.9 Debugging Multiple Inferiors and Programs 2057============================================= 2058 2059GDB lets you run and debug multiple programs in a single session. In 2060addition, GDB on some systems may let you run several programs 2061simultaneously (otherwise you have to exit from one before starting 2062another). In the most general case, you can have multiple threads of 2063execution in each of multiple processes, launched from multiple 2064executables. 2065 2066 GDB represents the state of each program execution with an object 2067called an "inferior". An inferior typically corresponds to a process, 2068but is more general and applies also to targets that do not have 2069processes. Inferiors may be created before a process runs, and may be 2070retained after a process exits. Inferiors have unique identifiers that 2071are different from process ids. Usually each inferior will also have 2072its own distinct address space, although some embedded targets may have 2073several inferiors running in different parts of a single address space. 2074Each inferior may in turn have multiple threads running in it. 2075 2076 To find out what inferiors exist at any moment, use `info inferiors': 2077 2078`info inferiors' 2079 Print a list of all inferiors currently being managed by GDB. 2080 2081 GDB displays for each inferior (in this order): 2082 2083 1. the inferior number assigned by GDB 2084 2085 2. the target system's inferior identifier 2086 2087 3. the name of the executable the inferior is running. 2088 2089 2090 An asterisk `*' preceding the GDB inferior number indicates the 2091 current inferior. 2092 2093 For example, 2094 2095 (gdb) info inferiors 2096 Num Description Executable 2097 2 process 2307 hello 2098 * 1 process 3401 goodbye 2099 2100 To switch focus between inferiors, use the `inferior' command: 2101 2102`inferior INFNO' 2103 Make inferior number INFNO the current inferior. The argument 2104 INFNO is the inferior number assigned by GDB, as shown in the 2105 first field of the `info inferiors' display. 2106 2107 You can get multiple executables into a debugging session via the 2108`add-inferior' and `clone-inferior' commands. On some systems GDB can 2109add inferiors to the debug session automatically by following calls to 2110`fork' and `exec'. To remove inferiors from the debugging session use 2111the `remove-inferiors' command. 2112 2113`add-inferior [ -copies N ] [ -exec EXECUTABLE ]' 2114 Adds N inferiors to be run using EXECUTABLE as the executable. N 2115 defaults to 1. If no executable is specified, the inferiors 2116 begins empty, with no program. You can still assign or change the 2117 program assigned to the inferior at any time by using the `file' 2118 command with the executable name as its argument. 2119 2120`clone-inferior [ -copies N ] [ INFNO ]' 2121 Adds N inferiors ready to execute the same program as inferior 2122 INFNO. N defaults to 1. INFNO defaults to the number of the 2123 current inferior. This is a convenient command when you want to 2124 run another instance of the inferior you are debugging. 2125 2126 (gdb) info inferiors 2127 Num Description Executable 2128 * 1 process 29964 helloworld 2129 (gdb) clone-inferior 2130 Added inferior 2. 2131 1 inferiors added. 2132 (gdb) info inferiors 2133 Num Description Executable 2134 2 <null> helloworld 2135 * 1 process 29964 helloworld 2136 2137 You can now simply switch focus to inferior 2 and run it. 2138 2139`remove-inferiors INFNO...' 2140 Removes the inferior or inferiors INFNO.... It is not possible to 2141 remove an inferior that is running with this command. For those, 2142 use the `kill' or `detach' command first. 2143 2144 2145 To quit debugging one of the running inferiors that is not the 2146current inferior, you can either detach from it by using the 2147`detach inferior' command (allowing it to run independently), or kill it 2148using the `kill inferiors' command: 2149 2150`detach inferior INFNO...' 2151 Detach from the inferior or inferiors identified by GDB inferior 2152 number(s) INFNO.... Note that the inferior's entry still stays on 2153 the list of inferiors shown by `info inferiors', but its 2154 Description will show `<null>'. 2155 2156`kill inferiors INFNO...' 2157 Kill the inferior or inferiors identified by GDB inferior 2158 number(s) INFNO.... Note that the inferior's entry still stays on 2159 the list of inferiors shown by `info inferiors', but its 2160 Description will show `<null>'. 2161 2162 After the successful completion of a command such as `detach', 2163`detach inferiors', `kill' or `kill inferiors', or after a normal 2164process exit, the inferior is still valid and listed with `info 2165inferiors', ready to be restarted. 2166 2167 To be notified when inferiors are started or exit under GDB's 2168control use `set print inferior-events': 2169 2170`set print inferior-events' 2171`set print inferior-events on' 2172`set print inferior-events off' 2173 The `set print inferior-events' command allows you to enable or 2174 disable printing of messages when GDB notices that new inferiors 2175 have started or that inferiors have exited or have been detached. 2176 By default, these messages will not be printed. 2177 2178`show print inferior-events' 2179 Show whether messages will be printed when GDB detects that 2180 inferiors have started, exited or have been detached. 2181 2182 Many commands will work the same with multiple programs as with a 2183single program: e.g., `print myglobal' will simply display the value of 2184`myglobal' in the current inferior. 2185 2186 Occasionaly, when debugging GDB itself, it may be useful to get more 2187info about the relationship of inferiors, programs, address spaces in a 2188debug session. You can do that with the `maint info program-spaces' 2189command. 2190 2191`maint info program-spaces' 2192 Print a list of all program spaces currently being managed by GDB. 2193 2194 GDB displays for each program space (in this order): 2195 2196 1. the program space number assigned by GDB 2197 2198 2. the name of the executable loaded into the program space, 2199 with e.g., the `file' command. 2200 2201 2202 An asterisk `*' preceding the GDB program space number indicates 2203 the current program space. 2204 2205 In addition, below each program space line, GDB prints extra 2206 information that isn't suitable to display in tabular form. For 2207 example, the list of inferiors bound to the program space. 2208 2209 (gdb) maint info program-spaces 2210 Id Executable 2211 2 goodbye 2212 Bound inferiors: ID 1 (process 21561) 2213 * 1 hello 2214 2215 Here we can see that no inferior is running the program `hello', 2216 while `process 21561' is running the program `goodbye'. On some 2217 targets, it is possible that multiple inferiors are bound to the 2218 same program space. The most common example is that of debugging 2219 both the parent and child processes of a `vfork' call. For 2220 example, 2221 2222 (gdb) maint info program-spaces 2223 Id Executable 2224 * 1 vfork-test 2225 Bound inferiors: ID 2 (process 18050), ID 1 (process 18045) 2226 2227 Here, both inferior 2 and inferior 1 are running in the same 2228 program space as a result of inferior 1 having executed a `vfork' 2229 call. 2230 2231 2232File: gdb.info, Node: Threads, Next: Forks, Prev: Inferiors and Programs, Up: Running 2233 22344.10 Debugging Programs with Multiple Threads 2235============================================= 2236 2237In some operating systems, such as HP-UX and Solaris, a single program 2238may have more than one "thread" of execution. The precise semantics of 2239threads differ from one operating system to another, but in general the 2240threads of a single program are akin to multiple processes--except that 2241they share one address space (that is, they can all examine and modify 2242the same variables). On the other hand, each thread has its own 2243registers and execution stack, and perhaps private memory. 2244 2245 GDB provides these facilities for debugging multi-thread programs: 2246 2247 * automatic notification of new threads 2248 2249 * `thread THREADNO', a command to switch among threads 2250 2251 * `info threads', a command to inquire about existing threads 2252 2253 * `thread apply [THREADNO] [ALL] ARGS', a command to apply a command 2254 to a list of threads 2255 2256 * thread-specific breakpoints 2257 2258 * `set print thread-events', which controls printing of messages on 2259 thread start and exit. 2260 2261 * `set libthread-db-search-path PATH', which lets the user specify 2262 which `libthread_db' to use if the default choice isn't compatible 2263 with the program. 2264 2265 _Warning:_ These facilities are not yet available on every GDB 2266 configuration where the operating system supports threads. If 2267 your GDB does not support threads, these commands have no effect. 2268 For example, a system without thread support shows no output from 2269 `info threads', and always rejects the `thread' command, like this: 2270 2271 (gdb) info threads 2272 (gdb) thread 1 2273 Thread ID 1 not known. Use the "info threads" command to 2274 see the IDs of currently known threads. 2275 2276 The GDB thread debugging facility allows you to observe all threads 2277while your program runs--but whenever GDB takes control, one thread in 2278particular is always the focus of debugging. This thread is called the 2279"current thread". Debugging commands show program information from the 2280perspective of the current thread. 2281 2282 Whenever GDB detects a new thread in your program, it displays the 2283target system's identification for the thread with a message in the 2284form `[New SYSTAG]'. SYSTAG is a thread identifier whose form varies 2285depending on the particular system. For example, on GNU/Linux, you 2286might see 2287 2288 [New Thread 0x41e02940 (LWP 25582)] 2289 2290when GDB notices a new thread. In contrast, on an SGI system, the 2291SYSTAG is simply something like `process 368', with no further 2292qualifier. 2293 2294 For debugging purposes, GDB associates its own thread number--always 2295a single integer--with each thread in your program. 2296 2297`info threads [ID...]' 2298 Display a summary of all threads currently in your program. 2299 Optional argument ID... is one or more thread ids separated by 2300 spaces, and means to print information only about the specified 2301 thread or threads. GDB displays for each thread (in this order): 2302 2303 1. the thread number assigned by GDB 2304 2305 2. the target system's thread identifier (SYSTAG) 2306 2307 3. the thread's name, if one is known. A thread can either be 2308 named by the user (see `thread name', below), or, in some 2309 cases, by the program itself. 2310 2311 4. the current stack frame summary for that thread 2312 2313 An asterisk `*' to the left of the GDB thread number indicates the 2314 current thread. 2315 2316 For example, 2317 2318 (gdb) info threads 2319 Id Target Id Frame 2320 3 process 35 thread 27 0x34e5 in sigpause () 2321 2 process 35 thread 23 0x34e5 in sigpause () 2322 * 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) 2323 at threadtest.c:68 2324 2325 On Solaris, you can display more information about user threads with 2326a Solaris-specific command: 2327 2328`maint info sol-threads' 2329 Display info on Solaris user threads. 2330 2331`thread THREADNO' 2332 Make thread number THREADNO the current thread. The command 2333 argument THREADNO is the internal GDB thread number, as shown in 2334 the first field of the `info threads' display. GDB responds by 2335 displaying the system identifier of the thread you selected, and 2336 its current stack frame summary: 2337 2338 (gdb) thread 2 2339 [Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))] 2340 #0 some_function (ignore=0x0) at example.c:8 2341 8 printf ("hello\n"); 2342 2343 As with the `[New ...]' message, the form of the text after 2344 `Switching to' depends on your system's conventions for identifying 2345 threads. 2346 2347 The debugger convenience variable `$_thread' contains the number 2348 of the current thread. You may find this useful in writing 2349 breakpoint conditional expressions, command scripts, and so forth. 2350 See *Note Convenience Variables: Convenience Vars, for general 2351 information on convenience variables. 2352 2353`thread apply [THREADNO | all] COMMAND' 2354 The `thread apply' command allows you to apply the named COMMAND 2355 to one or more threads. Specify the numbers of the threads that 2356 you want affected with the command argument THREADNO. It can be a 2357 single thread number, one of the numbers shown in the first field 2358 of the `info threads' display; or it could be a range of thread 2359 numbers, as in `2-4'. To apply a command to all threads, type 2360 `thread apply all COMMAND'. 2361 2362`thread name [NAME]' 2363 This command assigns a name to the current thread. If no argument 2364 is given, any existing user-specified name is removed. The thread 2365 name appears in the `info threads' display. 2366 2367 On some systems, such as GNU/Linux, GDB is able to determine the 2368 name of the thread as given by the OS. On these systems, a name 2369 specified with `thread name' will override the system-give name, 2370 and removing the user-specified name will cause GDB to once again 2371 display the system-specified name. 2372 2373`thread find [REGEXP]' 2374 Search for and display thread ids whose name or SYSTAG matches the 2375 supplied regular expression. 2376 2377 As well as being the complement to the `thread name' command, this 2378 command also allows you to identify a thread by its target SYSTAG. 2379 For instance, on GNU/Linux, the target SYSTAG is the LWP id. 2380 2381 (GDB) thread find 26688 2382 Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)' 2383 (GDB) info thread 4 2384 Id Target Id Frame 2385 4 Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select () 2386 2387`set print thread-events' 2388`set print thread-events on' 2389`set print thread-events off' 2390 The `set print thread-events' command allows you to enable or 2391 disable printing of messages when GDB notices that new threads have 2392 started or that threads have exited. By default, these messages 2393 will be printed if detection of these events is supported by the 2394 target. Note that these messages cannot be disabled on all 2395 targets. 2396 2397`show print thread-events' 2398 Show whether messages will be printed when GDB detects that threads 2399 have started and exited. 2400 2401 *Note Stopping and Starting Multi-thread Programs: Thread Stops, for 2402more information about how GDB behaves when you stop and start programs 2403with multiple threads. 2404 2405 *Note Setting Watchpoints: Set Watchpoints, for information about 2406watchpoints in programs with multiple threads. 2407 2408`set libthread-db-search-path [PATH]' 2409 If this variable is set, PATH is a colon-separated list of 2410 directories GDB will use to search for `libthread_db'. If you 2411 omit PATH, `libthread-db-search-path' will be reset to its default 2412 value (`$sdir:$pdir' on GNU/Linux and Solaris systems). 2413 Internally, the default value comes from the 2414 `LIBTHREAD_DB_SEARCH_PATH' macro. 2415 2416 On GNU/Linux and Solaris systems, GDB uses a "helper" 2417 `libthread_db' library to obtain information about threads in the 2418 inferior process. GDB will use `libthread-db-search-path' to find 2419 `libthread_db'. GDB also consults first if inferior specific 2420 thread debugging library loading is enabled by `set auto-load 2421 libthread-db' (*note libthread_db.so.1 file::). 2422 2423 A special entry `$sdir' for `libthread-db-search-path' refers to 2424 the default system directories that are normally searched for 2425 loading shared libraries. The `$sdir' entry is the only kind not 2426 needing to be enabled by `set auto-load libthread-db' (*note 2427 libthread_db.so.1 file::). 2428 2429 A special entry `$pdir' for `libthread-db-search-path' refers to 2430 the directory from which `libpthread' was loaded in the inferior 2431 process. 2432 2433 For any `libthread_db' library GDB finds in above directories, GDB 2434 attempts to initialize it with the current inferior process. If 2435 this initialization fails (which could happen because of a version 2436 mismatch between `libthread_db' and `libpthread'), GDB will unload 2437 `libthread_db', and continue with the next directory. If none of 2438 `libthread_db' libraries initialize successfully, GDB will issue a 2439 warning and thread debugging will be disabled. 2440 2441 Setting `libthread-db-search-path' is currently implemented only 2442 on some platforms. 2443 2444`show libthread-db-search-path' 2445 Display current libthread_db search path. 2446 2447`set debug libthread-db' 2448`show debug libthread-db' 2449 Turns on or off display of `libthread_db'-related events. Use `1' 2450 to enable, `0' to disable. 2451 2452 2453File: gdb.info, Node: Forks, Next: Checkpoint/Restart, Prev: Threads, Up: Running 2454 24554.11 Debugging Forks 2456==================== 2457 2458On most systems, GDB has no special support for debugging programs 2459which create additional processes using the `fork' function. When a 2460program forks, GDB will continue to debug the parent process and the 2461child process will run unimpeded. If you have set a breakpoint in any 2462code which the child then executes, the child will get a `SIGTRAP' 2463signal which (unless it catches the signal) will cause it to terminate. 2464 2465 However, if you want to debug the child process there is a workaround 2466which isn't too painful. Put a call to `sleep' in the code which the 2467child process executes after the fork. It may be useful to sleep only 2468if a certain environment variable is set, or a certain file exists, so 2469that the delay need not occur when you don't want to run GDB on the 2470child. While the child is sleeping, use the `ps' program to get its 2471process ID. Then tell GDB (a new invocation of GDB if you are also 2472debugging the parent process) to attach to the child process (*note 2473Attach::). From that point on you can debug the child process just 2474like any other process which you attached to. 2475 2476 On some systems, GDB provides support for debugging programs that 2477create additional processes using the `fork' or `vfork' functions. 2478Currently, the only platforms with this feature are HP-UX (11.x and 2479later only?) and GNU/Linux (kernel version 2.5.60 and later). 2480 2481 By default, when a program forks, GDB will continue to debug the 2482parent process and the child process will run unimpeded. 2483 2484 If you want to follow the child process instead of the parent 2485process, use the command `set follow-fork-mode'. 2486 2487`set follow-fork-mode MODE' 2488 Set the debugger response to a program call of `fork' or `vfork'. 2489 A call to `fork' or `vfork' creates a new process. The MODE 2490 argument can be: 2491 2492 `parent' 2493 The original process is debugged after a fork. The child 2494 process runs unimpeded. This is the default. 2495 2496 `child' 2497 The new process is debugged after a fork. The parent process 2498 runs unimpeded. 2499 2500 2501`show follow-fork-mode' 2502 Display the current debugger response to a `fork' or `vfork' call. 2503 2504 On Linux, if you want to debug both the parent and child processes, 2505use the command `set detach-on-fork'. 2506 2507`set detach-on-fork MODE' 2508 Tells gdb whether to detach one of the processes after a fork, or 2509 retain debugger control over them both. 2510 2511 `on' 2512 The child process (or parent process, depending on the value 2513 of `follow-fork-mode') will be detached and allowed to run 2514 independently. This is the default. 2515 2516 `off' 2517 Both processes will be held under the control of GDB. One 2518 process (child or parent, depending on the value of 2519 `follow-fork-mode') is debugged as usual, while the other is 2520 held suspended. 2521 2522 2523`show detach-on-fork' 2524 Show whether detach-on-fork mode is on/off. 2525 2526 If you choose to set `detach-on-fork' mode off, then GDB will retain 2527control of all forked processes (including nested forks). You can list 2528the forked processes under the control of GDB by using the 2529`info inferiors' command, and switch from one fork to another by using 2530the `inferior' command (*note Debugging Multiple Inferiors and 2531Programs: Inferiors and Programs.). 2532 2533 To quit debugging one of the forked processes, you can either detach 2534from it by using the `detach inferiors' command (allowing it to run 2535independently), or kill it using the `kill inferiors' command. *Note 2536Debugging Multiple Inferiors and Programs: Inferiors and Programs. 2537 2538 If you ask to debug a child process and a `vfork' is followed by an 2539`exec', GDB executes the new target up to the first breakpoint in the 2540new target. If you have a breakpoint set on `main' in your original 2541program, the breakpoint will also be set on the child process's `main'. 2542 2543 On some systems, when a child process is spawned by `vfork', you 2544cannot debug the child or parent until an `exec' call completes. 2545 2546 If you issue a `run' command to GDB after an `exec' call executes, 2547the new target restarts. To restart the parent process, use the `file' 2548command with the parent executable name as its argument. By default, 2549after an `exec' call executes, GDB discards the symbols of the previous 2550executable image. You can change this behaviour with the 2551`set follow-exec-mode' command. 2552 2553`set follow-exec-mode MODE' 2554 Set debugger response to a program call of `exec'. An `exec' call 2555 replaces the program image of a process. 2556 2557 `follow-exec-mode' can be: 2558 2559 `new' 2560 GDB creates a new inferior and rebinds the process to this 2561 new inferior. The program the process was running before the 2562 `exec' call can be restarted afterwards by restarting the 2563 original inferior. 2564 2565 For example: 2566 2567 (gdb) info inferiors 2568 (gdb) info inferior 2569 Id Description Executable 2570 * 1 <null> prog1 2571 (gdb) run 2572 process 12020 is executing new program: prog2 2573 Program exited normally. 2574 (gdb) info inferiors 2575 Id Description Executable 2576 * 2 <null> prog2 2577 1 <null> prog1 2578 2579 `same' 2580 GDB keeps the process bound to the same inferior. The new 2581 executable image replaces the previous executable loaded in 2582 the inferior. Restarting the inferior after the `exec' call, 2583 with e.g., the `run' command, restarts the executable the 2584 process was running after the `exec' call. This is the 2585 default mode. 2586 2587 For example: 2588 2589 (gdb) info inferiors 2590 Id Description Executable 2591 * 1 <null> prog1 2592 (gdb) run 2593 process 12020 is executing new program: prog2 2594 Program exited normally. 2595 (gdb) info inferiors 2596 Id Description Executable 2597 * 1 <null> prog2 2598 2599 2600 You can use the `catch' command to make GDB stop whenever a `fork', 2601`vfork', or `exec' call is made. *Note Setting Catchpoints: Set 2602Catchpoints. 2603 2604 2605File: gdb.info, Node: Checkpoint/Restart, Prev: Forks, Up: Running 2606 26074.12 Setting a _Bookmark_ to Return to Later 2608============================================ 2609 2610On certain operating systems(1), GDB is able to save a "snapshot" of a 2611program's state, called a "checkpoint", and come back to it later. 2612 2613 Returning to a checkpoint effectively undoes everything that has 2614happened in the program since the `checkpoint' was saved. This 2615includes changes in memory, registers, and even (within some limits) 2616system state. Effectively, it is like going back in time to the moment 2617when the checkpoint was saved. 2618 2619 Thus, if you're stepping thru a program and you think you're getting 2620close to the point where things go wrong, you can save a checkpoint. 2621Then, if you accidentally go too far and miss the critical statement, 2622instead of having to restart your program from the beginning, you can 2623just go back to the checkpoint and start again from there. 2624 2625 This can be especially useful if it takes a lot of time or steps to 2626reach the point where you think the bug occurs. 2627 2628 To use the `checkpoint'/`restart' method of debugging: 2629 2630`checkpoint' 2631 Save a snapshot of the debugged program's current execution state. 2632 The `checkpoint' command takes no arguments, but each checkpoint 2633 is assigned a small integer id, similar to a breakpoint id. 2634 2635`info checkpoints' 2636 List the checkpoints that have been saved in the current debugging 2637 session. For each checkpoint, the following information will be 2638 listed: 2639 2640 `Checkpoint ID' 2641 2642 `Process ID' 2643 2644 `Code Address' 2645 2646 `Source line, or label' 2647 2648`restart CHECKPOINT-ID' 2649 Restore the program state that was saved as checkpoint number 2650 CHECKPOINT-ID. All program variables, registers, stack frames 2651 etc. will be returned to the values that they had when the 2652 checkpoint was saved. In essence, gdb will "wind back the clock" 2653 to the point in time when the checkpoint was saved. 2654 2655 Note that breakpoints, GDB variables, command history etc. are 2656 not affected by restoring a checkpoint. In general, a checkpoint 2657 only restores things that reside in the program being debugged, 2658 not in the debugger. 2659 2660`delete checkpoint CHECKPOINT-ID' 2661 Delete the previously-saved checkpoint identified by CHECKPOINT-ID. 2662 2663 2664 Returning to a previously saved checkpoint will restore the user 2665state of the program being debugged, plus a significant subset of the 2666system (OS) state, including file pointers. It won't "un-write" data 2667from a file, but it will rewind the file pointer to the previous 2668location, so that the previously written data can be overwritten. For 2669files opened in read mode, the pointer will also be restored so that the 2670previously read data can be read again. 2671 2672 Of course, characters that have been sent to a printer (or other 2673external device) cannot be "snatched back", and characters received 2674from eg. a serial device can be removed from internal program buffers, 2675but they cannot be "pushed back" into the serial pipeline, ready to be 2676received again. Similarly, the actual contents of files that have been 2677changed cannot be restored (at this time). 2678 2679 However, within those constraints, you actually can "rewind" your 2680program to a previously saved point in time, and begin debugging it 2681again -- and you can change the course of events so as to debug a 2682different execution path this time. 2683 2684 Finally, there is one bit of internal program state that will be 2685different when you return to a checkpoint -- the program's process id. 2686Each checkpoint will have a unique process id (or PID), and each will 2687be different from the program's original PID. If your program has 2688saved a local copy of its process id, this could potentially pose a 2689problem. 2690 26914.12.1 A Non-obvious Benefit of Using Checkpoints 2692------------------------------------------------- 2693 2694On some systems such as GNU/Linux, address space randomization is 2695performed on new processes for security reasons. This makes it 2696difficult or impossible to set a breakpoint, or watchpoint, on an 2697absolute address if you have to restart the program, since the absolute 2698location of a symbol will change from one execution to the next. 2699 2700 A checkpoint, however, is an _identical_ copy of a process. 2701Therefore if you create a checkpoint at (eg.) the start of main, and 2702simply return to that checkpoint instead of restarting the process, you 2703can avoid the effects of address randomization and your symbols will 2704all stay in the same place. 2705 2706 ---------- Footnotes ---------- 2707 2708 (1) Currently, only GNU/Linux. 2709 2710 2711File: gdb.info, Node: Stopping, Next: Reverse Execution, Prev: Running, Up: Top 2712 27135 Stopping and Continuing 2714************************* 2715 2716The principal purposes of using a debugger are so that you can stop your 2717program before it terminates; or so that, if your program runs into 2718trouble, you can investigate and find out why. 2719 2720 Inside GDB, your program may stop for any of several reasons, such 2721as a signal, a breakpoint, or reaching a new line after a GDB command 2722such as `step'. You may then examine and change variables, set new 2723breakpoints or remove old ones, and then continue execution. Usually, 2724the messages shown by GDB provide ample explanation of the status of 2725your program--but you can also explicitly request this information at 2726any time. 2727 2728`info program' 2729 Display information about the status of your program: whether it is 2730 running or not, what process it is, and why it stopped. 2731 2732* Menu: 2733 2734* Breakpoints:: Breakpoints, watchpoints, and catchpoints 2735* Continuing and Stepping:: Resuming execution 2736* Skipping Over Functions and Files:: 2737 Skipping over functions and files 2738* Signals:: Signals 2739* Thread Stops:: Stopping and starting multi-thread programs 2740 2741 2742File: gdb.info, Node: Breakpoints, Next: Continuing and Stepping, Up: Stopping 2743 27445.1 Breakpoints, Watchpoints, and Catchpoints 2745============================================= 2746 2747A "breakpoint" makes your program stop whenever a certain point in the 2748program is reached. For each breakpoint, you can add conditions to 2749control in finer detail whether your program stops. You can set 2750breakpoints with the `break' command and its variants (*note Setting 2751Breakpoints: Set Breaks.), to specify the place where your program 2752should stop by line number, function name or exact address in the 2753program. 2754 2755 On some systems, you can set breakpoints in shared libraries before 2756the executable is run. There is a minor limitation on HP-UX systems: 2757you must wait until the executable is run in order to set breakpoints 2758in shared library routines that are not called directly by the program 2759(for example, routines that are arguments in a `pthread_create' call). 2760 2761 A "watchpoint" is a special breakpoint that stops your program when 2762the value of an expression changes. The expression may be a value of a 2763variable, or it could involve values of one or more variables combined 2764by operators, such as `a + b'. This is sometimes called "data 2765breakpoints". You must use a different command to set watchpoints 2766(*note Setting Watchpoints: Set Watchpoints.), but aside from that, you 2767can manage a watchpoint like any other breakpoint: you enable, disable, 2768and delete both breakpoints and watchpoints using the same commands. 2769 2770 You can arrange to have values from your program displayed 2771automatically whenever GDB stops at a breakpoint. *Note Automatic 2772Display: Auto Display. 2773 2774 A "catchpoint" is another special breakpoint that stops your program 2775when a certain kind of event occurs, such as the throwing of a C++ 2776exception or the loading of a library. As with watchpoints, you use a 2777different command to set a catchpoint (*note Setting Catchpoints: Set 2778Catchpoints.), but aside from that, you can manage a catchpoint like any 2779other breakpoint. (To stop when your program receives a signal, use the 2780`handle' command; see *note Signals: Signals.) 2781 2782 GDB assigns a number to each breakpoint, watchpoint, or catchpoint 2783when you create it; these numbers are successive integers starting with 2784one. In many of the commands for controlling various features of 2785breakpoints you use the breakpoint number to say which breakpoint you 2786want to change. Each breakpoint may be "enabled" or "disabled"; if 2787disabled, it has no effect on your program until you enable it again. 2788 2789 Some GDB commands accept a range of breakpoints on which to operate. 2790A breakpoint range is either a single breakpoint number, like `5', or 2791two such numbers, in increasing order, separated by a hyphen, like 2792`5-7'. When a breakpoint range is given to a command, all breakpoints 2793in that range are operated on. 2794 2795* Menu: 2796 2797* Set Breaks:: Setting breakpoints 2798* Set Watchpoints:: Setting watchpoints 2799* Set Catchpoints:: Setting catchpoints 2800* Delete Breaks:: Deleting breakpoints 2801* Disabling:: Disabling breakpoints 2802* Conditions:: Break conditions 2803* Break Commands:: Breakpoint command lists 2804* Dynamic Printf:: Dynamic printf 2805* Save Breakpoints:: How to save breakpoints in a file 2806* Static Probe Points:: Listing static probe points 2807* Error in Breakpoints:: ``Cannot insert breakpoints'' 2808* Breakpoint-related Warnings:: ``Breakpoint address adjusted...'' 2809 2810 2811File: gdb.info, Node: Set Breaks, Next: Set Watchpoints, Up: Breakpoints 2812 28135.1.1 Setting Breakpoints 2814------------------------- 2815 2816Breakpoints are set with the `break' command (abbreviated `b'). The 2817debugger convenience variable `$bpnum' records the number of the 2818breakpoint you've set most recently; see *note Convenience Variables: 2819Convenience Vars, for a discussion of what you can do with convenience 2820variables. 2821 2822`break LOCATION' 2823 Set a breakpoint at the given LOCATION, which can specify a 2824 function name, a line number, or an address of an instruction. 2825 (*Note Specify Location::, for a list of all the possible ways to 2826 specify a LOCATION.) The breakpoint will stop your program just 2827 before it executes any of the code in the specified LOCATION. 2828 2829 When using source languages that permit overloading of symbols, 2830 such as C++, a function name may refer to more than one possible 2831 place to break. *Note Ambiguous Expressions: Ambiguous 2832 Expressions, for a discussion of that situation. 2833 2834 It is also possible to insert a breakpoint that will stop the 2835 program only if a specific thread (*note Thread-Specific 2836 Breakpoints::) or a specific task (*note Ada Tasks::) hits that 2837 breakpoint. 2838 2839`break' 2840 When called without any arguments, `break' sets a breakpoint at 2841 the next instruction to be executed in the selected stack frame 2842 (*note Examining the Stack: Stack.). In any selected frame but the 2843 innermost, this makes your program stop as soon as control returns 2844 to that frame. This is similar to the effect of a `finish' 2845 command in the frame inside the selected frame--except that 2846 `finish' does not leave an active breakpoint. If you use `break' 2847 without an argument in the innermost frame, GDB stops the next 2848 time it reaches the current location; this may be useful inside 2849 loops. 2850 2851 GDB normally ignores breakpoints when it resumes execution, until 2852 at least one instruction has been executed. If it did not do 2853 this, you would be unable to proceed past a breakpoint without 2854 first disabling the breakpoint. This rule applies whether or not 2855 the breakpoint already existed when your program stopped. 2856 2857`break ... if COND' 2858 Set a breakpoint with condition COND; evaluate the expression COND 2859 each time the breakpoint is reached, and stop only if the value is 2860 nonzero--that is, if COND evaluates as true. `...' stands for one 2861 of the possible arguments described above (or no argument) 2862 specifying where to break. *Note Break Conditions: Conditions, 2863 for more information on breakpoint conditions. 2864 2865`tbreak ARGS' 2866 Set a breakpoint enabled only for one stop. ARGS are the same as 2867 for the `break' command, and the breakpoint is set in the same 2868 way, but the breakpoint is automatically deleted after the first 2869 time your program stops there. *Note Disabling Breakpoints: 2870 Disabling. 2871 2872`hbreak ARGS' 2873 Set a hardware-assisted breakpoint. ARGS are the same as for the 2874 `break' command and the breakpoint is set in the same way, but the 2875 breakpoint requires hardware support and some target hardware may 2876 not have this support. The main purpose of this is EPROM/ROM code 2877 debugging, so you can set a breakpoint at an instruction without 2878 changing the instruction. This can be used with the new 2879 trap-generation provided by SPARClite DSU and most x86-based 2880 targets. These targets will generate traps when a program 2881 accesses some data or instruction address that is assigned to the 2882 debug registers. However the hardware breakpoint registers can 2883 take a limited number of breakpoints. For example, on the DSU, 2884 only two data breakpoints can be set at a time, and GDB will 2885 reject this command if more than two are used. Delete or disable 2886 unused hardware breakpoints before setting new ones (*note 2887 Disabling Breakpoints: Disabling.). *Note Break Conditions: 2888 Conditions. For remote targets, you can restrict the number of 2889 hardware breakpoints GDB will use, see *note set remote 2890 hardware-breakpoint-limit::. 2891 2892`thbreak ARGS' 2893 Set a hardware-assisted breakpoint enabled only for one stop. ARGS 2894 are the same as for the `hbreak' command and the breakpoint is set 2895 in the same way. However, like the `tbreak' command, the 2896 breakpoint is automatically deleted after the first time your 2897 program stops there. Also, like the `hbreak' command, the 2898 breakpoint requires hardware support and some target hardware may 2899 not have this support. *Note Disabling Breakpoints: Disabling. 2900 See also *note Break Conditions: Conditions. 2901 2902`rbreak REGEX' 2903 Set breakpoints on all functions matching the regular expression 2904 REGEX. This command sets an unconditional breakpoint on all 2905 matches, printing a list of all breakpoints it set. Once these 2906 breakpoints are set, they are treated just like the breakpoints 2907 set with the `break' command. You can delete them, disable them, 2908 or make them conditional the same way as any other breakpoint. 2909 2910 The syntax of the regular expression is the standard one used with 2911 tools like `grep'. Note that this is different from the syntax 2912 used by shells, so for instance `foo*' matches all functions that 2913 include an `fo' followed by zero or more `o's. There is an 2914 implicit `.*' leading and trailing the regular expression you 2915 supply, so to match only functions that begin with `foo', use 2916 `^foo'. 2917 2918 When debugging C++ programs, `rbreak' is useful for setting 2919 breakpoints on overloaded functions that are not members of any 2920 special classes. 2921 2922 The `rbreak' command can be used to set breakpoints in *all* the 2923 functions in a program, like this: 2924 2925 (gdb) rbreak . 2926 2927`rbreak FILE:REGEX' 2928 If `rbreak' is called with a filename qualification, it limits the 2929 search for functions matching the given regular expression to the 2930 specified FILE. This can be used, for example, to set breakpoints 2931 on every function in a given file: 2932 2933 (gdb) rbreak file.c:. 2934 2935 The colon separating the filename qualifier from the regex may 2936 optionally be surrounded by spaces. 2937 2938`info breakpoints [N...]' 2939`info break [N...]' 2940 Print a table of all breakpoints, watchpoints, and catchpoints set 2941 and not deleted. Optional argument N means print information only 2942 about the specified breakpoint(s) (or watchpoint(s) or 2943 catchpoint(s)). For each breakpoint, following columns are 2944 printed: 2945 2946 _Breakpoint Numbers_ 2947 2948 _Type_ 2949 Breakpoint, watchpoint, or catchpoint. 2950 2951 _Disposition_ 2952 Whether the breakpoint is marked to be disabled or deleted 2953 when hit. 2954 2955 _Enabled or Disabled_ 2956 Enabled breakpoints are marked with `y'. `n' marks 2957 breakpoints that are not enabled. 2958 2959 _Address_ 2960 Where the breakpoint is in your program, as a memory address. 2961 For a pending breakpoint whose address is not yet known, this 2962 field will contain `<PENDING>'. Such breakpoint won't fire 2963 until a shared library that has the symbol or line referred 2964 by breakpoint is loaded. See below for details. A 2965 breakpoint with several locations will have `<MULTIPLE>' in 2966 this field--see below for details. 2967 2968 _What_ 2969 Where the breakpoint is in the source for your program, as a 2970 file and line number. For a pending breakpoint, the original 2971 string passed to the breakpoint command will be listed as it 2972 cannot be resolved until the appropriate shared library is 2973 loaded in the future. 2974 2975 If a breakpoint is conditional, there are two evaluation modes: 2976 "host" and "target". If mode is "host", breakpoint condition 2977 evaluation is done by GDB on the host's side. If it is "target", 2978 then the condition is evaluated by the target. The `info break' 2979 command shows the condition on the line following the affected 2980 breakpoint, together with its condition evaluation mode in between 2981 parentheses. 2982 2983 Breakpoint commands, if any, are listed after that. A pending 2984 breakpoint is allowed to have a condition specified for it. The 2985 condition is not parsed for validity until a shared library is 2986 loaded that allows the pending breakpoint to resolve to a valid 2987 location. 2988 2989 `info break' with a breakpoint number N as argument lists only 2990 that breakpoint. The convenience variable `$_' and the default 2991 examining-address for the `x' command are set to the address of 2992 the last breakpoint listed (*note Examining Memory: Memory.). 2993 2994 `info break' displays a count of the number of times the breakpoint 2995 has been hit. This is especially useful in conjunction with the 2996 `ignore' command. You can ignore a large number of breakpoint 2997 hits, look at the breakpoint info to see how many times the 2998 breakpoint was hit, and then run again, ignoring one less than 2999 that number. This will get you quickly to the last hit of that 3000 breakpoint. 3001 3002 For a breakpoints with an enable count (xref) greater than 1, 3003 `info break' also displays that count. 3004 3005 3006 GDB allows you to set any number of breakpoints at the same place in 3007your program. There is nothing silly or meaningless about this. When 3008the breakpoints are conditional, this is even useful (*note Break 3009Conditions: Conditions.). 3010 3011 It is possible that a breakpoint corresponds to several locations in 3012your program. Examples of this situation are: 3013 3014 * Multiple functions in the program may have the same name. 3015 3016 * For a C++ constructor, the GCC compiler generates several 3017 instances of the function body, used in different cases. 3018 3019 * For a C++ template function, a given line in the function can 3020 correspond to any number of instantiations. 3021 3022 * For an inlined function, a given source line can correspond to 3023 several places where that function is inlined. 3024 3025 In all those cases, GDB will insert a breakpoint at all the relevant 3026locations. 3027 3028 A breakpoint with multiple locations is displayed in the breakpoint 3029table using several rows--one header row, followed by one row for each 3030breakpoint location. The header row has `<MULTIPLE>' in the address 3031column. The rows for individual locations contain the actual addresses 3032for locations, and show the functions to which those locations belong. 3033The number column for a location is of the form 3034BREAKPOINT-NUMBER.LOCATION-NUMBER. 3035 3036 For example: 3037 3038 Num Type Disp Enb Address What 3039 1 breakpoint keep y <MULTIPLE> 3040 stop only if i==1 3041 breakpoint already hit 1 time 3042 1.1 y 0x080486a2 in void foo<int>() at t.cc:8 3043 1.2 y 0x080486ca in void foo<double>() at t.cc:8 3044 3045 Each location can be individually enabled or disabled by passing 3046BREAKPOINT-NUMBER.LOCATION-NUMBER as argument to the `enable' and 3047`disable' commands. Note that you cannot delete the individual 3048locations from the list, you can only delete the entire list of 3049locations that belong to their parent breakpoint (with the `delete NUM' 3050command, where NUM is the number of the parent breakpoint, 1 in the 3051above example). Disabling or enabling the parent breakpoint (*note 3052Disabling::) affects all of the locations that belong to that 3053breakpoint. 3054 3055 It's quite common to have a breakpoint inside a shared library. 3056Shared libraries can be loaded and unloaded explicitly, and possibly 3057repeatedly, as the program is executed. To support this use case, GDB 3058updates breakpoint locations whenever any shared library is loaded or 3059unloaded. Typically, you would set a breakpoint in a shared library at 3060the beginning of your debugging session, when the library is not 3061loaded, and when the symbols from the library are not available. When 3062you try to set breakpoint, GDB will ask you if you want to set a so 3063called "pending breakpoint"--breakpoint whose address is not yet 3064resolved. 3065 3066 After the program is run, whenever a new shared library is loaded, 3067GDB reevaluates all the breakpoints. When a newly loaded shared 3068library contains the symbol or line referred to by some pending 3069breakpoint, that breakpoint is resolved and becomes an ordinary 3070breakpoint. When a library is unloaded, all breakpoints that refer to 3071its symbols or source lines become pending again. 3072 3073 This logic works for breakpoints with multiple locations, too. For 3074example, if you have a breakpoint in a C++ template function, and a 3075newly loaded shared library has an instantiation of that template, a 3076new location is added to the list of locations for the breakpoint. 3077 3078 Except for having unresolved address, pending breakpoints do not 3079differ from regular breakpoints. You can set conditions or commands, 3080enable and disable them and perform other breakpoint operations. 3081 3082 GDB provides some additional commands for controlling what happens 3083when the `break' command cannot resolve breakpoint address 3084specification to an address: 3085 3086`set breakpoint pending auto' 3087 This is the default behavior. When GDB cannot find the breakpoint 3088 location, it queries you whether a pending breakpoint should be 3089 created. 3090 3091`set breakpoint pending on' 3092 This indicates that an unrecognized breakpoint location should 3093 automatically result in a pending breakpoint being created. 3094 3095`set breakpoint pending off' 3096 This indicates that pending breakpoints are not to be created. Any 3097 unrecognized breakpoint location results in an error. This 3098 setting does not affect any pending breakpoints previously created. 3099 3100`show breakpoint pending' 3101 Show the current behavior setting for creating pending breakpoints. 3102 3103 The settings above only affect the `break' command and its variants. 3104Once breakpoint is set, it will be automatically updated as shared 3105libraries are loaded and unloaded. 3106 3107 For some targets, GDB can automatically decide if hardware or 3108software breakpoints should be used, depending on whether the 3109breakpoint address is read-only or read-write. This applies to 3110breakpoints set with the `break' command as well as to internal 3111breakpoints set by commands like `next' and `finish'. For breakpoints 3112set with `hbreak', GDB will always use hardware breakpoints. 3113 3114 You can control this automatic behaviour with the following 3115commands:: 3116 3117`set breakpoint auto-hw on' 3118 This is the default behavior. When GDB sets a breakpoint, it will 3119 try to use the target memory map to decide if software or hardware 3120 breakpoint must be used. 3121 3122`set breakpoint auto-hw off' 3123 This indicates GDB should not automatically select breakpoint 3124 type. If the target provides a memory map, GDB will warn when 3125 trying to set software breakpoint at a read-only address. 3126 3127 GDB normally implements breakpoints by replacing the program code at 3128the breakpoint address with a special instruction, which, when 3129executed, given control to the debugger. By default, the program code 3130is so modified only when the program is resumed. As soon as the 3131program stops, GDB restores the original instructions. This behaviour 3132guards against leaving breakpoints inserted in the target should gdb 3133abrubptly disconnect. However, with slow remote targets, inserting and 3134removing breakpoint can reduce the performance. This behavior can be 3135controlled with the following commands:: 3136 3137`set breakpoint always-inserted off' 3138 All breakpoints, including newly added by the user, are inserted in 3139 the target only when the target is resumed. All breakpoints are 3140 removed from the target when it stops. 3141 3142`set breakpoint always-inserted on' 3143 Causes all breakpoints to be inserted in the target at all times. 3144 If the user adds a new breakpoint, or changes an existing 3145 breakpoint, the breakpoints in the target are updated immediately. 3146 A breakpoint is removed from the target only when breakpoint 3147 itself is removed. 3148 3149`set breakpoint always-inserted auto' 3150 This is the default mode. If GDB is controlling the inferior in 3151 non-stop mode (*note Non-Stop Mode::), gdb behaves as if 3152 `breakpoint always-inserted' mode is on. If GDB is controlling 3153 the inferior in all-stop mode, GDB behaves as if `breakpoint 3154 always-inserted' mode is off. 3155 3156 GDB handles conditional breakpoints by evaluating these conditions 3157when a breakpoint breaks. If the condition is true, then the process 3158being debugged stops, otherwise the process is resumed. 3159 3160 If the target supports evaluating conditions on its end, GDB may 3161download the breakpoint, together with its conditions, to it. 3162 3163 This feature can be controlled via the following commands: 3164 3165`set breakpoint condition-evaluation host' 3166 This option commands GDB to evaluate the breakpoint conditions on 3167 the host's side. Unconditional breakpoints are sent to the target 3168 which in turn receives the triggers and reports them back to GDB 3169 for condition evaluation. This is the standard evaluation mode. 3170 3171`set breakpoint condition-evaluation target' 3172 This option commands GDB to download breakpoint conditions to the 3173 target at the moment of their insertion. The target is 3174 responsible for evaluating the conditional expression and reporting 3175 breakpoint stop events back to GDB whenever the condition is true. 3176 Due to limitations of target-side evaluation, some conditions 3177 cannot be evaluated there, e.g., conditions that depend on local 3178 data that is only known to the host. Examples include conditional 3179 expressions involving convenience variables, complex types that 3180 cannot be handled by the agent expression parser and expressions 3181 that are too long to be sent over to the target, specially when the 3182 target is a remote system. In these cases, the conditions will be 3183 evaluated by GDB. 3184 3185`set breakpoint condition-evaluation auto' 3186 This is the default mode. If the target supports evaluating 3187 breakpoint conditions on its end, GDB will download breakpoint 3188 conditions to the target (limitations mentioned previously apply). 3189 If the target does not support breakpoint condition evaluation, 3190 then GDB will fallback to evaluating all these conditions on the 3191 host's side. 3192 3193 GDB itself sometimes sets breakpoints in your program for special 3194purposes, such as proper handling of `longjmp' (in C programs). These 3195internal breakpoints are assigned negative numbers, starting with `-1'; 3196`info breakpoints' does not display them. You can see these 3197breakpoints with the GDB maintenance command `maint info breakpoints' 3198(*note maint info breakpoints::). 3199 3200 3201File: gdb.info, Node: Set Watchpoints, Next: Set Catchpoints, Prev: Set Breaks, Up: Breakpoints 3202 32035.1.2 Setting Watchpoints 3204------------------------- 3205 3206You can use a watchpoint to stop execution whenever the value of an 3207expression changes, without having to predict a particular place where 3208this may happen. (This is sometimes called a "data breakpoint".) The 3209expression may be as simple as the value of a single variable, or as 3210complex as many variables combined by operators. Examples include: 3211 3212 * A reference to the value of a single variable. 3213 3214 * An address cast to an appropriate data type. For example, `*(int 3215 *)0x12345678' will watch a 4-byte region at the specified address 3216 (assuming an `int' occupies 4 bytes). 3217 3218 * An arbitrarily complex expression, such as `a*b + c/d'. The 3219 expression can use any operators valid in the program's native 3220 language (*note Languages::). 3221 3222 You can set a watchpoint on an expression even if the expression can 3223not be evaluated yet. For instance, you can set a watchpoint on 3224`*global_ptr' before `global_ptr' is initialized. GDB will stop when 3225your program sets `global_ptr' and the expression produces a valid 3226value. If the expression becomes valid in some other way than changing 3227a variable (e.g. if the memory pointed to by `*global_ptr' becomes 3228readable as the result of a `malloc' call), GDB may not stop until the 3229next time the expression changes. 3230 3231 Depending on your system, watchpoints may be implemented in software 3232or hardware. GDB does software watchpointing by single-stepping your 3233program and testing the variable's value each time, which is hundreds of 3234times slower than normal execution. (But this may still be worth it, to 3235catch errors where you have no clue what part of your program is the 3236culprit.) 3237 3238 On some systems, such as HP-UX, PowerPC, GNU/Linux and most other 3239x86-based targets, GDB includes support for hardware watchpoints, which 3240do not slow down the running of your program. 3241 3242`watch [-l|-location] EXPR [thread THREADNUM] [mask MASKVALUE]' 3243 Set a watchpoint for an expression. GDB will break when the 3244 expression EXPR is written into by the program and its value 3245 changes. The simplest (and the most popular) use of this command 3246 is to watch the value of a single variable: 3247 3248 (gdb) watch foo 3249 3250 If the command includes a `[thread THREADNUM]' argument, GDB 3251 breaks only when the thread identified by THREADNUM changes the 3252 value of EXPR. If any other threads change the value of EXPR, GDB 3253 will not break. Note that watchpoints restricted to a single 3254 thread in this way only work with Hardware Watchpoints. 3255 3256 Ordinarily a watchpoint respects the scope of variables in EXPR 3257 (see below). The `-location' argument tells GDB to instead watch 3258 the memory referred to by EXPR. In this case, GDB will evaluate 3259 EXPR, take the address of the result, and watch the memory at that 3260 address. The type of the result is used to determine the size of 3261 the watched memory. If the expression's result does not have an 3262 address, then GDB will print an error. 3263 3264 The `[mask MASKVALUE]' argument allows creation of masked 3265 watchpoints, if the current architecture supports this feature 3266 (e.g., PowerPC Embedded architecture, see *note PowerPC 3267 Embedded::.) A "masked watchpoint" specifies a mask in addition 3268 to an address to watch. The mask specifies that some bits of an 3269 address (the bits which are reset in the mask) should be ignored 3270 when matching the address accessed by the inferior against the 3271 watchpoint address. Thus, a masked watchpoint watches many 3272 addresses simultaneously--those addresses whose unmasked bits are 3273 identical to the unmasked bits in the watchpoint address. The 3274 `mask' argument implies `-location'. Examples: 3275 3276 (gdb) watch foo mask 0xffff00ff 3277 (gdb) watch *0xdeadbeef mask 0xffffff00 3278 3279`rwatch [-l|-location] EXPR [thread THREADNUM] [mask MASKVALUE]' 3280 Set a watchpoint that will break when the value of EXPR is read by 3281 the program. 3282 3283`awatch [-l|-location] EXPR [thread THREADNUM] [mask MASKVALUE]' 3284 Set a watchpoint that will break when EXPR is either read from or 3285 written into by the program. 3286 3287`info watchpoints [N...]' 3288 This command prints a list of watchpoints, using the same format as 3289 `info break' (*note Set Breaks::). 3290 3291 If you watch for a change in a numerically entered address you need 3292to dereference it, as the address itself is just a constant number 3293which will never change. GDB refuses to create a watchpoint that 3294watches a never-changing value: 3295 3296 (gdb) watch 0x600850 3297 Cannot watch constant value 0x600850. 3298 (gdb) watch *(int *) 0x600850 3299 Watchpoint 1: *(int *) 6293584 3300 3301 GDB sets a "hardware watchpoint" if possible. Hardware watchpoints 3302execute very quickly, and the debugger reports a change in value at the 3303exact instruction where the change occurs. If GDB cannot set a 3304hardware watchpoint, it sets a software watchpoint, which executes more 3305slowly and reports the change in value at the next _statement_, not the 3306instruction, after the change occurs. 3307 3308 You can force GDB to use only software watchpoints with the `set 3309can-use-hw-watchpoints 0' command. With this variable set to zero, GDB 3310will never try to use hardware watchpoints, even if the underlying 3311system supports them. (Note that hardware-assisted watchpoints that 3312were set _before_ setting `can-use-hw-watchpoints' to zero will still 3313use the hardware mechanism of watching expression values.) 3314 3315`set can-use-hw-watchpoints' 3316 Set whether or not to use hardware watchpoints. 3317 3318`show can-use-hw-watchpoints' 3319 Show the current mode of using hardware watchpoints. 3320 3321 For remote targets, you can restrict the number of hardware 3322watchpoints GDB will use, see *note set remote 3323hardware-breakpoint-limit::. 3324 3325 When you issue the `watch' command, GDB reports 3326 3327 Hardware watchpoint NUM: EXPR 3328 3329if it was able to set a hardware watchpoint. 3330 3331 Currently, the `awatch' and `rwatch' commands can only set hardware 3332watchpoints, because accesses to data that don't change the value of 3333the watched expression cannot be detected without examining every 3334instruction as it is being executed, and GDB does not do that 3335currently. If GDB finds that it is unable to set a hardware breakpoint 3336with the `awatch' or `rwatch' command, it will print a message like 3337this: 3338 3339 Expression cannot be implemented with read/access watchpoint. 3340 3341 Sometimes, GDB cannot set a hardware watchpoint because the data 3342type of the watched expression is wider than what a hardware watchpoint 3343on the target machine can handle. For example, some systems can only 3344watch regions that are up to 4 bytes wide; on such systems you cannot 3345set hardware watchpoints for an expression that yields a 3346double-precision floating-point number (which is typically 8 bytes 3347wide). As a work-around, it might be possible to break the large region 3348into a series of smaller ones and watch them with separate watchpoints. 3349 3350 If you set too many hardware watchpoints, GDB might be unable to 3351insert all of them when you resume the execution of your program. 3352Since the precise number of active watchpoints is unknown until such 3353time as the program is about to be resumed, GDB might not be able to 3354warn you about this when you set the watchpoints, and the warning will 3355be printed only when the program is resumed: 3356 3357 Hardware watchpoint NUM: Could not insert watchpoint 3358 3359If this happens, delete or disable some of the watchpoints. 3360 3361 Watching complex expressions that reference many variables can also 3362exhaust the resources available for hardware-assisted watchpoints. 3363That's because GDB needs to watch every variable in the expression with 3364separately allocated resources. 3365 3366 If you call a function interactively using `print' or `call', any 3367watchpoints you have set will be inactive until GDB reaches another 3368kind of breakpoint or the call completes. 3369 3370 GDB automatically deletes watchpoints that watch local (automatic) 3371variables, or expressions that involve such variables, when they go out 3372of scope, that is, when the execution leaves the block in which these 3373variables were defined. In particular, when the program being debugged 3374terminates, _all_ local variables go out of scope, and so only 3375watchpoints that watch global variables remain set. If you rerun the 3376program, you will need to set all such watchpoints again. One way of 3377doing that would be to set a code breakpoint at the entry to the `main' 3378function and when it breaks, set all the watchpoints. 3379 3380 In multi-threaded programs, watchpoints will detect changes to the 3381watched expression from every thread. 3382 3383 _Warning:_ In multi-threaded programs, software watchpoints have 3384 only limited usefulness. If GDB creates a software watchpoint, it 3385 can only watch the value of an expression _in a single thread_. 3386 If you are confident that the expression can only change due to 3387 the current thread's activity (and if you are also confident that 3388 no other thread can become current), then you can use software 3389 watchpoints as usual. However, GDB may not notice when a 3390 non-current thread's activity changes the expression. (Hardware 3391 watchpoints, in contrast, watch an expression in all threads.) 3392 3393 *Note set remote hardware-watchpoint-limit::. 3394 3395 3396File: gdb.info, Node: Set Catchpoints, Next: Delete Breaks, Prev: Set Watchpoints, Up: Breakpoints 3397 33985.1.3 Setting Catchpoints 3399------------------------- 3400 3401You can use "catchpoints" to cause the debugger to stop for certain 3402kinds of program events, such as C++ exceptions or the loading of a 3403shared library. Use the `catch' command to set a catchpoint. 3404 3405`catch EVENT' 3406 Stop when EVENT occurs. EVENT can be any of the following: 3407 `throw' 3408 The throwing of a C++ exception. 3409 3410 `catch' 3411 The catching of a C++ exception. 3412 3413 `exception' 3414 An Ada exception being raised. If an exception name is 3415 specified at the end of the command (eg `catch exception 3416 Program_Error'), the debugger will stop only when this 3417 specific exception is raised. Otherwise, the debugger stops 3418 execution when any Ada exception is raised. 3419 3420 When inserting an exception catchpoint on a user-defined 3421 exception whose name is identical to one of the exceptions 3422 defined by the language, the fully qualified name must be 3423 used as the exception name. Otherwise, GDB will assume that 3424 it should stop on the pre-defined exception rather than the 3425 user-defined one. For instance, assuming an exception called 3426 `Constraint_Error' is defined in package `Pck', then the 3427 command to use to catch such exceptions is `catch exception 3428 Pck.Constraint_Error'. 3429 3430 `exception unhandled' 3431 An exception that was raised but is not handled by the 3432 program. 3433 3434 `assert' 3435 A failed Ada assertion. 3436 3437 `exec' 3438 A call to `exec'. This is currently only available for HP-UX 3439 and GNU/Linux. 3440 3441 `syscall' 3442 `syscall [NAME | NUMBER] ...' 3443 A call to or return from a system call, a.k.a. "syscall". A 3444 syscall is a mechanism for application programs to request a 3445 service from the operating system (OS) or one of the OS 3446 system services. GDB can catch some or all of the syscalls 3447 issued by the debuggee, and show the related information for 3448 each syscall. If no argument is specified, calls to and 3449 returns from all system calls will be caught. 3450 3451 NAME can be any system call name that is valid for the 3452 underlying OS. Just what syscalls are valid depends on the 3453 OS. On GNU and Unix systems, you can find the full list of 3454 valid syscall names on `/usr/include/asm/unistd.h'. 3455 3456 Normally, GDB knows in advance which syscalls are valid for 3457 each OS, so you can use the GDB command-line completion 3458 facilities (*note command completion: Completion.) to list the 3459 available choices. 3460 3461 You may also specify the system call numerically. A syscall's 3462 number is the value passed to the OS's syscall dispatcher to 3463 identify the requested service. When you specify the syscall 3464 by its name, GDB uses its database of syscalls to convert the 3465 name into the corresponding numeric code, but using the 3466 number directly may be useful if GDB's database does not have 3467 the complete list of syscalls on your system (e.g., because 3468 GDB lags behind the OS upgrades). 3469 3470 The example below illustrates how this command works if you 3471 don't provide arguments to it: 3472 3473 (gdb) catch syscall 3474 Catchpoint 1 (syscall) 3475 (gdb) r 3476 Starting program: /tmp/catch-syscall 3477 3478 Catchpoint 1 (call to syscall 'close'), \ 3479 0xffffe424 in __kernel_vsyscall () 3480 (gdb) c 3481 Continuing. 3482 3483 Catchpoint 1 (returned from syscall 'close'), \ 3484 0xffffe424 in __kernel_vsyscall () 3485 (gdb) 3486 3487 Here is an example of catching a system call by name: 3488 3489 (gdb) catch syscall chroot 3490 Catchpoint 1 (syscall 'chroot' [61]) 3491 (gdb) r 3492 Starting program: /tmp/catch-syscall 3493 3494 Catchpoint 1 (call to syscall 'chroot'), \ 3495 0xffffe424 in __kernel_vsyscall () 3496 (gdb) c 3497 Continuing. 3498 3499 Catchpoint 1 (returned from syscall 'chroot'), \ 3500 0xffffe424 in __kernel_vsyscall () 3501 (gdb) 3502 3503 An example of specifying a system call numerically. In the 3504 case below, the syscall number has a corresponding entry in 3505 the XML file, so GDB finds its name and prints it: 3506 3507 (gdb) catch syscall 252 3508 Catchpoint 1 (syscall(s) 'exit_group') 3509 (gdb) r 3510 Starting program: /tmp/catch-syscall 3511 3512 Catchpoint 1 (call to syscall 'exit_group'), \ 3513 0xffffe424 in __kernel_vsyscall () 3514 (gdb) c 3515 Continuing. 3516 3517 Program exited normally. 3518 (gdb) 3519 3520 However, there can be situations when there is no 3521 corresponding name in XML file for that syscall number. In 3522 this case, GDB prints a warning message saying that it was 3523 not able to find the syscall name, but the catchpoint will be 3524 set anyway. See the example below: 3525 3526 (gdb) catch syscall 764 3527 warning: The number '764' does not represent a known syscall. 3528 Catchpoint 2 (syscall 764) 3529 (gdb) 3530 3531 If you configure GDB using the `--without-expat' option, it 3532 will not be able to display syscall names. Also, if your 3533 architecture does not have an XML file describing its system 3534 calls, you will not be able to see the syscall names. It is 3535 important to notice that these two features are used for 3536 accessing the syscall name database. In either case, you 3537 will see a warning like this: 3538 3539 (gdb) catch syscall 3540 warning: Could not open "syscalls/i386-linux.xml" 3541 warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'. 3542 GDB will not be able to display syscall names. 3543 Catchpoint 1 (syscall) 3544 (gdb) 3545 3546 Of course, the file name will change depending on your 3547 architecture and system. 3548 3549 Still using the example above, you can also try to catch a 3550 syscall by its number. In this case, you would see something 3551 like: 3552 3553 (gdb) catch syscall 252 3554 Catchpoint 1 (syscall(s) 252) 3555 3556 Again, in this case GDB would not be able to display 3557 syscall's names. 3558 3559 `fork' 3560 A call to `fork'. This is currently only available for HP-UX 3561 and GNU/Linux. 3562 3563 `vfork' 3564 A call to `vfork'. This is currently only available for HP-UX 3565 and GNU/Linux. 3566 3567 `load [regexp]' 3568 `unload [regexp]' 3569 The loading or unloading of a shared library. If REGEXP is 3570 given, then the catchpoint will stop only if the regular 3571 expression matches one of the affected libraries. 3572 3573 `signal [SIGNAL... | `all']' 3574 The delivery of a signal. 3575 3576 With no arguments, this catchpoint will catch any signal that 3577 is not used internally by GDB, specifically, all signals 3578 except `SIGTRAP' and `SIGINT'. 3579 3580 With the argument `all', all signals, including those used by 3581 GDB, will be caught. This argument cannot be used with other 3582 signal names. 3583 3584 Otherwise, the arguments are a list of signal names as given 3585 to `handle' (*note Signals::). Only signals specified in 3586 this list will be caught. 3587 3588 One reason that `catch signal' can be more useful than 3589 `handle' is that you can attach commands and conditions to the 3590 catchpoint. 3591 3592 When a signal is caught by a catchpoint, the signal's `stop' 3593 and `print' settings, as specified by `handle', are ignored. 3594 However, whether the signal is still delivered to the 3595 inferior depends on the `pass' setting; this can be changed 3596 in the catchpoint's commands. 3597 3598 3599`tcatch EVENT' 3600 Set a catchpoint that is enabled only for one stop. The 3601 catchpoint is automatically deleted after the first time the event 3602 is caught. 3603 3604 3605 Use the `info break' command to list the current catchpoints. 3606 3607 There are currently some limitations to C++ exception handling 3608(`catch throw' and `catch catch') in GDB: 3609 3610 * If you call a function interactively, GDB normally returns control 3611 to you when the function has finished executing. If the call 3612 raises an exception, however, the call may bypass the mechanism 3613 that returns control to you and cause your program either to abort 3614 or to simply continue running until it hits a breakpoint, catches 3615 a signal that GDB is listening for, or exits. This is the case 3616 even if you set a catchpoint for the exception; catchpoints on 3617 exceptions are disabled within interactive calls. 3618 3619 * You cannot raise an exception interactively. 3620 3621 * You cannot install an exception handler interactively. 3622 3623 Sometimes `catch' is not the best way to debug exception handling: 3624if you need to know exactly where an exception is raised, it is better 3625to stop _before_ the exception handler is called, since that way you 3626can see the stack before any unwinding takes place. If you set a 3627breakpoint in an exception handler instead, it may not be easy to find 3628out where the exception was raised. 3629 3630 To stop just before an exception handler is called, you need some 3631knowledge of the implementation. In the case of GNU C++, exceptions are 3632raised by calling a library function named `__raise_exception' which 3633has the following ANSI C interface: 3634 3635 /* ADDR is where the exception identifier is stored. 3636 ID is the exception identifier. */ 3637 void __raise_exception (void **addr, void *id); 3638 3639To make the debugger catch all exceptions before any stack unwinding 3640takes place, set a breakpoint on `__raise_exception' (*note 3641Breakpoints; Watchpoints; and Exceptions: Breakpoints.). 3642 3643 With a conditional breakpoint (*note Break Conditions: Conditions.) 3644that depends on the value of ID, you can stop your program when a 3645specific exception is raised. You can use multiple conditional 3646breakpoints to stop your program when any of a number of exceptions are 3647raised. 3648 3649 3650File: gdb.info, Node: Delete Breaks, Next: Disabling, Prev: Set Catchpoints, Up: Breakpoints 3651 36525.1.4 Deleting Breakpoints 3653-------------------------- 3654 3655It is often necessary to eliminate a breakpoint, watchpoint, or 3656catchpoint once it has done its job and you no longer want your program 3657to stop there. This is called "deleting" the breakpoint. A breakpoint 3658that has been deleted no longer exists; it is forgotten. 3659 3660 With the `clear' command you can delete breakpoints according to 3661where they are in your program. With the `delete' command you can 3662delete individual breakpoints, watchpoints, or catchpoints by specifying 3663their breakpoint numbers. 3664 3665 It is not necessary to delete a breakpoint to proceed past it. GDB 3666automatically ignores breakpoints on the first instruction to be 3667executed when you continue execution without changing the execution 3668address. 3669 3670`clear' 3671 Delete any breakpoints at the next instruction to be executed in 3672 the selected stack frame (*note Selecting a Frame: Selection.). 3673 When the innermost frame is selected, this is a good way to delete 3674 a breakpoint where your program just stopped. 3675 3676`clear LOCATION' 3677 Delete any breakpoints set at the specified LOCATION. *Note 3678 Specify Location::, for the various forms of LOCATION; the most 3679 useful ones are listed below: 3680 3681 `clear FUNCTION' 3682 `clear FILENAME:FUNCTION' 3683 Delete any breakpoints set at entry to the named FUNCTION. 3684 3685 `clear LINENUM' 3686 `clear FILENAME:LINENUM' 3687 Delete any breakpoints set at or within the code of the 3688 specified LINENUM of the specified FILENAME. 3689 3690`delete [breakpoints] [RANGE...]' 3691 Delete the breakpoints, watchpoints, or catchpoints of the 3692 breakpoint ranges specified as arguments. If no argument is 3693 specified, delete all breakpoints (GDB asks confirmation, unless 3694 you have `set confirm off'). You can abbreviate this command as 3695 `d'. 3696 3697 3698File: gdb.info, Node: Disabling, Next: Conditions, Prev: Delete Breaks, Up: Breakpoints 3699 37005.1.5 Disabling Breakpoints 3701--------------------------- 3702 3703Rather than deleting a breakpoint, watchpoint, or catchpoint, you might 3704prefer to "disable" it. This makes the breakpoint inoperative as if it 3705had been deleted, but remembers the information on the breakpoint so 3706that you can "enable" it again later. 3707 3708 You disable and enable breakpoints, watchpoints, and catchpoints with 3709the `enable' and `disable' commands, optionally specifying one or more 3710breakpoint numbers as arguments. Use `info break' to print a list of 3711all breakpoints, watchpoints, and catchpoints if you do not know which 3712numbers to use. 3713 3714 Disabling and enabling a breakpoint that has multiple locations 3715affects all of its locations. 3716 3717 A breakpoint, watchpoint, or catchpoint can have any of several 3718different states of enablement: 3719 3720 * Enabled. The breakpoint stops your program. A breakpoint set 3721 with the `break' command starts out in this state. 3722 3723 * Disabled. The breakpoint has no effect on your program. 3724 3725 * Enabled once. The breakpoint stops your program, but then becomes 3726 disabled. 3727 3728 * Enabled for a count. The breakpoint stops your program for the 3729 next N times, then becomes disabled. 3730 3731 * Enabled for deletion. The breakpoint stops your program, but 3732 immediately after it does so it is deleted permanently. A 3733 breakpoint set with the `tbreak' command starts out in this state. 3734 3735 You can use the following commands to enable or disable breakpoints, 3736watchpoints, and catchpoints: 3737 3738`disable [breakpoints] [RANGE...]' 3739 Disable the specified breakpoints--or all breakpoints, if none are 3740 listed. A disabled breakpoint has no effect but is not forgotten. 3741 All options such as ignore-counts, conditions and commands are 3742 remembered in case the breakpoint is enabled again later. You may 3743 abbreviate `disable' as `dis'. 3744 3745`enable [breakpoints] [RANGE...]' 3746 Enable the specified breakpoints (or all defined breakpoints). 3747 They become effective once again in stopping your program. 3748 3749`enable [breakpoints] once RANGE...' 3750 Enable the specified breakpoints temporarily. GDB disables any of 3751 these breakpoints immediately after stopping your program. 3752 3753`enable [breakpoints] count COUNT RANGE...' 3754 Enable the specified breakpoints temporarily. GDB records COUNT 3755 with each of the specified breakpoints, and decrements a 3756 breakpoint's count when it is hit. When any count reaches 0, GDB 3757 disables that breakpoint. If a breakpoint has an ignore count 3758 (*note Break Conditions: Conditions.), that will be decremented to 3759 0 before COUNT is affected. 3760 3761`enable [breakpoints] delete RANGE...' 3762 Enable the specified breakpoints to work once, then die. GDB 3763 deletes any of these breakpoints as soon as your program stops 3764 there. Breakpoints set by the `tbreak' command start out in this 3765 state. 3766 3767 Except for a breakpoint set with `tbreak' (*note Setting 3768Breakpoints: Set Breaks.), breakpoints that you set are initially 3769enabled; subsequently, they become disabled or enabled only when you 3770use one of the commands above. (The command `until' can set and delete 3771a breakpoint of its own, but it does not change the state of your other 3772breakpoints; see *note Continuing and Stepping: Continuing and 3773Stepping.) 3774 3775 3776File: gdb.info, Node: Conditions, Next: Break Commands, Prev: Disabling, Up: Breakpoints 3777 37785.1.6 Break Conditions 3779---------------------- 3780 3781The simplest sort of breakpoint breaks every time your program reaches a 3782specified place. You can also specify a "condition" for a breakpoint. 3783A condition is just a Boolean expression in your programming language 3784(*note Expressions: Expressions.). A breakpoint with a condition 3785evaluates the expression each time your program reaches it, and your 3786program stops only if the condition is _true_. 3787 3788 This is the converse of using assertions for program validation; in 3789that situation, you want to stop when the assertion is violated--that 3790is, when the condition is false. In C, if you want to test an 3791assertion expressed by the condition ASSERT, you should set the 3792condition `! ASSERT' on the appropriate breakpoint. 3793 3794 Conditions are also accepted for watchpoints; you may not need them, 3795since a watchpoint is inspecting the value of an expression anyhow--but 3796it might be simpler, say, to just set a watchpoint on a variable name, 3797and specify a condition that tests whether the new value is an 3798interesting one. 3799 3800 Break conditions can have side effects, and may even call functions 3801in your program. This can be useful, for example, to activate functions 3802that log program progress, or to use your own print functions to format 3803special data structures. The effects are completely predictable unless 3804there is another enabled breakpoint at the same address. (In that 3805case, GDB might see the other breakpoint first and stop your program 3806without checking the condition of this one.) Note that breakpoint 3807commands are usually more convenient and flexible than break conditions 3808for the purpose of performing side effects when a breakpoint is reached 3809(*note Breakpoint Command Lists: Break Commands.). 3810 3811 Breakpoint conditions can also be evaluated on the target's side if 3812the target supports it. Instead of evaluating the conditions locally, 3813GDB encodes the expression into an agent expression (*note Agent 3814Expressions::) suitable for execution on the target, independently of 3815GDB. Global variables become raw memory locations, locals become stack 3816accesses, and so forth. 3817 3818 In this case, GDB will only be notified of a breakpoint trigger when 3819its condition evaluates to true. This mechanism may provide faster 3820response times depending on the performance characteristics of the 3821target since it does not need to keep GDB informed about every 3822breakpoint trigger, even those with false conditions. 3823 3824 Break conditions can be specified when a breakpoint is set, by using 3825`if' in the arguments to the `break' command. *Note Setting 3826Breakpoints: Set Breaks. They can also be changed at any time with the 3827`condition' command. 3828 3829 You can also use the `if' keyword with the `watch' command. The 3830`catch' command does not recognize the `if' keyword; `condition' is the 3831only way to impose a further condition on a catchpoint. 3832 3833`condition BNUM EXPRESSION' 3834 Specify EXPRESSION as the break condition for breakpoint, 3835 watchpoint, or catchpoint number BNUM. After you set a condition, 3836 breakpoint BNUM stops your program only if the value of EXPRESSION 3837 is true (nonzero, in C). When you use `condition', GDB checks 3838 EXPRESSION immediately for syntactic correctness, and to determine 3839 whether symbols in it have referents in the context of your 3840 breakpoint. If EXPRESSION uses symbols not referenced in the 3841 context of the breakpoint, GDB prints an error message: 3842 3843 No symbol "foo" in current context. 3844 3845 GDB does not actually evaluate EXPRESSION at the time the 3846 `condition' command (or a command that sets a breakpoint with a 3847 condition, like `break if ...') is given, however. *Note 3848 Expressions: Expressions. 3849 3850`condition BNUM' 3851 Remove the condition from breakpoint number BNUM. It becomes an 3852 ordinary unconditional breakpoint. 3853 3854 A special case of a breakpoint condition is to stop only when the 3855breakpoint has been reached a certain number of times. This is so 3856useful that there is a special way to do it, using the "ignore count" 3857of the breakpoint. Every breakpoint has an ignore count, which is an 3858integer. Most of the time, the ignore count is zero, and therefore has 3859no effect. But if your program reaches a breakpoint whose ignore count 3860is positive, then instead of stopping, it just decrements the ignore 3861count by one and continues. As a result, if the ignore count value is 3862N, the breakpoint does not stop the next N times your program reaches 3863it. 3864 3865`ignore BNUM COUNT' 3866 Set the ignore count of breakpoint number BNUM to COUNT. The next 3867 COUNT times the breakpoint is reached, your program's execution 3868 does not stop; other than to decrement the ignore count, GDB takes 3869 no action. 3870 3871 To make the breakpoint stop the next time it is reached, specify a 3872 count of zero. 3873 3874 When you use `continue' to resume execution of your program from a 3875 breakpoint, you can specify an ignore count directly as an 3876 argument to `continue', rather than using `ignore'. *Note 3877 Continuing and Stepping: Continuing and Stepping. 3878 3879 If a breakpoint has a positive ignore count and a condition, the 3880 condition is not checked. Once the ignore count reaches zero, GDB 3881 resumes checking the condition. 3882 3883 You could achieve the effect of the ignore count with a condition 3884 such as `$foo-- <= 0' using a debugger convenience variable that 3885 is decremented each time. *Note Convenience Variables: 3886 Convenience Vars. 3887 3888 Ignore counts apply to breakpoints, watchpoints, and catchpoints. 3889 3890 3891File: gdb.info, Node: Break Commands, Next: Dynamic Printf, Prev: Conditions, Up: Breakpoints 3892 38935.1.7 Breakpoint Command Lists 3894------------------------------ 3895 3896You can give any breakpoint (or watchpoint or catchpoint) a series of 3897commands to execute when your program stops due to that breakpoint. For 3898example, you might want to print the values of certain expressions, or 3899enable other breakpoints. 3900 3901`commands [RANGE...]' 3902`... COMMAND-LIST ...' 3903`end' 3904 Specify a list of commands for the given breakpoints. The commands 3905 themselves appear on the following lines. Type a line containing 3906 just `end' to terminate the commands. 3907 3908 To remove all commands from a breakpoint, type `commands' and 3909 follow it immediately with `end'; that is, give no commands. 3910 3911 With no argument, `commands' refers to the last breakpoint, 3912 watchpoint, or catchpoint set (not to the breakpoint most recently 3913 encountered). If the most recent breakpoints were set with a 3914 single command, then the `commands' will apply to all the 3915 breakpoints set by that command. This applies to breakpoints set 3916 by `rbreak', and also applies when a single `break' command 3917 creates multiple breakpoints (*note Ambiguous Expressions: 3918 Ambiguous Expressions.). 3919 3920 Pressing <RET> as a means of repeating the last GDB command is 3921disabled within a COMMAND-LIST. 3922 3923 You can use breakpoint commands to start your program up again. 3924Simply use the `continue' command, or `step', or any other command that 3925resumes execution. 3926 3927 Any other commands in the command list, after a command that resumes 3928execution, are ignored. This is because any time you resume execution 3929(even with a simple `next' or `step'), you may encounter another 3930breakpoint--which could have its own command list, leading to 3931ambiguities about which list to execute. 3932 3933 If the first command you specify in a command list is `silent', the 3934usual message about stopping at a breakpoint is not printed. This may 3935be desirable for breakpoints that are to print a specific message and 3936then continue. If none of the remaining commands print anything, you 3937see no sign that the breakpoint was reached. `silent' is meaningful 3938only at the beginning of a breakpoint command list. 3939 3940 The commands `echo', `output', and `printf' allow you to print 3941precisely controlled output, and are often useful in silent 3942breakpoints. *Note Commands for Controlled Output: Output. 3943 3944 For example, here is how you could use breakpoint commands to print 3945the value of `x' at entry to `foo' whenever `x' is positive. 3946 3947 break foo if x>0 3948 commands 3949 silent 3950 printf "x is %d\n",x 3951 cont 3952 end 3953 3954 One application for breakpoint commands is to compensate for one bug 3955so you can test for another. Put a breakpoint just after the erroneous 3956line of code, give it a condition to detect the case in which something 3957erroneous has been done, and give it commands to assign correct values 3958to any variables that need them. End with the `continue' command so 3959that your program does not stop, and start with the `silent' command so 3960that no output is produced. Here is an example: 3961 3962 break 403 3963 commands 3964 silent 3965 set x = y + 4 3966 cont 3967 end 3968 3969 3970File: gdb.info, Node: Dynamic Printf, Next: Save Breakpoints, Prev: Break Commands, Up: Breakpoints 3971 39725.1.8 Dynamic Printf 3973-------------------- 3974 3975The dynamic printf command `dprintf' combines a breakpoint with 3976formatted printing of your program's data to give you the effect of 3977inserting `printf' calls into your program on-the-fly, without having 3978to recompile it. 3979 3980 In its most basic form, the output goes to the GDB console. However, 3981you can set the variable `dprintf-style' for alternate handling. For 3982instance, you can ask to format the output by calling your program's 3983`printf' function. This has the advantage that the characters go to 3984the program's output device, so they can recorded in redirects to files 3985and so forth. 3986 3987 If you are doing remote debugging with a stub or agent, you can also 3988ask to have the printf handled by the remote agent. In addition to 3989ensuring that the output goes to the remote program's device along with 3990any other output the program might produce, you can also ask that the 3991dprintf remain active even after disconnecting from the remote target. 3992Using the stub/agent is also more efficient, as it can do everything 3993without needing to communicate with GDB. 3994 3995`dprintf LOCATION,TEMPLATE,EXPRESSION[,EXPRESSION...]' 3996 Whenever execution reaches LOCATION, print the values of one or 3997 more EXPRESSIONS under the control of the string TEMPLATE. To 3998 print several values, separate them with commas. 3999 4000`set dprintf-style STYLE' 4001 Set the dprintf output to be handled in one of several different 4002 styles enumerated below. A change of style affects all existing 4003 dynamic printfs immediately. (If you need individual control over 4004 the print commands, simply define normal breakpoints with 4005 explicitly-supplied command lists.) 4006 4007`gdb' 4008 Handle the output using the GDB `printf' command. 4009 4010`call' 4011 Handle the output by calling a function in your program (normally 4012 `printf'). 4013 4014`agent' 4015 Have the remote debugging agent (such as `gdbserver') handle the 4016 output itself. This style is only available for agents that 4017 support running commands on the target. 4018 4019`set dprintf-function FUNCTION' 4020 Set the function to call if the dprintf style is `call'. By 4021 default its value is `printf'. You may set it to any expression. 4022 that GDB can evaluate to a function, as per the `call' command. 4023 4024`set dprintf-channel CHANNEL' 4025 Set a "channel" for dprintf. If set to a non-empty value, GDB 4026 will evaluate it as an expression and pass the result as a first 4027 argument to the `dprintf-function', in the manner of `fprintf' and 4028 similar functions. Otherwise, the dprintf format string will be 4029 the first argument, in the manner of `printf'. 4030 4031 As an example, if you wanted `dprintf' output to go to a logfile 4032 that is a standard I/O stream assigned to the variable `mylog', 4033 you could do the following: 4034 4035 (gdb) set dprintf-style call 4036 (gdb) set dprintf-function fprintf 4037 (gdb) set dprintf-channel mylog 4038 (gdb) dprintf 25,"at line 25, glob=%d\n",glob 4039 Dprintf 1 at 0x123456: file main.c, line 25. 4040 (gdb) info break 4041 1 dprintf keep y 0x00123456 in main at main.c:25 4042 call (void) fprintf (mylog,"at line 25, glob=%d\n",glob) 4043 continue 4044 (gdb) 4045 4046 Note that the `info break' displays the dynamic printf commands as 4047 normal breakpoint commands; you can thus easily see the effect of 4048 the variable settings. 4049 4050`set disconnected-dprintf on' 4051`set disconnected-dprintf off' 4052 Choose whether `dprintf' commands should continue to run if GDB 4053 has disconnected from the target. This only applies if the 4054 `dprintf-style' is `agent'. 4055 4056`show disconnected-dprintf off' 4057 Show the current choice for disconnected `dprintf'. 4058 4059 4060 GDB does not check the validity of function and channel, relying on 4061you to supply values that are meaningful for the contexts in which they 4062are being used. For instance, the function and channel may be the 4063values of local variables, but if that is the case, then all enabled 4064dynamic prints must be at locations within the scope of those locals. 4065If evaluation fails, GDB will report an error. 4066 4067 4068File: gdb.info, Node: Save Breakpoints, Next: Static Probe Points, Prev: Dynamic Printf, Up: Breakpoints 4069 40705.1.9 How to save breakpoints to a file 4071--------------------------------------- 4072 4073To save breakpoint definitions to a file use the `save breakpoints' 4074command. 4075 4076`save breakpoints [FILENAME]' 4077 This command saves all current breakpoint definitions together with 4078 their commands and ignore counts, into a file `FILENAME' suitable 4079 for use in a later debugging session. This includes all types of 4080 breakpoints (breakpoints, watchpoints, catchpoints, tracepoints). 4081 To read the saved breakpoint definitions, use the `source' command 4082 (*note Command Files::). Note that watchpoints with expressions 4083 involving local variables may fail to be recreated because it may 4084 not be possible to access the context where the watchpoint is 4085 valid anymore. Because the saved breakpoint definitions are 4086 simply a sequence of GDB commands that recreate the breakpoints, 4087 you can edit the file in your favorite editing program, and remove 4088 the breakpoint definitions you're not interested in, or that can 4089 no longer be recreated. 4090 4091 4092File: gdb.info, Node: Static Probe Points, Next: Error in Breakpoints, Prev: Save Breakpoints, Up: Breakpoints 4093 40945.1.10 Static Probe Points 4095-------------------------- 4096 4097GDB supports "SDT" probes in the code. SDT stands for Statically 4098Defined Tracing, and the probes are designed to have a tiny runtime 4099code and data footprint, and no dynamic relocations. They are usable 4100from assembly, C and C++ languages. See 4101`http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation' for 4102a good reference on how the SDT probes are implemented. 4103 4104 Currently, `SystemTap' (`http://sourceware.org/systemtap/') SDT 4105probes are supported on ELF-compatible systems. See 4106`http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps' for 4107more information on how to add `SystemTap' SDT probes in your 4108applications. 4109 4110 Some probes have an associated semaphore variable; for instance, this 4111happens automatically if you defined your probe using a DTrace-style 4112`.d' file. If your probe has a semaphore, GDB will automatically 4113enable it when you specify a breakpoint using the `-probe-stap' 4114notation. But, if you put a breakpoint at a probe's location by some 4115other method (e.g., `break file:line'), then GDB will not automatically 4116set the semaphore. 4117 4118 You can examine the available static static probes using `info 4119probes', with optional arguments: 4120 4121`info probes stap [PROVIDER [NAME [OBJFILE]]]' 4122 If given, PROVIDER is a regular expression used to match against 4123 provider names when selecting which probes to list. If omitted, 4124 probes by all probes from all providers are listed. 4125 4126 If given, NAME is a regular expression to match against probe names 4127 when selecting which probes to list. If omitted, probe names are 4128 not considered when deciding whether to display them. 4129 4130 If given, OBJFILE is a regular expression used to select which 4131 object files (executable or shared libraries) to examine. If not 4132 given, all object files are considered. 4133 4134`info probes all' 4135 List the available static probes, from all types. 4136 4137 A probe may specify up to twelve arguments. These are available at 4138the point at which the probe is defined--that is, when the current PC is 4139at the probe's location. The arguments are available using the 4140convenience variables (*note Convenience Vars::) 4141`$_probe_arg0'...`$_probe_arg11'. Each probe argument is an integer of 4142the appropriate size; types are not preserved. The convenience 4143variable `$_probe_argc' holds the number of arguments at the current 4144probe point. 4145 4146 These variables are always available, but attempts to access them at 4147any location other than a probe point will cause GDB to give an error 4148message. 4149 4150 4151File: gdb.info, Node: Error in Breakpoints, Next: Breakpoint-related Warnings, Prev: Static Probe Points, Up: Breakpoints 4152 41535.1.11 "Cannot insert breakpoints" 4154---------------------------------- 4155 4156If you request too many active hardware-assisted breakpoints and 4157watchpoints, you will see this error message: 4158 4159 Stopped; cannot insert breakpoints. 4160 You may have requested too many hardware breakpoints and watchpoints. 4161 4162This message is printed when you attempt to resume the program, since 4163only then GDB knows exactly how many hardware breakpoints and 4164watchpoints it needs to insert. 4165 4166 When this message is printed, you need to disable or remove some of 4167the hardware-assisted breakpoints and watchpoints, and then continue. 4168 4169 4170File: gdb.info, Node: Breakpoint-related Warnings, Prev: Error in Breakpoints, Up: Breakpoints 4171 41725.1.12 "Breakpoint address adjusted..." 4173--------------------------------------- 4174 4175Some processor architectures place constraints on the addresses at 4176which breakpoints may be placed. For architectures thus constrained, 4177GDB will attempt to adjust the breakpoint's address to comply with the 4178constraints dictated by the architecture. 4179 4180 One example of such an architecture is the Fujitsu FR-V. The FR-V is 4181a VLIW architecture in which a number of RISC-like instructions may be 4182bundled together for parallel execution. The FR-V architecture 4183constrains the location of a breakpoint instruction within such a 4184bundle to the instruction with the lowest address. GDB honors this 4185constraint by adjusting a breakpoint's address to the first in the 4186bundle. 4187 4188 It is not uncommon for optimized code to have bundles which contain 4189instructions from different source statements, thus it may happen that 4190a breakpoint's address will be adjusted from one source statement to 4191another. Since this adjustment may significantly alter GDB's 4192breakpoint related behavior from what the user expects, a warning is 4193printed when the breakpoint is first set and also when the breakpoint 4194is hit. 4195 4196 A warning like the one below is printed when setting a breakpoint 4197that's been subject to address adjustment: 4198 4199 warning: Breakpoint address adjusted from 0x00010414 to 0x00010410. 4200 4201 Such warnings are printed both for user settable and GDB's internal 4202breakpoints. If you see one of these warnings, you should verify that 4203a breakpoint set at the adjusted address will have the desired affect. 4204If not, the breakpoint in question may be removed and other breakpoints 4205may be set which will have the desired behavior. E.g., it may be 4206sufficient to place the breakpoint at a later instruction. A 4207conditional breakpoint may also be useful in some cases to prevent the 4208breakpoint from triggering too often. 4209 4210 GDB will also issue a warning when stopping at one of these adjusted 4211breakpoints: 4212 4213 warning: Breakpoint 1 address previously adjusted from 0x00010414 4214 to 0x00010410. 4215 4216 When this warning is encountered, it may be too late to take remedial 4217action except in cases where the breakpoint is hit earlier or more 4218frequently than expected. 4219 4220 4221File: gdb.info, Node: Continuing and Stepping, Next: Skipping Over Functions and Files, Prev: Breakpoints, Up: Stopping 4222 42235.2 Continuing and Stepping 4224=========================== 4225 4226"Continuing" means resuming program execution until your program 4227completes normally. In contrast, "stepping" means executing just one 4228more "step" of your program, where "step" may mean either one line of 4229source code, or one machine instruction (depending on what particular 4230command you use). Either when continuing or when stepping, your 4231program may stop even sooner, due to a breakpoint or a signal. (If it 4232stops due to a signal, you may want to use `handle', or use `signal 0' 4233to resume execution. *Note Signals: Signals.) 4234 4235`continue [IGNORE-COUNT]' 4236`c [IGNORE-COUNT]' 4237`fg [IGNORE-COUNT]' 4238 Resume program execution, at the address where your program last 4239 stopped; any breakpoints set at that address are bypassed. The 4240 optional argument IGNORE-COUNT allows you to specify a further 4241 number of times to ignore a breakpoint at this location; its 4242 effect is like that of `ignore' (*note Break Conditions: 4243 Conditions.). 4244 4245 The argument IGNORE-COUNT is meaningful only when your program 4246 stopped due to a breakpoint. At other times, the argument to 4247 `continue' is ignored. 4248 4249 The synonyms `c' and `fg' (for "foreground", as the debugged 4250 program is deemed to be the foreground program) are provided 4251 purely for convenience, and have exactly the same behavior as 4252 `continue'. 4253 4254 To resume execution at a different place, you can use `return' 4255(*note Returning from a Function: Returning.) to go back to the calling 4256function; or `jump' (*note Continuing at a Different Address: Jumping.) 4257to go to an arbitrary location in your program. 4258 4259 A typical technique for using stepping is to set a breakpoint (*note 4260Breakpoints; Watchpoints; and Catchpoints: Breakpoints.) at the 4261beginning of the function or the section of your program where a problem 4262is believed to lie, run your program until it stops at that breakpoint, 4263and then step through the suspect area, examining the variables that are 4264interesting, until you see the problem happen. 4265 4266`step' 4267 Continue running your program until control reaches a different 4268 source line, then stop it and return control to GDB. This command 4269 is abbreviated `s'. 4270 4271 _Warning:_ If you use the `step' command while control is 4272 within a function that was compiled without debugging 4273 information, execution proceeds until control reaches a 4274 function that does have debugging information. Likewise, it 4275 will not step into a function which is compiled without 4276 debugging information. To step through functions without 4277 debugging information, use the `stepi' command, described 4278 below. 4279 4280 The `step' command only stops at the first instruction of a source 4281 line. This prevents the multiple stops that could otherwise occur 4282 in `switch' statements, `for' loops, etc. `step' continues to 4283 stop if a function that has debugging information is called within 4284 the line. In other words, `step' _steps inside_ any functions 4285 called within the line. 4286 4287 Also, the `step' command only enters a function if there is line 4288 number information for the function. Otherwise it acts like the 4289 `next' command. This avoids problems when using `cc -gl' on MIPS 4290 machines. Previously, `step' entered subroutines if there was any 4291 debugging information about the routine. 4292 4293`step COUNT' 4294 Continue running as in `step', but do so COUNT times. If a 4295 breakpoint is reached, or a signal not related to stepping occurs 4296 before COUNT steps, stepping stops right away. 4297 4298`next [COUNT]' 4299 Continue to the next source line in the current (innermost) stack 4300 frame. This is similar to `step', but function calls that appear 4301 within the line of code are executed without stopping. Execution 4302 stops when control reaches a different line of code at the 4303 original stack level that was executing when you gave the `next' 4304 command. This command is abbreviated `n'. 4305 4306 An argument COUNT is a repeat count, as for `step'. 4307 4308 The `next' command only stops at the first instruction of a source 4309 line. This prevents multiple stops that could otherwise occur in 4310 `switch' statements, `for' loops, etc. 4311 4312`set step-mode' 4313`set step-mode on' 4314 The `set step-mode on' command causes the `step' command to stop 4315 at the first instruction of a function which contains no debug line 4316 information rather than stepping over it. 4317 4318 This is useful in cases where you may be interested in inspecting 4319 the machine instructions of a function which has no symbolic info 4320 and do not want GDB to automatically skip over this function. 4321 4322`set step-mode off' 4323 Causes the `step' command to step over any functions which 4324 contains no debug information. This is the default. 4325 4326`show step-mode' 4327 Show whether GDB will stop in or step over functions without 4328 source line debug information. 4329 4330`finish' 4331 Continue running until just after function in the selected stack 4332 frame returns. Print the returned value (if any). This command 4333 can be abbreviated as `fin'. 4334 4335 Contrast this with the `return' command (*note Returning from a 4336 Function: Returning.). 4337 4338`until' 4339`u' 4340 Continue running until a source line past the current line, in the 4341 current stack frame, is reached. This command is used to avoid 4342 single stepping through a loop more than once. It is like the 4343 `next' command, except that when `until' encounters a jump, it 4344 automatically continues execution until the program counter is 4345 greater than the address of the jump. 4346 4347 This means that when you reach the end of a loop after single 4348 stepping though it, `until' makes your program continue execution 4349 until it exits the loop. In contrast, a `next' command at the end 4350 of a loop simply steps back to the beginning of the loop, which 4351 forces you to step through the next iteration. 4352 4353 `until' always stops your program if it attempts to exit the 4354 current stack frame. 4355 4356 `until' may produce somewhat counterintuitive results if the order 4357 of machine code does not match the order of the source lines. For 4358 example, in the following excerpt from a debugging session, the `f' 4359 (`frame') command shows that execution is stopped at line `206'; 4360 yet when we use `until', we get to line `195': 4361 4362 (gdb) f 4363 #0 main (argc=4, argv=0xf7fffae8) at m4.c:206 4364 206 expand_input(); 4365 (gdb) until 4366 195 for ( ; argc > 0; NEXTARG) { 4367 4368 This happened because, for execution efficiency, the compiler had 4369 generated code for the loop closure test at the end, rather than 4370 the start, of the loop--even though the test in a C `for'-loop is 4371 written before the body of the loop. The `until' command appeared 4372 to step back to the beginning of the loop when it advanced to this 4373 expression; however, it has not really gone to an earlier 4374 statement--not in terms of the actual machine code. 4375 4376 `until' with no argument works by means of single instruction 4377 stepping, and hence is slower than `until' with an argument. 4378 4379`until LOCATION' 4380`u LOCATION' 4381 Continue running your program until either the specified location 4382 is reached, or the current stack frame returns. LOCATION is any of 4383 the forms described in *note Specify Location::. This form of the 4384 command uses temporary breakpoints, and hence is quicker than 4385 `until' without an argument. The specified location is actually 4386 reached only if it is in the current frame. This implies that 4387 `until' can be used to skip over recursive function invocations. 4388 For instance in the code below, if the current location is line 4389 `96', issuing `until 99' will execute the program up to line `99' 4390 in the same invocation of factorial, i.e., after the inner 4391 invocations have returned. 4392 4393 94 int factorial (int value) 4394 95 { 4395 96 if (value > 1) { 4396 97 value *= factorial (value - 1); 4397 98 } 4398 99 return (value); 4399 100 } 4400 4401`advance LOCATION' 4402 Continue running the program up to the given LOCATION. An 4403 argument is required, which should be of one of the forms 4404 described in *note Specify Location::. Execution will also stop 4405 upon exit from the current stack frame. This command is similar 4406 to `until', but `advance' will not skip over recursive function 4407 calls, and the target location doesn't have to be in the same 4408 frame as the current one. 4409 4410`stepi' 4411`stepi ARG' 4412`si' 4413 Execute one machine instruction, then stop and return to the 4414 debugger. 4415 4416 It is often useful to do `display/i $pc' when stepping by machine 4417 instructions. This makes GDB automatically display the next 4418 instruction to be executed, each time your program stops. *Note 4419 Automatic Display: Auto Display. 4420 4421 An argument is a repeat count, as in `step'. 4422 4423`nexti' 4424`nexti ARG' 4425`ni' 4426 Execute one machine instruction, but if it is a function call, 4427 proceed until the function returns. 4428 4429 An argument is a repeat count, as in `next'. 4430 4431 4432File: gdb.info, Node: Skipping Over Functions and Files, Next: Signals, Prev: Continuing and Stepping, Up: Stopping 4433 44345.3 Skipping Over Functions and Files 4435===================================== 4436 4437The program you are debugging may contain some functions which are 4438uninteresting to debug. The `skip' comand lets you tell GDB to skip a 4439function or all functions in a file when stepping. 4440 4441 For example, consider the following C function: 4442 4443 101 int func() 4444 102 { 4445 103 foo(boring()); 4446 104 bar(boring()); 4447 105 } 4448 4449Suppose you wish to step into the functions `foo' and `bar', but you 4450are not interested in stepping through `boring'. If you run `step' at 4451line 103, you'll enter `boring()', but if you run `next', you'll step 4452over both `foo' and `boring'! 4453 4454 One solution is to `step' into `boring' and use the `finish' command 4455to immediately exit it. But this can become tedious if `boring' is 4456called from many places. 4457 4458 A more flexible solution is to execute `skip boring'. This instructs 4459GDB never to step into `boring'. Now when you execute `step' at line 4460103, you'll step over `boring' and directly into `foo'. 4461 4462 You can also instruct GDB to skip all functions in a file, with, for 4463example, `skip file boring.c'. 4464 4465`skip [LINESPEC]' 4466`skip function [LINESPEC]' 4467 After running this command, the function named by LINESPEC or the 4468 function containing the line named by LINESPEC will be skipped 4469 over when stepping. *Note Specify Location::. 4470 4471 If you do not specify LINESPEC, the function you're currently 4472 debugging will be skipped. 4473 4474 (If you have a function called `file' that you want to skip, use 4475 `skip function file'.) 4476 4477`skip file [FILENAME]' 4478 After running this command, any function whose source lives in 4479 FILENAME will be skipped over when stepping. 4480 4481 If you do not specify FILENAME, functions whose source lives in 4482 the file you're currently debugging will be skipped. 4483 4484 Skips can be listed, deleted, disabled, and enabled, much like 4485breakpoints. These are the commands for managing your list of skips: 4486 4487`info skip [RANGE]' 4488 Print details about the specified skip(s). If RANGE is not 4489 specified, print a table with details about all functions and 4490 files marked for skipping. `info skip' prints the following 4491 information about each skip: 4492 4493 _Identifier_ 4494 A number identifying this skip. 4495 4496 _Type_ 4497 The type of this skip, either `function' or `file'. 4498 4499 _Enabled or Disabled_ 4500 Enabled skips are marked with `y'. Disabled skips are marked 4501 with `n'. 4502 4503 _Address_ 4504 For function skips, this column indicates the address in 4505 memory of the function being skipped. If you've set a 4506 function skip on a function which has not yet been loaded, 4507 this field will contain `<PENDING>'. Once a shared library 4508 which has the function is loaded, `info skip' will show the 4509 function's address here. 4510 4511 _What_ 4512 For file skips, this field contains the filename being 4513 skipped. For functions skips, this field contains the 4514 function name and its line number in the file where it is 4515 defined. 4516 4517`skip delete [RANGE]' 4518 Delete the specified skip(s). If RANGE is not specified, delete 4519 all skips. 4520 4521`skip enable [RANGE]' 4522 Enable the specified skip(s). If RANGE is not specified, enable 4523 all skips. 4524 4525`skip disable [RANGE]' 4526 Disable the specified skip(s). If RANGE is not specified, disable 4527 all skips. 4528 4529 4530 4531File: gdb.info, Node: Signals, Next: Thread Stops, Prev: Skipping Over Functions and Files, Up: Stopping 4532 45335.4 Signals 4534=========== 4535 4536A signal is an asynchronous event that can happen in a program. The 4537operating system defines the possible kinds of signals, and gives each 4538kind a name and a number. For example, in Unix `SIGINT' is the signal 4539a program gets when you type an interrupt character (often `Ctrl-c'); 4540`SIGSEGV' is the signal a program gets from referencing a place in 4541memory far away from all the areas in use; `SIGALRM' occurs when the 4542alarm clock timer goes off (which happens only if your program has 4543requested an alarm). 4544 4545 Some signals, including `SIGALRM', are a normal part of the 4546functioning of your program. Others, such as `SIGSEGV', indicate 4547errors; these signals are "fatal" (they kill your program immediately) 4548if the program has not specified in advance some other way to handle 4549the signal. `SIGINT' does not indicate an error in your program, but 4550it is normally fatal so it can carry out the purpose of the interrupt: 4551to kill the program. 4552 4553 GDB has the ability to detect any occurrence of a signal in your 4554program. You can tell GDB in advance what to do for each kind of 4555signal. 4556 4557 Normally, GDB is set up to let the non-erroneous signals like 4558`SIGALRM' be silently passed to your program (so as not to interfere 4559with their role in the program's functioning) but to stop your program 4560immediately whenever an error signal happens. You can change these 4561settings with the `handle' command. 4562 4563`info signals' 4564`info handle' 4565 Print a table of all the kinds of signals and how GDB has been 4566 told to handle each one. You can use this to see the signal 4567 numbers of all the defined types of signals. 4568 4569`info signals SIG' 4570 Similar, but print information only about the specified signal 4571 number. 4572 4573 `info handle' is an alias for `info signals'. 4574 4575`catch signal [SIGNAL... | `all']' 4576 Set a catchpoint for the indicated signals. *Note Set 4577 Catchpoints::, for details about this command. 4578 4579`handle SIGNAL [KEYWORDS...]' 4580 Change the way GDB handles signal SIGNAL. SIGNAL can be the 4581 number of a signal or its name (with or without the `SIG' at the 4582 beginning); a list of signal numbers of the form `LOW-HIGH'; or 4583 the word `all', meaning all the known signals. Optional arguments 4584 KEYWORDS, described below, say what change to make. 4585 4586 The keywords allowed by the `handle' command can be abbreviated. 4587Their full names are: 4588 4589`nostop' 4590 GDB should not stop your program when this signal happens. It may 4591 still print a message telling you that the signal has come in. 4592 4593`stop' 4594 GDB should stop your program when this signal happens. This 4595 implies the `print' keyword as well. 4596 4597`print' 4598 GDB should print a message when this signal happens. 4599 4600`noprint' 4601 GDB should not mention the occurrence of the signal at all. This 4602 implies the `nostop' keyword as well. 4603 4604`pass' 4605`noignore' 4606 GDB should allow your program to see this signal; your program can 4607 handle the signal, or else it may terminate if the signal is fatal 4608 and not handled. `pass' and `noignore' are synonyms. 4609 4610`nopass' 4611`ignore' 4612 GDB should not allow your program to see this signal. `nopass' 4613 and `ignore' are synonyms. 4614 4615 When a signal stops your program, the signal is not visible to the 4616program until you continue. Your program sees the signal then, if 4617`pass' is in effect for the signal in question _at that time_. In 4618other words, after GDB reports a signal, you can use the `handle' 4619command with `pass' or `nopass' to control whether your program sees 4620that signal when you continue. 4621 4622 The default is set to `nostop', `noprint', `pass' for non-erroneous 4623signals such as `SIGALRM', `SIGWINCH' and `SIGCHLD', and to `stop', 4624`print', `pass' for the erroneous signals. 4625 4626 You can also use the `signal' command to prevent your program from 4627seeing a signal, or cause it to see a signal it normally would not see, 4628or to give it any signal at any time. For example, if your program 4629stopped due to some sort of memory reference error, you might store 4630correct values into the erroneous variables and continue, hoping to see 4631more execution; but your program would probably terminate immediately as 4632a result of the fatal signal once it saw the signal. To prevent this, 4633you can continue with `signal 0'. *Note Giving your Program a Signal: 4634Signaling. 4635 4636 On some targets, GDB can inspect extra signal information associated 4637with the intercepted signal, before it is actually delivered to the 4638program being debugged. This information is exported by the 4639convenience variable `$_siginfo', and consists of data that is passed 4640by the kernel to the signal handler at the time of the receipt of a 4641signal. The data type of the information itself is target dependent. 4642You can see the data type using the `ptype $_siginfo' command. On Unix 4643systems, it typically corresponds to the standard `siginfo_t' type, as 4644defined in the `signal.h' system header. 4645 4646 Here's an example, on a GNU/Linux system, printing the stray 4647referenced address that raised a segmentation fault. 4648 4649 (gdb) continue 4650 Program received signal SIGSEGV, Segmentation fault. 4651 0x0000000000400766 in main () 4652 69 *(int *)p = 0; 4653 (gdb) ptype $_siginfo 4654 type = struct { 4655 int si_signo; 4656 int si_errno; 4657 int si_code; 4658 union { 4659 int _pad[28]; 4660 struct {...} _kill; 4661 struct {...} _timer; 4662 struct {...} _rt; 4663 struct {...} _sigchld; 4664 struct {...} _sigfault; 4665 struct {...} _sigpoll; 4666 } _sifields; 4667 } 4668 (gdb) ptype $_siginfo._sifields._sigfault 4669 type = struct { 4670 void *si_addr; 4671 } 4672 (gdb) p $_siginfo._sifields._sigfault.si_addr 4673 $1 = (void *) 0x7ffff7ff7000 4674 4675 Depending on target support, `$_siginfo' may also be writable. 4676 4677 4678File: gdb.info, Node: Thread Stops, Prev: Signals, Up: Stopping 4679 46805.5 Stopping and Starting Multi-thread Programs 4681=============================================== 4682 4683GDB supports debugging programs with multiple threads (*note Debugging 4684Programs with Multiple Threads: Threads.). There are two modes of 4685controlling execution of your program within the debugger. In the 4686default mode, referred to as "all-stop mode", when any thread in your 4687program stops (for example, at a breakpoint or while being stepped), 4688all other threads in the program are also stopped by GDB. On some 4689targets, GDB also supports "non-stop mode", in which other threads can 4690continue to run freely while you examine the stopped thread in the 4691debugger. 4692 4693* Menu: 4694 4695* All-Stop Mode:: All threads stop when GDB takes control 4696* Non-Stop Mode:: Other threads continue to execute 4697* Background Execution:: Running your program asynchronously 4698* Thread-Specific Breakpoints:: Controlling breakpoints 4699* Interrupted System Calls:: GDB may interfere with system calls 4700* Observer Mode:: GDB does not alter program behavior 4701 4702 4703File: gdb.info, Node: All-Stop Mode, Next: Non-Stop Mode, Up: Thread Stops 4704 47055.5.1 All-Stop Mode 4706------------------- 4707 4708In all-stop mode, whenever your program stops under GDB for any reason, 4709_all_ threads of execution stop, not just the current thread. This 4710allows you to examine the overall state of the program, including 4711switching between threads, without worrying that things may change 4712underfoot. 4713 4714 Conversely, whenever you restart the program, _all_ threads start 4715executing. _This is true even when single-stepping_ with commands like 4716`step' or `next'. 4717 4718 In particular, GDB cannot single-step all threads in lockstep. 4719Since thread scheduling is up to your debugging target's operating 4720system (not controlled by GDB), other threads may execute more than one 4721statement while the current thread completes a single step. Moreover, 4722in general other threads stop in the middle of a statement, rather than 4723at a clean statement boundary, when the program stops. 4724 4725 You might even find your program stopped in another thread after 4726continuing or even single-stepping. This happens whenever some other 4727thread runs into a breakpoint, a signal, or an exception before the 4728first thread completes whatever you requested. 4729 4730 Whenever GDB stops your program, due to a breakpoint or a signal, it 4731automatically selects the thread where that breakpoint or signal 4732happened. GDB alerts you to the context switch with a message such as 4733`[Switching to Thread N]' to identify the thread. 4734 4735 On some OSes, you can modify GDB's default behavior by locking the 4736OS scheduler to allow only a single thread to run. 4737 4738`set scheduler-locking MODE' 4739 Set the scheduler locking mode. If it is `off', then there is no 4740 locking and any thread may run at any time. If `on', then only the 4741 current thread may run when the inferior is resumed. The `step' 4742 mode optimizes for single-stepping; it prevents other threads from 4743 preempting the current thread while you are stepping, so that the 4744 focus of debugging does not change unexpectedly. Other threads 4745 only rarely (or never) get a chance to run when you step. They 4746 are more likely to run when you `next' over a function call, and 4747 they are completely free to run when you use commands like 4748 `continue', `until', or `finish'. However, unless another thread 4749 hits a breakpoint during its timeslice, GDB does not change the 4750 current thread away from the thread that you are debugging. 4751 4752`show scheduler-locking' 4753 Display the current scheduler locking mode. 4754 4755 By default, when you issue one of the execution commands such as 4756`continue', `next' or `step', GDB allows only threads of the current 4757inferior to run. For example, if GDB is attached to two inferiors, 4758each with two threads, the `continue' command resumes only the two 4759threads of the current inferior. This is useful, for example, when you 4760debug a program that forks and you want to hold the parent stopped (so 4761that, for instance, it doesn't run to exit), while you debug the child. 4762In other situations, you may not be interested in inspecting the 4763current state of any of the processes GDB is attached to, and you may 4764want to resume them all until some breakpoint is hit. In the latter 4765case, you can instruct GDB to allow all threads of all the inferiors to 4766run with the `set schedule-multiple' command. 4767 4768`set schedule-multiple' 4769 Set the mode for allowing threads of multiple processes to be 4770 resumed when an execution command is issued. When `on', all 4771 threads of all processes are allowed to run. When `off', only the 4772 threads of the current process are resumed. The default is `off'. 4773 The `scheduler-locking' mode takes precedence when set to `on', or 4774 while you are stepping and set to `step'. 4775 4776`show schedule-multiple' 4777 Display the current mode for resuming the execution of threads of 4778 multiple processes. 4779 4780 4781File: gdb.info, Node: Non-Stop Mode, Next: Background Execution, Prev: All-Stop Mode, Up: Thread Stops 4782 47835.5.2 Non-Stop Mode 4784------------------- 4785 4786For some multi-threaded targets, GDB supports an optional mode of 4787operation in which you can examine stopped program threads in the 4788debugger while other threads continue to execute freely. This 4789minimizes intrusion when debugging live systems, such as programs where 4790some threads have real-time constraints or must continue to respond to 4791external events. This is referred to as "non-stop" mode. 4792 4793 In non-stop mode, when a thread stops to report a debugging event, 4794_only_ that thread is stopped; GDB does not stop other threads as well, 4795in contrast to the all-stop mode behavior. Additionally, execution 4796commands such as `continue' and `step' apply by default only to the 4797current thread in non-stop mode, rather than all threads as in all-stop 4798mode. This allows you to control threads explicitly in ways that are 4799not possible in all-stop mode -- for example, stepping one thread while 4800allowing others to run freely, stepping one thread while holding all 4801others stopped, or stepping several threads independently and 4802simultaneously. 4803 4804 To enter non-stop mode, use this sequence of commands before you run 4805or attach to your program: 4806 4807 # Enable the async interface. 4808 set target-async 1 4809 4810 # If using the CLI, pagination breaks non-stop. 4811 set pagination off 4812 4813 # Finally, turn it on! 4814 set non-stop on 4815 4816 You can use these commands to manipulate the non-stop mode setting: 4817 4818`set non-stop on' 4819 Enable selection of non-stop mode. 4820 4821`set non-stop off' 4822 Disable selection of non-stop mode. 4823 4824`show non-stop' 4825 Show the current non-stop enablement setting. 4826 4827 Note these commands only reflect whether non-stop mode is enabled, 4828not whether the currently-executing program is being run in non-stop 4829mode. In particular, the `set non-stop' preference is only consulted 4830when GDB starts or connects to the target program, and it is generally 4831not possible to switch modes once debugging has started. Furthermore, 4832since not all targets support non-stop mode, even when you have enabled 4833non-stop mode, GDB may still fall back to all-stop operation by default. 4834 4835 In non-stop mode, all execution commands apply only to the current 4836thread by default. That is, `continue' only continues one thread. To 4837continue all threads, issue `continue -a' or `c -a'. 4838 4839 You can use GDB's background execution commands (*note Background 4840Execution::) to run some threads in the background while you continue 4841to examine or step others from GDB. The MI execution commands (*note 4842GDB/MI Program Execution::) are always executed asynchronously in 4843non-stop mode. 4844 4845 Suspending execution is done with the `interrupt' command when 4846running in the background, or `Ctrl-c' during foreground execution. In 4847all-stop mode, this stops the whole process; but in non-stop mode the 4848interrupt applies only to the current thread. To stop the whole 4849program, use `interrupt -a'. 4850 4851 Other execution commands do not currently support the `-a' option. 4852 4853 In non-stop mode, when a thread stops, GDB doesn't automatically make 4854that thread current, as it does in all-stop mode. This is because the 4855thread stop notifications are asynchronous with respect to GDB's 4856command interpreter, and it would be confusing if GDB unexpectedly 4857changed to a different thread just as you entered a command to operate 4858on the previously current thread. 4859 4860 4861File: gdb.info, Node: Background Execution, Next: Thread-Specific Breakpoints, Prev: Non-Stop Mode, Up: Thread Stops 4862 48635.5.3 Background Execution 4864-------------------------- 4865 4866GDB's execution commands have two variants: the normal foreground 4867(synchronous) behavior, and a background (asynchronous) behavior. In 4868foreground execution, GDB waits for the program to report that some 4869thread has stopped before prompting for another command. In background 4870execution, GDB immediately gives a command prompt so that you can issue 4871other commands while your program runs. 4872 4873 You need to explicitly enable asynchronous mode before you can use 4874background execution commands. You can use these commands to 4875manipulate the asynchronous mode setting: 4876 4877`set target-async on' 4878 Enable asynchronous mode. 4879 4880`set target-async off' 4881 Disable asynchronous mode. 4882 4883`show target-async' 4884 Show the current target-async setting. 4885 4886 If the target doesn't support async mode, GDB issues an error 4887message if you attempt to use the background execution commands. 4888 4889 To specify background execution, add a `&' to the command. For 4890example, the background form of the `continue' command is `continue&', 4891or just `c&'. The execution commands that accept background execution 4892are: 4893 4894`run' 4895 *Note Starting your Program: Starting. 4896 4897`attach' 4898 *Note Debugging an Already-running Process: Attach. 4899 4900`step' 4901 *Note step: Continuing and Stepping. 4902 4903`stepi' 4904 *Note stepi: Continuing and Stepping. 4905 4906`next' 4907 *Note next: Continuing and Stepping. 4908 4909`nexti' 4910 *Note nexti: Continuing and Stepping. 4911 4912`continue' 4913 *Note continue: Continuing and Stepping. 4914 4915`finish' 4916 *Note finish: Continuing and Stepping. 4917 4918`until' 4919 *Note until: Continuing and Stepping. 4920 4921 4922 Background execution is especially useful in conjunction with 4923non-stop mode for debugging programs with multiple threads; see *note 4924Non-Stop Mode::. However, you can also use these commands in the 4925normal all-stop mode with the restriction that you cannot issue another 4926execution command until the previous one finishes. Examples of 4927commands that are valid in all-stop mode while the program is running 4928include `help' and `info break'. 4929 4930 You can interrupt your program while it is running in the background 4931by using the `interrupt' command. 4932 4933`interrupt' 4934`interrupt -a' 4935 Suspend execution of the running program. In all-stop mode, 4936 `interrupt' stops the whole process, but in non-stop mode, it stops 4937 only the current thread. To stop the whole program in non-stop 4938 mode, use `interrupt -a'. 4939 4940 4941File: gdb.info, Node: Thread-Specific Breakpoints, Next: Interrupted System Calls, Prev: Background Execution, Up: Thread Stops 4942 49435.5.4 Thread-Specific Breakpoints 4944--------------------------------- 4945 4946When your program has multiple threads (*note Debugging Programs with 4947Multiple Threads: Threads.), you can choose whether to set breakpoints 4948on all threads, or on a particular thread. 4949 4950`break LINESPEC thread THREADNO' 4951`break LINESPEC thread THREADNO if ...' 4952 LINESPEC specifies source lines; there are several ways of writing 4953 them (*note Specify Location::), but the effect is always to 4954 specify some source line. 4955 4956 Use the qualifier `thread THREADNO' with a breakpoint command to 4957 specify that you only want GDB to stop the program when a 4958 particular thread reaches this breakpoint. THREADNO is one of the 4959 numeric thread identifiers assigned by GDB, shown in the first 4960 column of the `info threads' display. 4961 4962 If you do not specify `thread THREADNO' when you set a breakpoint, 4963 the breakpoint applies to _all_ threads of your program. 4964 4965 You can use the `thread' qualifier on conditional breakpoints as 4966 well; in this case, place `thread THREADNO' before or after the 4967 breakpoint condition, like this: 4968 4969 (gdb) break frik.c:13 thread 28 if bartab > lim 4970 4971 4972 4973File: gdb.info, Node: Interrupted System Calls, Next: Observer Mode, Prev: Thread-Specific Breakpoints, Up: Thread Stops 4974 49755.5.5 Interrupted System Calls 4976------------------------------ 4977 4978There is an unfortunate side effect when using GDB to debug 4979multi-threaded programs. If one thread stops for a breakpoint, or for 4980some other reason, and another thread is blocked in a system call, then 4981the system call may return prematurely. This is a consequence of the 4982interaction between multiple threads and the signals that GDB uses to 4983implement breakpoints and other events that stop execution. 4984 4985 To handle this problem, your program should check the return value of 4986each system call and react appropriately. This is good programming 4987style anyways. 4988 4989 For example, do not write code like this: 4990 4991 sleep (10); 4992 4993 The call to `sleep' will return early if a different thread stops at 4994a breakpoint or for some other reason. 4995 4996 Instead, write this: 4997 4998 int unslept = 10; 4999 while (unslept > 0) 5000 unslept = sleep (unslept); 5001 5002 A system call is allowed to return early, so the system is still 5003conforming to its specification. But GDB does cause your 5004multi-threaded program to behave differently than it would without GDB. 5005 5006 Also, GDB uses internal breakpoints in the thread library to monitor 5007certain events such as thread creation and thread destruction. When 5008such an event happens, a system call in another thread may return 5009prematurely, even though your program does not appear to stop. 5010 5011 5012File: gdb.info, Node: Observer Mode, Prev: Interrupted System Calls, Up: Thread Stops 5013 50145.5.6 Observer Mode 5015------------------- 5016 5017If you want to build on non-stop mode and observe program behavior 5018without any chance of disruption by GDB, you can set variables to 5019disable all of the debugger's attempts to modify state, whether by 5020writing memory, inserting breakpoints, etc. These operate at a low 5021level, intercepting operations from all commands. 5022 5023 When all of these are set to `off', then GDB is said to be "observer 5024mode". As a convenience, the variable `observer' can be set to disable 5025these, plus enable non-stop mode. 5026 5027 Note that GDB will not prevent you from making nonsensical 5028combinations of these settings. For instance, if you have enabled 5029`may-insert-breakpoints' but disabled `may-write-memory', then 5030breakpoints that work by writing trap instructions into the code stream 5031will still not be able to be placed. 5032 5033`set observer on' 5034`set observer off' 5035 When set to `on', this disables all the permission variables below 5036 (except for `insert-fast-tracepoints'), plus enables non-stop 5037 debugging. Setting this to `off' switches back to normal 5038 debugging, though remaining in non-stop mode. 5039 5040`show observer' 5041 Show whether observer mode is on or off. 5042 5043`set may-write-registers on' 5044`set may-write-registers off' 5045 This controls whether GDB will attempt to alter the values of 5046 registers, such as with assignment expressions in `print', or the 5047 `jump' command. It defaults to `on'. 5048 5049`show may-write-registers' 5050 Show the current permission to write registers. 5051 5052`set may-write-memory on' 5053`set may-write-memory off' 5054 This controls whether GDB will attempt to alter the contents of 5055 memory, such as with assignment expressions in `print'. It 5056 defaults to `on'. 5057 5058`show may-write-memory' 5059 Show the current permission to write memory. 5060 5061`set may-insert-breakpoints on' 5062`set may-insert-breakpoints off' 5063 This controls whether GDB will attempt to insert breakpoints. 5064 This affects all breakpoints, including internal breakpoints 5065 defined by GDB. It defaults to `on'. 5066 5067`show may-insert-breakpoints' 5068 Show the current permission to insert breakpoints. 5069 5070`set may-insert-tracepoints on' 5071`set may-insert-tracepoints off' 5072 This controls whether GDB will attempt to insert (regular) 5073 tracepoints at the beginning of a tracing experiment. It affects 5074 only non-fast tracepoints, fast tracepoints being under the 5075 control of `may-insert-fast-tracepoints'. It defaults to `on'. 5076 5077`show may-insert-tracepoints' 5078 Show the current permission to insert tracepoints. 5079 5080`set may-insert-fast-tracepoints on' 5081`set may-insert-fast-tracepoints off' 5082 This controls whether GDB will attempt to insert fast tracepoints 5083 at the beginning of a tracing experiment. It affects only fast 5084 tracepoints, regular (non-fast) tracepoints being under the 5085 control of `may-insert-tracepoints'. It defaults to `on'. 5086 5087`show may-insert-fast-tracepoints' 5088 Show the current permission to insert fast tracepoints. 5089 5090`set may-interrupt on' 5091`set may-interrupt off' 5092 This controls whether GDB will attempt to interrupt or stop 5093 program execution. When this variable is `off', the `interrupt' 5094 command will have no effect, nor will `Ctrl-c'. It defaults to 5095 `on'. 5096 5097`show may-interrupt' 5098 Show the current permission to interrupt or stop the program. 5099 5100 5101 5102File: gdb.info, Node: Reverse Execution, Next: Process Record and Replay, Prev: Stopping, Up: Top 5103 51046 Running programs backward 5105*************************** 5106 5107When you are debugging a program, it is not unusual to realize that you 5108have gone too far, and some event of interest has already happened. If 5109the target environment supports it, GDB can allow you to "rewind" the 5110program by running it backward. 5111 5112 A target environment that supports reverse execution should be able 5113to "undo" the changes in machine state that have taken place as the 5114program was executing normally. Variables, registers etc. should 5115revert to their previous values. Obviously this requires a great deal 5116of sophistication on the part of the target environment; not all target 5117environments can support reverse execution. 5118 5119 When a program is executed in reverse, the instructions that have 5120most recently been executed are "un-executed", in reverse order. The 5121program counter runs backward, following the previous thread of 5122execution in reverse. As each instruction is "un-executed", the values 5123of memory and/or registers that were changed by that instruction are 5124reverted to their previous states. After executing a piece of source 5125code in reverse, all side effects of that code should be "undone", and 5126all variables should be returned to their prior values(1). 5127 5128 If you are debugging in a target environment that supports reverse 5129execution, GDB provides the following commands. 5130 5131`reverse-continue [IGNORE-COUNT]' 5132`rc [IGNORE-COUNT]' 5133 Beginning at the point where your program last stopped, start 5134 executing in reverse. Reverse execution will stop for breakpoints 5135 and synchronous exceptions (signals), just like normal execution. 5136 Behavior of asynchronous signals depends on the target environment. 5137 5138`reverse-step [COUNT]' 5139 Run the program backward until control reaches the start of a 5140 different source line; then stop it, and return control to GDB. 5141 5142 Like the `step' command, `reverse-step' will only stop at the 5143 beginning of a source line. It "un-executes" the previously 5144 executed source line. If the previous source line included calls 5145 to debuggable functions, `reverse-step' will step (backward) into 5146 the called function, stopping at the beginning of the _last_ 5147 statement in the called function (typically a return statement). 5148 5149 Also, as with the `step' command, if non-debuggable functions are 5150 called, `reverse-step' will run thru them backward without 5151 stopping. 5152 5153`reverse-stepi [COUNT]' 5154 Reverse-execute one machine instruction. Note that the instruction 5155 to be reverse-executed is _not_ the one pointed to by the program 5156 counter, but the instruction executed prior to that one. For 5157 instance, if the last instruction was a jump, `reverse-stepi' will 5158 take you back from the destination of the jump to the jump 5159 instruction itself. 5160 5161`reverse-next [COUNT]' 5162 Run backward to the beginning of the previous line executed in the 5163 current (innermost) stack frame. If the line contains function 5164 calls, they will be "un-executed" without stopping. Starting from 5165 the first line of a function, `reverse-next' will take you back to 5166 the caller of that function, _before_ the function was called, 5167 just as the normal `next' command would take you from the last 5168 line of a function back to its return to its caller (2). 5169 5170`reverse-nexti [COUNT]' 5171 Like `nexti', `reverse-nexti' executes a single instruction in 5172 reverse, except that called functions are "un-executed" atomically. 5173 That is, if the previously executed instruction was a return from 5174 another function, `reverse-nexti' will continue to execute in 5175 reverse until the call to that function (from the current stack 5176 frame) is reached. 5177 5178`reverse-finish' 5179 Just as the `finish' command takes you to the point where the 5180 current function returns, `reverse-finish' takes you to the point 5181 where it was called. Instead of ending up at the end of the 5182 current function invocation, you end up at the beginning. 5183 5184`set exec-direction' 5185 Set the direction of target execution. 5186 5187`set exec-direction reverse' 5188 GDB will perform all execution commands in reverse, until the 5189 exec-direction mode is changed to "forward". Affected commands 5190 include `step, stepi, next, nexti, continue, and finish'. The 5191 `return' command cannot be used in reverse mode. 5192 5193`set exec-direction forward' 5194 GDB will perform all execution commands in the normal fashion. 5195 This is the default. 5196 5197 ---------- Footnotes ---------- 5198 5199 (1) Note that some side effects are easier to undo than others. For 5200instance, memory and registers are relatively easy, but device I/O is 5201hard. Some targets may be able undo things like device I/O, and some 5202may not. 5203 5204 The contract between GDB and the reverse executing target requires 5205only that the target do something reasonable when GDB tells it to 5206execute backwards, and then report the results back to GDB. Whatever 5207the target reports back to GDB, GDB will report back to the user. GDB 5208assumes that the memory and registers that the target reports are in a 5209consistant state, but GDB accepts whatever it is given. 5210 5211 (2) Unless the code is too heavily optimized. 5212 5213 5214File: gdb.info, Node: Process Record and Replay, Next: Stack, Prev: Reverse Execution, Up: Top 5215 52167 Recording Inferior's Execution and Replaying It 5217************************************************* 5218 5219On some platforms, GDB provides a special "process record and replay" 5220target that can record a log of the process execution, and replay it 5221later with both forward and reverse execution commands. 5222 5223 When this target is in use, if the execution log includes the record 5224for the next instruction, GDB will debug in "replay mode". In the 5225replay mode, the inferior does not really execute code instructions. 5226Instead, all the events that normally happen during code execution are 5227taken from the execution log. While code is not really executed in 5228replay mode, the values of registers (including the program counter 5229register) and the memory of the inferior are still changed as they 5230normally would. Their contents are taken from the execution log. 5231 5232 If the record for the next instruction is not in the execution log, 5233GDB will debug in "record mode". In this mode, the inferior executes 5234normally, and GDB records the execution log for future replay. 5235 5236 The process record and replay target supports reverse execution 5237(*note Reverse Execution::), even if the platform on which the inferior 5238runs does not. However, the reverse execution is limited in this case 5239by the range of the instructions recorded in the execution log. In 5240other words, reverse execution on platforms that don't support it 5241directly can only be done in the replay mode. 5242 5243 When debugging in the reverse direction, GDB will work in replay 5244mode as long as the execution log includes the record for the previous 5245instruction; otherwise, it will work in record mode, if the platform 5246supports reverse execution, or stop if not. 5247 5248 For architecture environments that support process record and replay, 5249GDB provides the following commands: 5250 5251`record METHOD' 5252 This command starts the process record and replay target. The 5253 recording method can be specified as parameter. Without a 5254 parameter the command uses the `full' recording method. The 5255 following recording methods are available: 5256 5257 `full' 5258 Full record/replay recording using GDB's software record and 5259 replay implementation. This method allows replaying and 5260 reverse execution. 5261 5262 `btrace' 5263 Hardware-supported instruction recording. This method does 5264 not allow replaying and reverse execution. 5265 5266 This recording method may not be available on all processors. 5267 5268 The process record and replay target can only debug a process that 5269 is already running. Therefore, you need first to start the 5270 process with the `run' or `start' commands, and then start the 5271 recording with the `record METHOD' command. 5272 5273 Both `record METHOD' and `rec METHOD' are aliases of `target 5274 record-METHOD'. 5275 5276 Displaced stepping (*note displaced stepping: Maintenance 5277 Commands.) will be automatically disabled when process record and 5278 replay target is started. That's because the process record and 5279 replay target doesn't support displaced stepping. 5280 5281 If the inferior is in the non-stop mode (*note Non-Stop Mode::) or 5282 in the asynchronous execution mode (*note Background Execution::), 5283 not all recording methods are available. The `full' recording 5284 method does not support these two modes. 5285 5286`record stop' 5287 Stop the process record and replay target. When process record and 5288 replay target stops, the entire execution log will be deleted and 5289 the inferior will either be terminated, or will remain in its 5290 final state. 5291 5292 When you stop the process record and replay target in record mode 5293 (at the end of the execution log), the inferior will be stopped at 5294 the next instruction that would have been recorded. In other 5295 words, if you record for a while and then stop recording, the 5296 inferior process will be left in the same state as if the 5297 recording never happened. 5298 5299 On the other hand, if the process record and replay target is 5300 stopped while in replay mode (that is, not at the end of the 5301 execution log, but at some earlier point), the inferior process 5302 will become "live" at that earlier state, and it will then be 5303 possible to continue the usual "live" debugging of the process 5304 from that state. 5305 5306 When the inferior process exits, or GDB detaches from it, process 5307 record and replay target will automatically stop itself. 5308 5309`record save FILENAME' 5310 Save the execution log to a file `FILENAME'. Default filename is 5311 `gdb_record.PROCESS_ID', where PROCESS_ID is the process ID of the 5312 inferior. 5313 5314 This command may not be available for all recording methods. 5315 5316`record restore FILENAME' 5317 Restore the execution log from a file `FILENAME'. File must have 5318 been created with `record save'. 5319 5320`set record full insn-number-max LIMIT' 5321 Set the limit of instructions to be recorded for the `full' 5322 recording method. Default value is 200000. 5323 5324 If LIMIT is a positive number, then GDB will start deleting 5325 instructions from the log once the number of the record 5326 instructions becomes greater than LIMIT. For every new recorded 5327 instruction, GDB will delete the earliest recorded instruction to 5328 keep the number of recorded instructions at the limit. (Since 5329 deleting recorded instructions loses information, GDB lets you 5330 control what happens when the limit is reached, by means of the 5331 `stop-at-limit' option, described below.) 5332 5333 If LIMIT is zero, GDB will never delete recorded instructions from 5334 the execution log. The number of recorded instructions is 5335 unlimited in this case. 5336 5337`show record full insn-number-max' 5338 Show the limit of instructions to be recorded with the `full' 5339 recording method. 5340 5341`set record full stop-at-limit' 5342 Control the behavior of the `full' recording method when the 5343 number of recorded instructions reaches the limit. If ON (the 5344 default), GDB will stop when the limit is reached for the first 5345 time and ask you whether you want to stop the inferior or continue 5346 running it and recording the execution log. If you decide to 5347 continue recording, each new recorded instruction will cause the 5348 oldest one to be deleted. 5349 5350 If this option is OFF, GDB will automatically delete the oldest 5351 record to make room for each new one, without asking. 5352 5353`show record full stop-at-limit' 5354 Show the current setting of `stop-at-limit'. 5355 5356`set record full memory-query' 5357 Control the behavior when GDB is unable to record memory changes 5358 caused by an instruction for the `full' recording method. If ON, 5359 GDB will query whether to stop the inferior in that case. 5360 5361 If this option is OFF (the default), GDB will automatically ignore 5362 the effect of such instructions on memory. Later, when GDB 5363 replays this execution log, it will mark the log of this 5364 instruction as not accessible, and it will not affect the replay 5365 results. 5366 5367`show record full memory-query' 5368 Show the current setting of `memory-query'. 5369 5370`info record' 5371 Show various statistics about the recording depending on the 5372 recording method: 5373 5374 `full' 5375 For the `full' recording method, it shows the state of process 5376 record and its in-memory execution log buffer, including: 5377 5378 * Whether in record mode or replay mode. 5379 5380 * Lowest recorded instruction number (counting from when 5381 the current execution log started recording 5382 instructions). 5383 5384 * Highest recorded instruction number. 5385 5386 * Current instruction about to be replayed (if in replay 5387 mode). 5388 5389 * Number of instructions contained in the execution log. 5390 5391 * Maximum number of instructions that may be contained in 5392 the execution log. 5393 5394 `btrace' 5395 For the `btrace' recording method, it shows the number of 5396 instructions that have been recorded and the number of blocks 5397 of sequential control-flow that is formed by the recorded 5398 instructions. 5399 5400`record delete' 5401 When record target runs in replay mode ("in the past"), delete the 5402 subsequent execution log and begin to record a new execution log 5403 starting from the current address. This means you will abandon 5404 the previously recorded "future" and begin recording a new 5405 "future". 5406 5407`record instruction-history' 5408 Disassembles instructions from the recorded execution log. By 5409 default, ten instructions are disassembled. This can be changed 5410 using the `set record instruction-history-size' command. 5411 Instructions are printed in execution order. There are several 5412 ways to specify what part of the execution log to disassemble: 5413 5414 `record instruction-history INSN' 5415 Disassembles ten instructions starting from instruction number 5416 INSN. 5417 5418 `record instruction-history INSN, +/-N' 5419 Disassembles N instructions around instruction number INSN. 5420 If N is preceded with `+', disassembles N instructions after 5421 instruction number INSN. If N is preceded with `-', 5422 disassembles N instructions before instruction number INSN. 5423 5424 `record instruction-history' 5425 Disassembles ten more instructions after the last disassembly. 5426 5427 `record instruction-history -' 5428 Disassembles ten more instructions before the last 5429 disassembly. 5430 5431 `record instruction-history BEGIN END' 5432 Disassembles instructions beginning with instruction number 5433 BEGIN until instruction number END. The instruction number 5434 END is not included. 5435 5436 This command may not be available for all recording methods. 5437 5438`set record instruction-history-size' 5439 Define how many instructions to disassemble in the `record 5440 instruction-history' command. The default value is 10. 5441 5442`show record instruction-history-size' 5443 Show how many instructions to disassemble in the `record 5444 instruction-history' command. 5445 5446`record function-call-history' 5447 Prints the execution history at function granularity. It prints one 5448 line for each sequence of instructions that belong to the same 5449 function giving the name of that function, the source lines for 5450 this instruction sequence (if the `/l' modifier is specified), and 5451 the instructions numbers that form the sequence (if the `/i' 5452 modifier is specified). 5453 5454 (gdb) list 1, 10 5455 1 void foo (void) 5456 2 { 5457 3 } 5458 4 5459 5 void bar (void) 5460 6 { 5461 7 ... 5462 8 foo (); 5463 9 ... 5464 10 } 5465 (gdb) record function-call-history /l 5466 1 foo.c:6-8 bar 5467 2 foo.c:2-3 foo 5468 3 foo.c:9-10 bar 5469 5470 By default, ten lines are printed. This can be changed using the 5471 `set record function-call-history-size' command. Functions are 5472 printed in execution order. There are several ways to specify what 5473 to print: 5474 5475 `record function-call-history FUNC' 5476 Prints ten functions starting from function number FUNC. 5477 5478 `record function-call-history FUNC, +/-N' 5479 Prints N functions around function number FUNC. If N is 5480 preceded with `+', prints N functions after function number 5481 FUNC. If N is preceded with `-', prints N functions before 5482 function number FUNC. 5483 5484 `record function-call-history' 5485 Prints ten more functions after the last ten-line print. 5486 5487 `record function-call-history -' 5488 Prints ten more functions before the last ten-line print. 5489 5490 `record function-call-history BEGIN END' 5491 Prints functions beginning with function number BEGIN until 5492 function number END. The function number END is not included. 5493 5494 This command may not be available for all recording methods. 5495 5496`set record function-call-history-size' 5497 Define how many lines to print in the `record 5498 function-call-history' command. The default value is 10. 5499 5500`show record function-call-history-size' 5501 Show how many lines to print in the `record function-call-history' 5502 command. 5503 5504 5505File: gdb.info, Node: Stack, Next: Source, Prev: Process Record and Replay, Up: Top 5506 55078 Examining the Stack 5508********************* 5509 5510When your program has stopped, the first thing you need to know is 5511where it stopped and how it got there. 5512 5513 Each time your program performs a function call, information about 5514the call is generated. That information includes the location of the 5515call in your program, the arguments of the call, and the local 5516variables of the function being called. The information is saved in a 5517block of data called a "stack frame". The stack frames are allocated 5518in a region of memory called the "call stack". 5519 5520 When your program stops, the GDB commands for examining the stack 5521allow you to see all of this information. 5522 5523 One of the stack frames is "selected" by GDB and many GDB commands 5524refer implicitly to the selected frame. In particular, whenever you 5525ask GDB for the value of a variable in your program, the value is found 5526in the selected frame. There are special GDB commands to select 5527whichever frame you are interested in. *Note Selecting a Frame: 5528Selection. 5529 5530 When your program stops, GDB automatically selects the currently 5531executing frame and describes it briefly, similar to the `frame' 5532command (*note Information about a Frame: Frame Info.). 5533 5534* Menu: 5535 5536* Frames:: Stack frames 5537* Backtrace:: Backtraces 5538* Selection:: Selecting a frame 5539* Frame Info:: Information on a frame 5540 5541 5542File: gdb.info, Node: Frames, Next: Backtrace, Up: Stack 5543 55448.1 Stack Frames 5545================ 5546 5547The call stack is divided up into contiguous pieces called "stack 5548frames", or "frames" for short; each frame is the data associated with 5549one call to one function. The frame contains the arguments given to 5550the function, the function's local variables, and the address at which 5551the function is executing. 5552 5553 When your program is started, the stack has only one frame, that of 5554the function `main'. This is called the "initial" frame or the 5555"outermost" frame. Each time a function is called, a new frame is 5556made. Each time a function returns, the frame for that function 5557invocation is eliminated. If a function is recursive, there can be 5558many frames for the same function. The frame for the function in which 5559execution is actually occurring is called the "innermost" frame. This 5560is the most recently created of all the stack frames that still exist. 5561 5562 Inside your program, stack frames are identified by their addresses. 5563A stack frame consists of many bytes, each of which has its own 5564address; each kind of computer has a convention for choosing one byte 5565whose address serves as the address of the frame. Usually this address 5566is kept in a register called the "frame pointer register" (*note $fp: 5567Registers.) while execution is going on in that frame. 5568 5569 GDB assigns numbers to all existing stack frames, starting with zero 5570for the innermost frame, one for the frame that called it, and so on 5571upward. These numbers do not really exist in your program; they are 5572assigned by GDB to give you a way of designating stack frames in GDB 5573commands. 5574 5575 Some compilers provide a way to compile functions so that they 5576operate without stack frames. (For example, the GCC option 5577 `-fomit-frame-pointer' 5578 generates functions without a frame.) This is occasionally done 5579with heavily used library functions to save the frame setup time. GDB 5580has limited facilities for dealing with these function invocations. If 5581the innermost function invocation has no stack frame, GDB nevertheless 5582regards it as though it had a separate frame, which is numbered zero as 5583usual, allowing correct tracing of the function call chain. However, 5584GDB has no provision for frameless functions elsewhere in the stack. 5585 5586`frame ARGS' 5587 The `frame' command allows you to move from one stack frame to 5588 another, and to print the stack frame you select. ARGS may be 5589 either the address of the frame or the stack frame number. 5590 Without an argument, `frame' prints the current stack frame. 5591 5592`select-frame' 5593 The `select-frame' command allows you to move from one stack frame 5594 to another without printing the frame. This is the silent version 5595 of `frame'. 5596 5597 5598File: gdb.info, Node: Backtrace, Next: Selection, Prev: Frames, Up: Stack 5599 56008.2 Backtraces 5601============== 5602 5603A backtrace is a summary of how your program got where it is. It shows 5604one line per frame, for many frames, starting with the currently 5605executing frame (frame zero), followed by its caller (frame one), and 5606on up the stack. 5607 5608`backtrace' 5609`bt' 5610 Print a backtrace of the entire stack: one line per frame for all 5611 frames in the stack. 5612 5613 You can stop the backtrace at any time by typing the system 5614 interrupt character, normally `Ctrl-c'. 5615 5616`backtrace N' 5617`bt N' 5618 Similar, but print only the innermost N frames. 5619 5620`backtrace -N' 5621`bt -N' 5622 Similar, but print only the outermost N frames. 5623 5624`backtrace full' 5625`bt full' 5626`bt full N' 5627`bt full -N' 5628 Print the values of the local variables also. N specifies the 5629 number of frames to print, as described above. 5630 5631 The names `where' and `info stack' (abbreviated `info s') are 5632additional aliases for `backtrace'. 5633 5634 In a multi-threaded program, GDB by default shows the backtrace only 5635for the current thread. To display the backtrace for several or all of 5636the threads, use the command `thread apply' (*note thread apply: 5637Threads.). For example, if you type `thread apply all backtrace', GDB 5638will display the backtrace for all the threads; this is handy when you 5639debug a core dump of a multi-threaded program. 5640 5641 Each line in the backtrace shows the frame number and the function 5642name. The program counter value is also shown--unless you use `set 5643print address off'. The backtrace also shows the source file name and 5644line number, as well as the arguments to the function. The program 5645counter value is omitted if it is at the beginning of the code for that 5646line number. 5647 5648 Here is an example of a backtrace. It was made with the command `bt 56493', so it shows the innermost three frames. 5650 5651 #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) 5652 at builtin.c:993 5653 #1 0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242 5654 #2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08) 5655 at macro.c:71 5656 (More stack frames follow...) 5657 5658The display for frame zero does not begin with a program counter value, 5659indicating that your program has stopped at the beginning of the code 5660for line `993' of `builtin.c'. 5661 5662The value of parameter `data' in frame 1 has been replaced by `...'. 5663By default, GDB prints the value of a parameter only if it is a scalar 5664(integer, pointer, enumeration, etc). See command `set print 5665frame-arguments' in *note Print Settings:: for more details on how to 5666configure the way function parameter values are printed. 5667 5668 If your program was compiled with optimizations, some compilers will 5669optimize away arguments passed to functions if those arguments are 5670never used after the call. Such optimizations generate code that 5671passes arguments through registers, but doesn't store those arguments 5672in the stack frame. GDB has no way of displaying such arguments in 5673stack frames other than the innermost one. Here's what such a 5674backtrace might look like: 5675 5676 #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) 5677 at builtin.c:993 5678 #1 0x6e38 in expand_macro (sym=<optimized out>) at macro.c:242 5679 #2 0x6840 in expand_token (obs=0x0, t=<optimized out>, td=0xf7fffb08) 5680 at macro.c:71 5681 (More stack frames follow...) 5682 5683The values of arguments that were not saved in their stack frames are 5684shown as `<optimized out>'. 5685 5686 If you need to display the values of such optimized-out arguments, 5687either deduce that from other variables whose values depend on the one 5688you are interested in, or recompile without optimizations. 5689 5690 Most programs have a standard user entry point--a place where system 5691libraries and startup code transition into user code. For C this is 5692`main'(1). When GDB finds the entry function in a backtrace it will 5693terminate the backtrace, to avoid tracing into highly system-specific 5694(and generally uninteresting) code. 5695 5696 If you need to examine the startup code, or limit the number of 5697levels in a backtrace, you can change this behavior: 5698 5699`set backtrace past-main' 5700`set backtrace past-main on' 5701 Backtraces will continue past the user entry point. 5702 5703`set backtrace past-main off' 5704 Backtraces will stop when they encounter the user entry point. 5705 This is the default. 5706 5707`show backtrace past-main' 5708 Display the current user entry point backtrace policy. 5709 5710`set backtrace past-entry' 5711`set backtrace past-entry on' 5712 Backtraces will continue past the internal entry point of an 5713 application. This entry point is encoded by the linker when the 5714 application is built, and is likely before the user entry point 5715 `main' (or equivalent) is called. 5716 5717`set backtrace past-entry off' 5718 Backtraces will stop when they encounter the internal entry point 5719 of an application. This is the default. 5720 5721`show backtrace past-entry' 5722 Display the current internal entry point backtrace policy. 5723 5724`set backtrace limit N' 5725`set backtrace limit 0' 5726 Limit the backtrace to N levels. A value of zero means unlimited. 5727 5728`show backtrace limit' 5729 Display the current limit on backtrace levels. 5730 5731 You can control how file names are displayed. 5732 5733`set filename-display' 5734`set filename-display relative' 5735 Display file names relative to the compilation directory. This is 5736 the default. 5737 5738`set filename-display basename' 5739 Display only basename of a filename. 5740 5741`set filename-display absolute' 5742 Display an absolute filename. 5743 5744`show filename-display' 5745 Show the current way to display filenames. 5746 5747 ---------- Footnotes ---------- 5748 5749 (1) Note that embedded programs (the so-called "free-standing" 5750environment) are not required to have a `main' function as the entry 5751point. They could even have multiple entry points. 5752 5753 5754File: gdb.info, Node: Selection, Next: Frame Info, Prev: Backtrace, Up: Stack 5755 57568.3 Selecting a Frame 5757===================== 5758 5759Most commands for examining the stack and other data in your program 5760work on whichever stack frame is selected at the moment. Here are the 5761commands for selecting a stack frame; all of them finish by printing a 5762brief description of the stack frame just selected. 5763 5764`frame N' 5765`f N' 5766 Select frame number N. Recall that frame zero is the innermost 5767 (currently executing) frame, frame one is the frame that called the 5768 innermost one, and so on. The highest-numbered frame is the one 5769 for `main'. 5770 5771`frame ADDR' 5772`f ADDR' 5773 Select the frame at address ADDR. This is useful mainly if the 5774 chaining of stack frames has been damaged by a bug, making it 5775 impossible for GDB to assign numbers properly to all frames. In 5776 addition, this can be useful when your program has multiple stacks 5777 and switches between them. 5778 5779 On the SPARC architecture, `frame' needs two addresses to select 5780 an arbitrary frame: a frame pointer and a stack pointer. 5781 5782 On the MIPS and Alpha architecture, it needs two addresses: a stack 5783 pointer and a program counter. 5784 5785 On the 29k architecture, it needs three addresses: a register stack 5786 pointer, a program counter, and a memory stack pointer. 5787 5788`up N' 5789 Move N frames up the stack. For positive numbers N, this advances 5790 toward the outermost frame, to higher frame numbers, to frames 5791 that have existed longer. N defaults to one. 5792 5793`down N' 5794 Move N frames down the stack. For positive numbers N, this 5795 advances toward the innermost frame, to lower frame numbers, to 5796 frames that were created more recently. N defaults to one. You 5797 may abbreviate `down' as `do'. 5798 5799 All of these commands end by printing two lines of output describing 5800the frame. The first line shows the frame number, the function name, 5801the arguments, and the source file and line number of execution in that 5802frame. The second line shows the text of that source line. 5803 5804 For example: 5805 5806 (gdb) up 5807 #1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) 5808 at env.c:10 5809 10 read_input_file (argv[i]); 5810 5811 After such a printout, the `list' command with no arguments prints 5812ten lines centered on the point of execution in the frame. You can 5813also edit the program at the point of execution with your favorite 5814editing program by typing `edit'. *Note Printing Source Lines: List, 5815for details. 5816 5817`up-silently N' 5818`down-silently N' 5819 These two commands are variants of `up' and `down', respectively; 5820 they differ in that they do their work silently, without causing 5821 display of the new frame. They are intended primarily for use in 5822 GDB command scripts, where the output might be unnecessary and 5823 distracting. 5824 5825 5826File: gdb.info, Node: Frame Info, Prev: Selection, Up: Stack 5827 58288.4 Information About a Frame 5829============================= 5830 5831There are several other commands to print information about the selected 5832stack frame. 5833 5834`frame' 5835`f' 5836 When used without any argument, this command does not change which 5837 frame is selected, but prints a brief description of the currently 5838 selected stack frame. It can be abbreviated `f'. With an 5839 argument, this command is used to select a stack frame. *Note 5840 Selecting a Frame: Selection. 5841 5842`info frame' 5843`info f' 5844 This command prints a verbose description of the selected stack 5845 frame, including: 5846 5847 * the address of the frame 5848 5849 * the address of the next frame down (called by this frame) 5850 5851 * the address of the next frame up (caller of this frame) 5852 5853 * the language in which the source code corresponding to this 5854 frame is written 5855 5856 * the address of the frame's arguments 5857 5858 * the address of the frame's local variables 5859 5860 * the program counter saved in it (the address of execution in 5861 the caller frame) 5862 5863 * which registers were saved in the frame 5864 5865 The verbose description is useful when something has gone wrong 5866 that has made the stack format fail to fit the usual conventions. 5867 5868`info frame ADDR' 5869`info f ADDR' 5870 Print a verbose description of the frame at address ADDR, without 5871 selecting that frame. The selected frame remains unchanged by this 5872 command. This requires the same kind of address (more than one 5873 for some architectures) that you specify in the `frame' command. 5874 *Note Selecting a Frame: Selection. 5875 5876`info args' 5877 Print the arguments of the selected frame, each on a separate line. 5878 5879`info locals' 5880 Print the local variables of the selected frame, each on a separate 5881 line. These are all variables (declared either static or 5882 automatic) accessible at the point of execution of the selected 5883 frame. 5884 5885 5886 5887File: gdb.info, Node: Source, Next: Data, Prev: Stack, Up: Top 5888 58899 Examining Source Files 5890************************ 5891 5892GDB can print parts of your program's source, since the debugging 5893information recorded in the program tells GDB what source files were 5894used to build it. When your program stops, GDB spontaneously prints 5895the line where it stopped. Likewise, when you select a stack frame 5896(*note Selecting a Frame: Selection.), GDB prints the line where 5897execution in that frame has stopped. You can print other portions of 5898source files by explicit command. 5899 5900 If you use GDB through its GNU Emacs interface, you may prefer to 5901use Emacs facilities to view source; see *note Using GDB under GNU 5902Emacs: Emacs. 5903 5904* Menu: 5905 5906* List:: Printing source lines 5907* Specify Location:: How to specify code locations 5908* Edit:: Editing source files 5909* Search:: Searching source files 5910* Source Path:: Specifying source directories 5911* Machine Code:: Source and machine code 5912 5913 5914File: gdb.info, Node: List, Next: Specify Location, Up: Source 5915 59169.1 Printing Source Lines 5917========================= 5918 5919To print lines from a source file, use the `list' command (abbreviated 5920`l'). By default, ten lines are printed. There are several ways to 5921specify what part of the file you want to print; see *note Specify 5922Location::, for the full list. 5923 5924 Here are the forms of the `list' command most commonly used: 5925 5926`list LINENUM' 5927 Print lines centered around line number LINENUM in the current 5928 source file. 5929 5930`list FUNCTION' 5931 Print lines centered around the beginning of function FUNCTION. 5932 5933`list' 5934 Print more lines. If the last lines printed were printed with a 5935 `list' command, this prints lines following the last lines 5936 printed; however, if the last line printed was a solitary line 5937 printed as part of displaying a stack frame (*note Examining the 5938 Stack: Stack.), this prints lines centered around that line. 5939 5940`list -' 5941 Print lines just before the lines last printed. 5942 5943 By default, GDB prints ten source lines with any of these forms of 5944the `list' command. You can change this using `set listsize': 5945 5946`set listsize COUNT' 5947 Make the `list' command display COUNT source lines (unless the 5948 `list' argument explicitly specifies some other number). Setting 5949 COUNT to 0 means there's no limit. 5950 5951`show listsize' 5952 Display the number of lines that `list' prints. 5953 5954 Repeating a `list' command with <RET> discards the argument, so it 5955is equivalent to typing just `list'. This is more useful than listing 5956the same lines again. An exception is made for an argument of `-'; 5957that argument is preserved in repetition so that each repetition moves 5958up in the source file. 5959 5960 In general, the `list' command expects you to supply zero, one or two 5961"linespecs". Linespecs specify source lines; there are several ways of 5962writing them (*note Specify Location::), but the effect is always to 5963specify some source line. 5964 5965 Here is a complete description of the possible arguments for `list': 5966 5967`list LINESPEC' 5968 Print lines centered around the line specified by LINESPEC. 5969 5970`list FIRST,LAST' 5971 Print lines from FIRST to LAST. Both arguments are linespecs. 5972 When a `list' command has two linespecs, and the source file of 5973 the second linespec is omitted, this refers to the same source 5974 file as the first linespec. 5975 5976`list ,LAST' 5977 Print lines ending with LAST. 5978 5979`list FIRST,' 5980 Print lines starting with FIRST. 5981 5982`list +' 5983 Print lines just after the lines last printed. 5984 5985`list -' 5986 Print lines just before the lines last printed. 5987 5988`list' 5989 As described in the preceding table. 5990 5991 5992File: gdb.info, Node: Specify Location, Next: Edit, Prev: List, Up: Source 5993 59949.2 Specifying a Location 5995========================= 5996 5997Several GDB commands accept arguments that specify a location of your 5998program's code. Since GDB is a source-level debugger, a location 5999usually specifies some line in the source code; for that reason, 6000locations are also known as "linespecs". 6001 6002 Here are all the different ways of specifying a code location that 6003GDB understands: 6004 6005`LINENUM' 6006 Specifies the line number LINENUM of the current source file. 6007 6008`-OFFSET' 6009`+OFFSET' 6010 Specifies the line OFFSET lines before or after the "current 6011 line". For the `list' command, the current line is the last one 6012 printed; for the breakpoint commands, this is the line at which 6013 execution stopped in the currently selected "stack frame" (*note 6014 Frames: Frames, for a description of stack frames.) When used as 6015 the second of the two linespecs in a `list' command, this 6016 specifies the line OFFSET lines up or down from the first linespec. 6017 6018`FILENAME:LINENUM' 6019 Specifies the line LINENUM in the source file FILENAME. If 6020 FILENAME is a relative file name, then it will match any source 6021 file name with the same trailing components. For example, if 6022 FILENAME is `gcc/expr.c', then it will match source file name of 6023 `/build/trunk/gcc/expr.c', but not `/build/trunk/libcpp/expr.c' or 6024 `/build/trunk/gcc/x-expr.c'. 6025 6026`FUNCTION' 6027 Specifies the line that begins the body of the function FUNCTION. 6028 For example, in C, this is the line with the open brace. 6029 6030`FUNCTION:LABEL' 6031 Specifies the line where LABEL appears in FUNCTION. 6032 6033`FILENAME:FUNCTION' 6034 Specifies the line that begins the body of the function FUNCTION 6035 in the file FILENAME. You only need the file name with a function 6036 name to avoid ambiguity when there are identically named functions 6037 in different source files. 6038 6039`LABEL' 6040 Specifies the line at which the label named LABEL appears. GDB 6041 searches for the label in the function corresponding to the 6042 currently selected stack frame. If there is no current selected 6043 stack frame (for instance, if the inferior is not running), then 6044 GDB will not search for a label. 6045 6046`*ADDRESS' 6047 Specifies the program address ADDRESS. For line-oriented 6048 commands, such as `list' and `edit', this specifies a source line 6049 that contains ADDRESS. For `break' and other breakpoint oriented 6050 commands, this can be used to set breakpoints in parts of your 6051 program which do not have debugging information or source files. 6052 6053 Here ADDRESS may be any expression valid in the current working 6054 language (*note working language: Languages.) that specifies a code 6055 address. In addition, as a convenience, GDB extends the semantics 6056 of expressions used in locations to cover the situations that 6057 frequently happen during debugging. Here are the various forms of 6058 ADDRESS: 6059 6060 `EXPRESSION' 6061 Any expression valid in the current working language. 6062 6063 `FUNCADDR' 6064 An address of a function or procedure derived from its name. 6065 In C, C++, Java, Objective-C, Fortran, minimal, and assembly, 6066 this is simply the function's name FUNCTION (and actually a 6067 special case of a valid expression). In Pascal and Modula-2, 6068 this is `&FUNCTION'. In Ada, this is `FUNCTION'Address' 6069 (although the Pascal form also works). 6070 6071 This form specifies the address of the function's first 6072 instruction, before the stack frame and arguments have been 6073 set up. 6074 6075 `'FILENAME'::FUNCADDR' 6076 Like FUNCADDR above, but also specifies the name of the source 6077 file explicitly. This is useful if the name of the function 6078 does not specify the function unambiguously, e.g., if there 6079 are several functions with identical names in different 6080 source files. 6081 6082`-pstap|-probe-stap [OBJFILE:[PROVIDER:]]NAME' 6083 The GNU/Linux tool `SystemTap' provides a way for applications to 6084 embed static probes. *Note Static Probe Points::, for more 6085 information on finding and using static probes. This form of 6086 linespec specifies the location of such a static probe. 6087 6088 If OBJFILE is given, only probes coming from that shared library 6089 or executable matching OBJFILE as a regular expression are 6090 considered. If PROVIDER is given, then only probes from that 6091 provider are considered. If several probes match the spec, GDB 6092 will insert a breakpoint at each one of those probes. 6093 6094 6095 6096File: gdb.info, Node: Edit, Next: Search, Prev: Specify Location, Up: Source 6097 60989.3 Editing Source Files 6099======================== 6100 6101To edit the lines in a source file, use the `edit' command. The 6102editing program of your choice is invoked with the current line set to 6103the active line in the program. Alternatively, there are several ways 6104to specify what part of the file you want to print if you want to see 6105other parts of the program: 6106 6107`edit LOCATION' 6108 Edit the source file specified by `location'. Editing starts at 6109 that LOCATION, e.g., at the specified source line of the specified 6110 file. *Note Specify Location::, for all the possible forms of the 6111 LOCATION argument; here are the forms of the `edit' command most 6112 commonly used: 6113 6114 `edit NUMBER' 6115 Edit the current source file with NUMBER as the active line 6116 number. 6117 6118 `edit FUNCTION' 6119 Edit the file containing FUNCTION at the beginning of its 6120 definition. 6121 6122 61239.3.1 Choosing your Editor 6124-------------------------- 6125 6126You can customize GDB to use any editor you want (1). By default, it 6127is `/bin/ex', but you can change this by setting the environment 6128variable `EDITOR' before using GDB. For example, to configure GDB to 6129use the `vi' editor, you could use these commands with the `sh' shell: 6130 EDITOR=/usr/bin/vi 6131 export EDITOR 6132 gdb ... 6133 or in the `csh' shell, 6134 setenv EDITOR /usr/bin/vi 6135 gdb ... 6136 6137 ---------- Footnotes ---------- 6138 6139 (1) The only restriction is that your editor (say `ex'), recognizes 6140the following command-line syntax: 6141 ex +NUMBER file 6142 The optional numeric value +NUMBER specifies the number of the line 6143in the file where to start editing. 6144 6145 6146File: gdb.info, Node: Search, Next: Source Path, Prev: Edit, Up: Source 6147 61489.4 Searching Source Files 6149========================== 6150 6151There are two commands for searching through the current source file 6152for a regular expression. 6153 6154`forward-search REGEXP' 6155`search REGEXP' 6156 The command `forward-search REGEXP' checks each line, starting 6157 with the one following the last line listed, for a match for 6158 REGEXP. It lists the line that is found. You can use the synonym 6159 `search REGEXP' or abbreviate the command name as `fo'. 6160 6161`reverse-search REGEXP' 6162 The command `reverse-search REGEXP' checks each line, starting 6163 with the one before the last line listed and going backward, for a 6164 match for REGEXP. It lists the line that is found. You can 6165 abbreviate this command as `rev'. 6166 6167 6168File: gdb.info, Node: Source Path, Next: Machine Code, Prev: Search, Up: Source 6169 61709.5 Specifying Source Directories 6171================================= 6172 6173Executable programs sometimes do not record the directories of the 6174source files from which they were compiled, just the names. Even when 6175they do, the directories could be moved between the compilation and 6176your debugging session. GDB has a list of directories to search for 6177source files; this is called the "source path". Each time GDB wants a 6178source file, it tries all the directories in the list, in the order 6179they are present in the list, until it finds a file with the desired 6180name. 6181 6182 For example, suppose an executable references the file 6183`/usr/src/foo-1.0/lib/foo.c', and our source path is `/mnt/cross'. The 6184file is first looked up literally; if this fails, 6185`/mnt/cross/usr/src/foo-1.0/lib/foo.c' is tried; if this fails, 6186`/mnt/cross/foo.c' is opened; if this fails, an error message is 6187printed. GDB does not look up the parts of the source file name, such 6188as `/mnt/cross/src/foo-1.0/lib/foo.c'. Likewise, the subdirectories of 6189the source path are not searched: if the source path is `/mnt/cross', 6190and the binary refers to `foo.c', GDB would not find it under 6191`/mnt/cross/usr/src/foo-1.0/lib'. 6192 6193 Plain file names, relative file names with leading directories, file 6194names containing dots, etc. are all treated as described above; for 6195instance, if the source path is `/mnt/cross', and the source file is 6196recorded as `../lib/foo.c', GDB would first try `../lib/foo.c', then 6197`/mnt/cross/../lib/foo.c', and after that--`/mnt/cross/foo.c'. 6198 6199 Note that the executable search path is _not_ used to locate the 6200source files. 6201 6202 Whenever you reset or rearrange the source path, GDB clears out any 6203information it has cached about where source files are found and where 6204each line is in the file. 6205 6206 When you start GDB, its source path includes only `cdir' and `cwd', 6207in that order. To add other directories, use the `directory' command. 6208 6209 The search path is used to find both program source files and GDB 6210script files (read using the `-command' option and `source' command). 6211 6212 In addition to the source path, GDB provides a set of commands that 6213manage a list of source path substitution rules. A "substitution rule" 6214specifies how to rewrite source directories stored in the program's 6215debug information in case the sources were moved to a different 6216directory between compilation and debugging. A rule is made of two 6217strings, the first specifying what needs to be rewritten in the path, 6218and the second specifying how it should be rewritten. In *note set 6219substitute-path::, we name these two parts FROM and TO respectively. 6220GDB does a simple string replacement of FROM with TO at the start of 6221the directory part of the source file name, and uses that result 6222instead of the original file name to look up the sources. 6223 6224 Using the previous example, suppose the `foo-1.0' tree has been 6225moved from `/usr/src' to `/mnt/cross', then you can tell GDB to replace 6226`/usr/src' in all source path names with `/mnt/cross'. The first 6227lookup will then be `/mnt/cross/foo-1.0/lib/foo.c' in place of the 6228original location of `/usr/src/foo-1.0/lib/foo.c'. To define a source 6229path substitution rule, use the `set substitute-path' command (*note 6230set substitute-path::). 6231 6232 To avoid unexpected substitution results, a rule is applied only if 6233the FROM part of the directory name ends at a directory separator. For 6234instance, a rule substituting `/usr/source' into `/mnt/cross' will be 6235applied to `/usr/source/foo-1.0' but not to `/usr/sourceware/foo-2.0'. 6236And because the substitution is applied only at the beginning of the 6237directory name, this rule will not be applied to 6238`/root/usr/source/baz.c' either. 6239 6240 In many cases, you can achieve the same result using the `directory' 6241command. However, `set substitute-path' can be more efficient in the 6242case where the sources are organized in a complex tree with multiple 6243subdirectories. With the `directory' command, you need to add each 6244subdirectory of your project. If you moved the entire tree while 6245preserving its internal organization, then `set substitute-path' allows 6246you to direct the debugger to all the sources with one single command. 6247 6248 `set substitute-path' is also more than just a shortcut command. 6249The source path is only used if the file at the original location no 6250longer exists. On the other hand, `set substitute-path' modifies the 6251debugger behavior to look at the rewritten location instead. So, if 6252for any reason a source file that is not relevant to your executable is 6253located at the original location, a substitution rule is the only 6254method available to point GDB at the new location. 6255 6256 You can configure a default source path substitution rule by 6257configuring GDB with the `--with-relocated-sources=DIR' option. The DIR 6258should be the name of a directory under GDB's configured prefix (set 6259with `--prefix' or `--exec-prefix'), and directory names in debug 6260information under DIR will be adjusted automatically if the installed 6261GDB is moved to a new location. This is useful if GDB, libraries or 6262executables with debug information and corresponding source code are 6263being moved together. 6264 6265`directory DIRNAME ...' 6266 6267`dir DIRNAME ...' 6268 Add directory DIRNAME to the front of the source path. Several 6269 directory names may be given to this command, separated by `:' 6270 (`;' on MS-DOS and MS-Windows, where `:' usually appears as part 6271 of absolute file names) or whitespace. You may specify a 6272 directory that is already in the source path; this moves it 6273 forward, so GDB searches it sooner. 6274 6275 You can use the string `$cdir' to refer to the compilation 6276 directory (if one is recorded), and `$cwd' to refer to the current 6277 working directory. `$cwd' is not the same as `.'--the former 6278 tracks the current working directory as it changes during your GDB 6279 session, while the latter is immediately expanded to the current 6280 directory at the time you add an entry to the source path. 6281 6282`directory' 6283 Reset the source path to its default value (`$cdir:$cwd' on Unix 6284 systems). This requires confirmation. 6285 6286`set directories PATH-LIST' 6287 Set the source path to PATH-LIST. `$cdir:$cwd' are added if 6288 missing. 6289 6290`show directories' 6291 Print the source path: show which directories it contains. 6292 6293`set substitute-path FROM TO' 6294 Define a source path substitution rule, and add it at the end of 6295 the current list of existing substitution rules. If a rule with 6296 the same FROM was already defined, then the old rule is also 6297 deleted. 6298 6299 For example, if the file `/foo/bar/baz.c' was moved to 6300 `/mnt/cross/baz.c', then the command 6301 6302 (gdb) set substitute-path /usr/src /mnt/cross 6303 6304 will tell GDB to replace `/usr/src' with `/mnt/cross', which will 6305 allow GDB to find the file `baz.c' even though it was moved. 6306 6307 In the case when more than one substitution rule have been defined, 6308 the rules are evaluated one by one in the order where they have 6309 been defined. The first one matching, if any, is selected to 6310 perform the substitution. 6311 6312 For instance, if we had entered the following commands: 6313 6314 (gdb) set substitute-path /usr/src/include /mnt/include 6315 (gdb) set substitute-path /usr/src /mnt/src 6316 6317 GDB would then rewrite `/usr/src/include/defs.h' into 6318 `/mnt/include/defs.h' by using the first rule. However, it would 6319 use the second rule to rewrite `/usr/src/lib/foo.c' into 6320 `/mnt/src/lib/foo.c'. 6321 6322`unset substitute-path [path]' 6323 If a path is specified, search the current list of substitution 6324 rules for a rule that would rewrite that path. Delete that rule 6325 if found. A warning is emitted by the debugger if no rule could 6326 be found. 6327 6328 If no path is specified, then all substitution rules are deleted. 6329 6330`show substitute-path [path]' 6331 If a path is specified, then print the source path substitution 6332 rule which would rewrite that path, if any. 6333 6334 If no path is specified, then print all existing source path 6335 substitution rules. 6336 6337 6338 If your source path is cluttered with directories that are no longer 6339of interest, GDB may sometimes cause confusion by finding the wrong 6340versions of source. You can correct the situation as follows: 6341 6342 1. Use `directory' with no argument to reset the source path to its 6343 default value. 6344 6345 2. Use `directory' with suitable arguments to reinstall the 6346 directories you want in the source path. You can add all the 6347 directories in one command. 6348 6349 6350File: gdb.info, Node: Machine Code, Prev: Source Path, Up: Source 6351 63529.6 Source and Machine Code 6353=========================== 6354 6355You can use the command `info line' to map source lines to program 6356addresses (and vice versa), and the command `disassemble' to display a 6357range of addresses as machine instructions. You can use the command 6358`set disassemble-next-line' to set whether to disassemble next source 6359line when execution stops. When run under GNU Emacs mode, the `info 6360line' command causes the arrow to point to the line specified. Also, 6361`info line' prints addresses in symbolic form as well as hex. 6362 6363`info line LINESPEC' 6364 Print the starting and ending addresses of the compiled code for 6365 source line LINESPEC. You can specify source lines in any of the 6366 ways documented in *note Specify Location::. 6367 6368 For example, we can use `info line' to discover the location of the 6369object code for the first line of function `m4_changequote': 6370 6371 (gdb) info line m4_changequote 6372 Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350. 6373 6374We can also inquire (using `*ADDR' as the form for LINESPEC) what 6375source line covers a particular address: 6376 (gdb) info line *0x63ff 6377 Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404. 6378 6379 After `info line', the default address for the `x' command is 6380changed to the starting address of the line, so that `x/i' is 6381sufficient to begin examining the machine code (*note Examining Memory: 6382Memory.). Also, this address is saved as the value of the convenience 6383variable `$_' (*note Convenience Variables: Convenience Vars.). 6384 6385`disassemble' 6386`disassemble /m' 6387`disassemble /r' 6388 This specialized command dumps a range of memory as machine 6389 instructions. It can also print mixed source+disassembly by 6390 specifying the `/m' modifier and print the raw instructions in hex 6391 as well as in symbolic form by specifying the `/r'. The default 6392 memory range is the function surrounding the program counter of 6393 the selected frame. A single argument to this command is a 6394 program counter value; GDB dumps the function surrounding this 6395 value. When two arguments are given, they should be separated by 6396 a comma, possibly surrounded by whitespace. The arguments specify 6397 a range of addresses to dump, in one of two forms: 6398 6399 `START,END' 6400 the addresses from START (inclusive) to END (exclusive) 6401 6402 `START,+LENGTH' 6403 the addresses from START (inclusive) to `START+LENGTH' 6404 (exclusive). 6405 6406 When 2 arguments are specified, the name of the function is also 6407 printed (since there could be several functions in the given 6408 range). 6409 6410 The argument(s) can be any expression yielding a numeric value, 6411 such as `0x32c4', `&main+10' or `$pc - 8'. 6412 6413 If the range of memory being disassembled contains current program 6414 counter, the instruction at that location is shown with a `=>' 6415 marker. 6416 6417 The following example shows the disassembly of a range of addresses 6418of HP PA-RISC 2.0 code: 6419 6420 (gdb) disas 0x32c4, 0x32e4 6421 Dump of assembler code from 0x32c4 to 0x32e4: 6422 0x32c4 <main+204>: addil 0,dp 6423 0x32c8 <main+208>: ldw 0x22c(sr0,r1),r26 6424 0x32cc <main+212>: ldil 0x3000,r31 6425 0x32d0 <main+216>: ble 0x3f8(sr4,r31) 6426 0x32d4 <main+220>: ldo 0(r31),rp 6427 0x32d8 <main+224>: addil -0x800,dp 6428 0x32dc <main+228>: ldo 0x588(r1),r26 6429 0x32e0 <main+232>: ldil 0x3000,r31 6430 End of assembler dump. 6431 6432 Here is an example showing mixed source+assembly for Intel x86, when 6433the program is stopped just after function prologue: 6434 6435 (gdb) disas /m main 6436 Dump of assembler code for function main: 6437 5 { 6438 0x08048330 <+0>: push %ebp 6439 0x08048331 <+1>: mov %esp,%ebp 6440 0x08048333 <+3>: sub $0x8,%esp 6441 0x08048336 <+6>: and $0xfffffff0,%esp 6442 0x08048339 <+9>: sub $0x10,%esp 6443 6444 6 printf ("Hello.\n"); 6445 => 0x0804833c <+12>: movl $0x8048440,(%esp) 6446 0x08048343 <+19>: call 0x8048284 <puts@plt> 6447 6448 7 return 0; 6449 8 } 6450 0x08048348 <+24>: mov $0x0,%eax 6451 0x0804834d <+29>: leave 6452 0x0804834e <+30>: ret 6453 6454 End of assembler dump. 6455 6456 Here is another example showing raw instructions in hex for AMD 6457x86-64, 6458 6459 (gdb) disas /r 0x400281,+10 6460 Dump of assembler code from 0x400281 to 0x40028b: 6461 0x0000000000400281: 38 36 cmp %dh,(%rsi) 6462 0x0000000000400283: 2d 36 34 2e 73 sub $0x732e3436,%eax 6463 0x0000000000400288: 6f outsl %ds:(%rsi),(%dx) 6464 0x0000000000400289: 2e 32 00 xor %cs:(%rax),%al 6465 End of assembler dump. 6466 6467 Addresses cannot be specified as a linespec (*note Specify 6468Location::). So, for example, if you want to disassemble function `bar' 6469in file `foo.c', you must type `disassemble 'foo.c'::bar' and not 6470`disassemble foo.c:bar'. 6471 6472 Some architectures have more than one commonly-used set of 6473instruction mnemonics or other syntax. 6474 6475 For programs that were dynamically linked and use shared libraries, 6476instructions that call functions or branch to locations in the shared 6477libraries might show a seemingly bogus location--it's actually a 6478location of the relocation table. On some architectures, GDB might be 6479able to resolve these to actual function names. 6480 6481`set disassembly-flavor INSTRUCTION-SET' 6482 Select the instruction set to use when disassembling the program 6483 via the `disassemble' or `x/i' commands. 6484 6485 Currently this command is only defined for the Intel x86 family. 6486 You can set INSTRUCTION-SET to either `intel' or `att'. The 6487 default is `att', the AT&T flavor used by default by Unix 6488 assemblers for x86-based targets. 6489 6490`show disassembly-flavor' 6491 Show the current setting of the disassembly flavor. 6492 6493`set disassemble-next-line' 6494`show disassemble-next-line' 6495 Control whether or not GDB will disassemble the next source line 6496 or instruction when execution stops. If ON, GDB will display 6497 disassembly of the next source line when execution of the program 6498 being debugged stops. This is _in addition_ to displaying the 6499 source line itself, which GDB always does if possible. If the 6500 next source line cannot be displayed for some reason (e.g., if GDB 6501 cannot find the source file, or there's no line info in the debug 6502 info), GDB will display disassembly of the next _instruction_ 6503 instead of showing the next source line. If AUTO, GDB will 6504 display disassembly of next instruction only if the source line 6505 cannot be displayed. This setting causes GDB to display some 6506 feedback when you step through a function with no line info or 6507 whose source file is unavailable. The default is OFF, which means 6508 never display the disassembly of the next line or instruction. 6509 6510 6511File: gdb.info, Node: Data, Next: Optimized Code, Prev: Source, Up: Top 6512 651310 Examining Data 6514***************** 6515 6516The usual way to examine data in your program is with the `print' 6517command (abbreviated `p'), or its synonym `inspect'. It evaluates and 6518prints the value of an expression of the language your program is 6519written in (*note Using GDB with Different Languages: Languages.). It 6520may also print the expression using a Python-based pretty-printer 6521(*note Pretty Printing::). 6522 6523`print EXPR' 6524`print /F EXPR' 6525 EXPR is an expression (in the source language). By default the 6526 value of EXPR is printed in a format appropriate to its data type; 6527 you can choose a different format by specifying `/F', where F is a 6528 letter specifying the format; see *note Output Formats: Output 6529 Formats. 6530 6531`print' 6532`print /F' 6533 If you omit EXPR, GDB displays the last value again (from the 6534 "value history"; *note Value History: Value History.). This 6535 allows you to conveniently inspect the same value in an 6536 alternative format. 6537 6538 A more low-level way of examining data is with the `x' command. It 6539examines data in memory at a specified address and prints it in a 6540specified format. *Note Examining Memory: Memory. 6541 6542 If you are interested in information about types, or about how the 6543fields of a struct or a class are declared, use the `ptype EXP' command 6544rather than `print'. *Note Examining the Symbol Table: Symbols. 6545 6546 Another way of examining values of expressions and type information 6547is through the Python extension command `explore' (available only if 6548the GDB build is configured with `--with-python'). It offers an 6549interactive way to start at the highest level (or, the most abstract 6550level) of the data type of an expression (or, the data type itself) and 6551explore all the way down to leaf scalar values/fields embedded in the 6552higher level data types. 6553 6554`explore ARG' 6555 ARG is either an expression (in the source language), or a type 6556 visible in the current context of the program being debugged. 6557 6558 The working of the `explore' command can be illustrated with an 6559example. If a data type `struct ComplexStruct' is defined in your C 6560program as 6561 6562 struct SimpleStruct 6563 { 6564 int i; 6565 double d; 6566 }; 6567 6568 struct ComplexStruct 6569 { 6570 struct SimpleStruct *ss_p; 6571 int arr[10]; 6572 }; 6573 6574followed by variable declarations as 6575 6576 struct SimpleStruct ss = { 10, 1.11 }; 6577 struct ComplexStruct cs = { &ss, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } }; 6578 6579then, the value of the variable `cs' can be explored using the 6580`explore' command as follows. 6581 6582 (gdb) explore cs 6583 The value of `cs' is a struct/class of type `struct ComplexStruct' with 6584 the following fields: 6585 6586 ss_p = <Enter 0 to explore this field of type `struct SimpleStruct *'> 6587 arr = <Enter 1 to explore this field of type `int [10]'> 6588 6589 Enter the field number of choice: 6590 6591Since the fields of `cs' are not scalar values, you are being prompted 6592to chose the field you want to explore. Let's say you choose the field 6593`ss_p' by entering `0'. Then, since this field is a pointer, you will 6594be asked if it is pointing to a single value. From the declaration of 6595`cs' above, it is indeed pointing to a single value, hence you enter 6596`y'. If you enter `n', then you will be asked if it were pointing to 6597an array of values, in which case this field will be explored as if it 6598were an array. 6599 6600 `cs.ss_p' is a pointer to a value of type `struct SimpleStruct' 6601 Continue exploring it as a pointer to a single value [y/n]: y 6602 The value of `*(cs.ss_p)' is a struct/class of type `struct 6603 SimpleStruct' with the following fields: 6604 6605 i = 10 .. (Value of type `int') 6606 d = 1.1100000000000001 .. (Value of type `double') 6607 6608 Press enter to return to parent value: 6609 6610If the field `arr' of `cs' was chosen for exploration by entering `1' 6611earlier, then since it is as array, you will be prompted to enter the 6612index of the element in the array that you want to explore. 6613 6614 `cs.arr' is an array of `int'. 6615 Enter the index of the element you want to explore in `cs.arr': 5 6616 6617 `(cs.arr)[5]' is a scalar value of type `int'. 6618 6619 (cs.arr)[5] = 4 6620 6621 Press enter to return to parent value: 6622 6623 In general, at any stage of exploration, you can go deeper towards 6624the leaf values by responding to the prompts appropriately, or hit the 6625return key to return to the enclosing data structure (the higher level 6626data structure). 6627 6628 Similar to exploring values, you can use the `explore' command to 6629explore types. Instead of specifying a value (which is typically a 6630variable name or an expression valid in the current context of the 6631program being debugged), you specify a type name. If you consider the 6632same example as above, your can explore the type `struct ComplexStruct' 6633by passing the argument `struct ComplexStruct' to the `explore' command. 6634 6635 (gdb) explore struct ComplexStruct 6636 6637By responding to the prompts appropriately in the subsequent interactive 6638session, you can explore the type `struct ComplexStruct' in a manner 6639similar to how the value `cs' was explored in the above example. 6640 6641 The `explore' command also has two sub-commands, `explore value' and 6642`explore type'. The former sub-command is a way to explicitly specify 6643that value exploration of the argument is being invoked, while the 6644latter is a way to explicitly specify that type exploration of the 6645argument is being invoked. 6646 6647`explore value EXPR' 6648 This sub-command of `explore' explores the value of the expression 6649 EXPR (if EXPR is an expression valid in the current context of the 6650 program being debugged). The behavior of this command is 6651 identical to that of the behavior of the `explore' command being 6652 passed the argument EXPR. 6653 6654`explore type ARG' 6655 This sub-command of `explore' explores the type of ARG (if ARG is 6656 a type visible in the current context of program being debugged), 6657 or the type of the value/expression ARG (if ARG is an expression 6658 valid in the current context of the program being debugged). If 6659 ARG is a type, then the behavior of this command is identical to 6660 that of the `explore' command being passed the argument ARG. If 6661 ARG is an expression, then the behavior of this command will be 6662 identical to that of the `explore' command being passed the type 6663 of ARG as the argument. 6664 6665* Menu: 6666 6667* Expressions:: Expressions 6668* Ambiguous Expressions:: Ambiguous Expressions 6669* Variables:: Program variables 6670* Arrays:: Artificial arrays 6671* Output Formats:: Output formats 6672* Memory:: Examining memory 6673* Auto Display:: Automatic display 6674* Print Settings:: Print settings 6675* Pretty Printing:: Python pretty printing 6676* Value History:: Value history 6677* Convenience Vars:: Convenience variables 6678* Convenience Funs:: Convenience functions 6679* Registers:: Registers 6680* Floating Point Hardware:: Floating point hardware 6681* Vector Unit:: Vector Unit 6682* OS Information:: Auxiliary data provided by operating system 6683* Memory Region Attributes:: Memory region attributes 6684* Dump/Restore Files:: Copy between memory and a file 6685* Core File Generation:: Cause a program dump its core 6686* Character Sets:: Debugging programs that use a different 6687 character set than GDB does 6688* Caching Remote Data:: Data caching for remote targets 6689* Searching Memory:: Searching memory for a sequence of bytes 6690 6691 6692File: gdb.info, Node: Expressions, Next: Ambiguous Expressions, Up: Data 6693 669410.1 Expressions 6695================ 6696 6697`print' and many other GDB commands accept an expression and compute 6698its value. Any kind of constant, variable or operator defined by the 6699programming language you are using is valid in an expression in GDB. 6700This includes conditional expressions, function calls, casts, and 6701string constants. It also includes preprocessor macros, if you 6702compiled your program to include this information; see *note 6703Compilation::. 6704 6705 GDB supports array constants in expressions input by the user. The 6706syntax is {ELEMENT, ELEMENT...}. For example, you can use the command 6707`print {1, 2, 3}' to create an array of three integers. If you pass an 6708array to a function or assign it to a program variable, GDB copies the 6709array to memory that is `malloc'ed in the target program. 6710 6711 Because C is so widespread, most of the expressions shown in 6712examples in this manual are in C. *Note Using GDB with Different 6713Languages: Languages, for information on how to use expressions in other 6714languages. 6715 6716 In this section, we discuss operators that you can use in GDB 6717expressions regardless of your programming language. 6718 6719 Casts are supported in all languages, not just in C, because it is so 6720useful to cast a number into a pointer in order to examine a structure 6721at that address in memory. 6722 6723 GDB supports these operators, in addition to those common to 6724programming languages: 6725 6726`@' 6727 `@' is a binary operator for treating parts of memory as arrays. 6728 *Note Artificial Arrays: Arrays, for more information. 6729 6730`::' 6731 `::' allows you to specify a variable in terms of the file or 6732 function where it is defined. *Note Program Variables: Variables. 6733 6734`{TYPE} ADDR' 6735 Refers to an object of type TYPE stored at address ADDR in memory. 6736 ADDR may be any expression whose value is an integer or pointer 6737 (but parentheses are required around binary operators, just as in 6738 a cast). This construct is allowed regardless of what kind of 6739 data is normally supposed to reside at ADDR. 6740 6741 6742File: gdb.info, Node: Ambiguous Expressions, Next: Variables, Prev: Expressions, Up: Data 6743 674410.2 Ambiguous Expressions 6745========================== 6746 6747Expressions can sometimes contain some ambiguous elements. For 6748instance, some programming languages (notably Ada, C++ and Objective-C) 6749permit a single function name to be defined several times, for 6750application in different contexts. This is called "overloading". 6751Another example involving Ada is generics. A "generic package" is 6752similar to C++ templates and is typically instantiated several times, 6753resulting in the same function name being defined in different contexts. 6754 6755 In some cases and depending on the language, it is possible to adjust 6756the expression to remove the ambiguity. For instance in C++, you can 6757specify the signature of the function you want to break on, as in 6758`break FUNCTION(TYPES)'. In Ada, using the fully qualified name of 6759your function often makes the expression unambiguous as well. 6760 6761 When an ambiguity that needs to be resolved is detected, the debugger 6762has the capability to display a menu of numbered choices for each 6763possibility, and then waits for the selection with the prompt `>'. The 6764first option is always `[0] cancel', and typing `0 <RET>' aborts the 6765current command. If the command in which the expression was used 6766allows more than one choice to be selected, the next option in the menu 6767is `[1] all', and typing `1 <RET>' selects all possible choices. 6768 6769 For example, the following session excerpt shows an attempt to set a 6770breakpoint at the overloaded symbol `String::after'. We choose three 6771particular definitions of that function name: 6772 6773 (gdb) b String::after 6774 [0] cancel 6775 [1] all 6776 [2] file:String.cc; line number:867 6777 [3] file:String.cc; line number:860 6778 [4] file:String.cc; line number:875 6779 [5] file:String.cc; line number:853 6780 [6] file:String.cc; line number:846 6781 [7] file:String.cc; line number:735 6782 > 2 4 6 6783 Breakpoint 1 at 0xb26c: file String.cc, line 867. 6784 Breakpoint 2 at 0xb344: file String.cc, line 875. 6785 Breakpoint 3 at 0xafcc: file String.cc, line 846. 6786 Multiple breakpoints were set. 6787 Use the "delete" command to delete unwanted 6788 breakpoints. 6789 (gdb) 6790 6791`set multiple-symbols MODE' 6792 This option allows you to adjust the debugger behavior when an 6793 expression is ambiguous. 6794 6795 By default, MODE is set to `all'. If the command with which the 6796 expression is used allows more than one choice, then GDB 6797 automatically selects all possible choices. For instance, 6798 inserting a breakpoint on a function using an ambiguous name 6799 results in a breakpoint inserted on each possible match. However, 6800 if a unique choice must be made, then GDB uses the menu to help 6801 you disambiguate the expression. For instance, printing the 6802 address of an overloaded function will result in the use of the 6803 menu. 6804 6805 When MODE is set to `ask', the debugger always uses the menu when 6806 an ambiguity is detected. 6807 6808 Finally, when MODE is set to `cancel', the debugger reports an 6809 error due to the ambiguity and the command is aborted. 6810 6811`show multiple-symbols' 6812 Show the current value of the `multiple-symbols' setting. 6813 6814 6815File: gdb.info, Node: Variables, Next: Arrays, Prev: Ambiguous Expressions, Up: Data 6816 681710.3 Program Variables 6818====================== 6819 6820The most common kind of expression to use is the name of a variable in 6821your program. 6822 6823 Variables in expressions are understood in the selected stack frame 6824(*note Selecting a Frame: Selection.); they must be either: 6825 6826 * global (or file-static) 6827 6828or 6829 6830 * visible according to the scope rules of the programming language 6831 from the point of execution in that frame 6832 6833This means that in the function 6834 6835 foo (a) 6836 int a; 6837 { 6838 bar (a); 6839 { 6840 int b = test (); 6841 bar (b); 6842 } 6843 } 6844 6845you can examine and use the variable `a' whenever your program is 6846executing within the function `foo', but you can only use or examine 6847the variable `b' while your program is executing inside the block where 6848`b' is declared. 6849 6850 There is an exception: you can refer to a variable or function whose 6851scope is a single source file even if the current execution point is not 6852in this file. But it is possible to have more than one such variable or 6853function with the same name (in different source files). If that 6854happens, referring to that name has unpredictable effects. If you wish, 6855you can specify a static variable in a particular function or file by 6856using the colon-colon (`::') notation: 6857 6858 FILE::VARIABLE 6859 FUNCTION::VARIABLE 6860 6861Here FILE or FUNCTION is the name of the context for the static 6862VARIABLE. In the case of file names, you can use quotes to make sure 6863GDB parses the file name as a single word--for example, to print a 6864global value of `x' defined in `f2.c': 6865 6866 (gdb) p 'f2.c'::x 6867 6868 The `::' notation is normally used for referring to static 6869variables, since you typically disambiguate uses of local variables in 6870functions by selecting the appropriate frame and using the simple name 6871of the variable. However, you may also use this notation to refer to 6872local variables in frames enclosing the selected frame: 6873 6874 void 6875 foo (int a) 6876 { 6877 if (a < 10) 6878 bar (a); 6879 else 6880 process (a); /* Stop here */ 6881 } 6882 6883 int 6884 bar (int a) 6885 { 6886 foo (a + 5); 6887 } 6888 6889For example, if there is a breakpoint at the commented line, here is 6890what you might see when the program stops after executing the call 6891`bar(0)': 6892 6893 (gdb) p a 6894 $1 = 10 6895 (gdb) p bar::a 6896 $2 = 5 6897 (gdb) up 2 6898 #2 0x080483d0 in foo (a=5) at foobar.c:12 6899 (gdb) p a 6900 $3 = 5 6901 (gdb) p bar::a 6902 $4 = 0 6903 6904 These uses of `::' are very rarely in conflict with the very similar 6905use of the same notation in C++. GDB also supports use of the C++ 6906scope resolution operator in GDB expressions. 6907 6908 _Warning:_ Occasionally, a local variable may appear to have the 6909 wrong value at certain points in a function--just after entry to a 6910 new scope, and just before exit. 6911 You may see this problem when you are stepping by machine 6912instructions. This is because, on most machines, it takes more than 6913one instruction to set up a stack frame (including local variable 6914definitions); if you are stepping by machine instructions, variables 6915may appear to have the wrong values until the stack frame is completely 6916built. On exit, it usually also takes more than one machine 6917instruction to destroy a stack frame; after you begin stepping through 6918that group of instructions, local variable definitions may be gone. 6919 6920 This may also happen when the compiler does significant 6921optimizations. To be sure of always seeing accurate values, turn off 6922all optimization when compiling. 6923 6924 Another possible effect of compiler optimizations is to optimize 6925unused variables out of existence, or assign variables to registers (as 6926opposed to memory addresses). Depending on the support for such cases 6927offered by the debug info format used by the compiler, GDB might not be 6928able to display values for such local variables. If that happens, GDB 6929will print a message like this: 6930 6931 No symbol "foo" in current context. 6932 6933 To solve such problems, either recompile without optimizations, or 6934use a different debug info format, if the compiler supports several such 6935formats. *Note Compilation::, for more information on choosing compiler 6936options. *Note C and C++: C, for more information about debug info 6937formats that are best suited to C++ programs. 6938 6939 If you ask to print an object whose contents are unknown to GDB, 6940e.g., because its data type is not completely specified by the debug 6941information, GDB will say `<incomplete type>'. *Note incomplete type: 6942Symbols, for more about this. 6943 6944 If you append `@entry' string to a function parameter name you get 6945its value at the time the function got called. If the value is not 6946available an error message is printed. Entry values are available only 6947with some compilers. Entry values are normally also printed at the 6948function parameter list according to *note set print entry-values::. 6949 6950 Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29 6951 29 i++; 6952 (gdb) next 6953 30 e (i); 6954 (gdb) print i 6955 $1 = 31 6956 (gdb) print i@entry 6957 $2 = 30 6958 6959 Strings are identified as arrays of `char' values without specified 6960signedness. Arrays of either `signed char' or `unsigned char' get 6961printed as arrays of 1 byte sized integers. `-fsigned-char' or 6962`-funsigned-char' GCC options have no effect as GDB defines literal 6963string type `"char"' as `char' without a sign. For program code 6964 6965 char var0[] = "A"; 6966 signed char var1[] = "A"; 6967 6968 You get during debugging 6969 (gdb) print var0 6970 $1 = "A" 6971 (gdb) print var1 6972 $2 = {65 'A', 0 '\0'} 6973 6974 6975File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data 6976 697710.4 Artificial Arrays 6978====================== 6979 6980It is often useful to print out several successive objects of the same 6981type in memory; a section of an array, or an array of dynamically 6982determined size for which only a pointer exists in the program. 6983 6984 You can do this by referring to a contiguous span of memory as an 6985"artificial array", using the binary operator `@'. The left operand of 6986`@' should be the first element of the desired array and be an 6987individual object. The right operand should be the desired length of 6988the array. The result is an array value whose elements are all of the 6989type of the left argument. The first element is actually the left 6990argument; the second element comes from bytes of memory immediately 6991following those that hold the first element, and so on. Here is an 6992example. If a program says 6993 6994 int *array = (int *) malloc (len * sizeof (int)); 6995 6996you can print the contents of `array' with 6997 6998 p *array@len 6999 7000 The left operand of `@' must reside in memory. Array values made 7001with `@' in this way behave just like other arrays in terms of 7002subscripting, and are coerced to pointers when used in expressions. 7003Artificial arrays most often appear in expressions via the value history 7004(*note Value History: Value History.), after printing one out. 7005 7006 Another way to create an artificial array is to use a cast. This 7007re-interprets a value as if it were an array. The value need not be in 7008memory: 7009 (gdb) p/x (short[2])0x12345678 7010 $1 = {0x1234, 0x5678} 7011 7012 As a convenience, if you leave the array length out (as in 7013`(TYPE[])VALUE') GDB calculates the size to fill the value (as 7014`sizeof(VALUE)/sizeof(TYPE)': 7015 (gdb) p/x (short[])0x12345678 7016 $2 = {0x1234, 0x5678} 7017 7018 Sometimes the artificial array mechanism is not quite enough; in 7019moderately complex data structures, the elements of interest may not 7020actually be adjacent--for example, if you are interested in the values 7021of pointers in an array. One useful work-around in this situation is 7022to use a convenience variable (*note Convenience Variables: Convenience 7023Vars.) as a counter in an expression that prints the first interesting 7024value, and then repeat that expression via <RET>. For instance, 7025suppose you have an array `dtab' of pointers to structures, and you are 7026interested in the values of a field `fv' in each structure. Here is an 7027example of what you might type: 7028 7029 set $i = 0 7030 p dtab[$i++]->fv 7031 <RET> 7032 <RET> 7033 ... 7034 7035 7036File: gdb.info, Node: Output Formats, Next: Memory, Prev: Arrays, Up: Data 7037 703810.5 Output Formats 7039=================== 7040 7041By default, GDB prints a value according to its data type. Sometimes 7042this is not what you want. For example, you might want to print a 7043number in hex, or a pointer in decimal. Or you might want to view data 7044in memory at a certain address as a character string or as an 7045instruction. To do these things, specify an "output format" when you 7046print a value. 7047 7048 The simplest use of output formats is to say how to print a value 7049already computed. This is done by starting the arguments of the 7050`print' command with a slash and a format letter. The format letters 7051supported are: 7052 7053`x' 7054 Regard the bits of the value as an integer, and print the integer 7055 in hexadecimal. 7056 7057`d' 7058 Print as integer in signed decimal. 7059 7060`u' 7061 Print as integer in unsigned decimal. 7062 7063`o' 7064 Print as integer in octal. 7065 7066`t' 7067 Print as integer in binary. The letter `t' stands for "two". (1) 7068 7069`a' 7070 Print as an address, both absolute in hexadecimal and as an offset 7071 from the nearest preceding symbol. You can use this format used 7072 to discover where (in what function) an unknown address is located: 7073 7074 (gdb) p/a 0x54320 7075 $3 = 0x54320 <_initialize_vx+396> 7076 7077 The command `info symbol 0x54320' yields similar results. *Note 7078 info symbol: Symbols. 7079 7080`c' 7081 Regard as an integer and print it as a character constant. This 7082 prints both the numerical value and its character representation. 7083 The character representation is replaced with the octal escape 7084 `\nnn' for characters outside the 7-bit ASCII range. 7085 7086 Without this format, GDB displays `char', `unsigned char', and 7087 `signed char' data as character constants. Single-byte members of 7088 vectors are displayed as integer data. 7089 7090`f' 7091 Regard the bits of the value as a floating point number and print 7092 using typical floating point syntax. 7093 7094`s' 7095 Regard as a string, if possible. With this format, pointers to 7096 single-byte data are displayed as null-terminated strings and 7097 arrays of single-byte data are displayed as fixed-length strings. 7098 Other values are displayed in their natural types. 7099 7100 Without this format, GDB displays pointers to and arrays of 7101 `char', `unsigned char', and `signed char' as strings. 7102 Single-byte members of a vector are displayed as an integer array. 7103 7104`r' 7105 Print using the `raw' formatting. By default, GDB will use a 7106 Python-based pretty-printer, if one is available (*note Pretty 7107 Printing::). This typically results in a higher-level display of 7108 the value's contents. The `r' format bypasses any Python 7109 pretty-printer which might exist. 7110 7111 For example, to print the program counter in hex (*note 7112Registers::), type 7113 7114 p/x $pc 7115 7116Note that no space is required before the slash; this is because command 7117names in GDB cannot contain a slash. 7118 7119 To reprint the last value in the value history with a different 7120format, you can use the `print' command with just a format and no 7121expression. For example, `p/x' reprints the last value in hex. 7122 7123 ---------- Footnotes ---------- 7124 7125 (1) `b' cannot be used because these format letters are also used 7126with the `x' command, where `b' stands for "byte"; see *note Examining 7127Memory: Memory. 7128 7129