1.\" Copyright (c) 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This man page is derived from documentation contributed to Berkeley by 5.\" Donn Seeley at UUNET Technologies, Inc. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)a.out.5 8.1 (Berkeley) 6/5/93 36.\" $FreeBSD: src/share/man/man5/a.out.5,v 1.10.2.4 2002/04/16 14:50:18 trhodes Exp $ 37.\" 38.Dd June 5, 1993 39.Dt A.OUT 5 40.Os 41.Sh NAME 42.Nm a.out 43.Nd format of executable binary files 44.Sh SYNOPSIS 45.In a.out.h 46.Sh DESCRIPTION 47The include file 48.Aq Pa a.out.h 49declares three structures and several macros. 50The structures describe the format of 51executable machine code files 52.Pq Sq binaries 53on the system. 54.Pp 55A binary file consists of up to 7 sections. 56In order, these sections are: 57.Bl -tag -width "text relocations" 58.It exec header 59Contains parameters used by the kernel 60to load a binary file into memory and execute it, 61and by the link editor 62.Xr ld 1 63to combine a binary file with other binary files. 64This section is the only mandatory one. 65.It text segment 66Contains machine code and related data 67that are loaded into memory when a program executes. 68May be loaded read-only. 69.It data segment 70Contains initialized data; always loaded into writable memory. 71.It text relocations 72Contains records used by the link editor 73to update pointers in the text segment when combining binary files. 74.It data relocations 75Like the text relocation section, but for data segment pointers. 76.It symbol table 77Contains records used by the link editor 78to cross reference the addresses of named variables and functions 79.Pq Sq symbols 80between binary files. 81.It string table 82Contains the character strings corresponding to the symbol names. 83.El 84.Pp 85Every binary file begins with an 86.Fa exec 87structure: 88.Bd -literal -offset indent 89struct exec { 90 unsigned long a_midmag; 91 unsigned long a_text; 92 unsigned long a_data; 93 unsigned long a_bss; 94 unsigned long a_syms; 95 unsigned long a_entry; 96 unsigned long a_trsize; 97 unsigned long a_drsize; 98}; 99.Ed 100.Pp 101The fields have the following functions: 102.Bl -tag -width a_trsize 103.It Fa a_midmag 104This field is stored in host byte-order. 105It has a number of sub-components accessed by the macros 106.Fn N_GETFLAG , 107.Fn N_GETMID , 108and 109.Fn N_GETMAGIC , 110and set by the macro 111.Fn N_SETMAGIC . 112.Pp 113The macro 114.Fn N_GETFLAG 115returns a few flags: 116.Bl -tag -width EX_DYNAMIC 117.It Dv EX_DYNAMIC 118indicates that the executable requires the services of the run-time link editor. 119.It Dv EX_PIC 120indicates that the object contains position independent code. 121This flag is 122set by 123.Xr as 1 124when given the 125.Sq -k 126flag and is preserved by 127.Xr ld 1 128if necessary. 129.El 130.Pp 131If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent 132executable image (eg. a shared library), which is to be loaded into the 133process address space by the run-time link editor. 134.Pp 135The macro 136.Fn N_GETMID 137returns the machine-id. 138This indicates which machine(s) the binary is intended to run on. 139.Pp 140.Fn N_GETMAGIC 141specifies the magic number, which uniquely identifies binary files 142and distinguishes different loading conventions. 143The field must contain one of the following values: 144.Bl -tag -width ZMAGIC 145.It Dv OMAGIC 146The text and data segments immediately follow the header 147and are contiguous. 148The kernel loads both text and data segments into writable memory. 149.It Dv NMAGIC 150As with 151.Dv OMAGIC , 152text and data segments immediately follow the header and are contiguous. 153However, the kernel loads the text into read-only memory 154and loads the data into writable memory at the next 155page boundary after the text. 156.It Dv ZMAGIC 157The kernel loads individual pages on demand from the binary. 158The header, text segment and data segment are all 159padded by the link editor to a multiple of the page size. 160Pages that the kernel loads from the text segment are read-only, 161while pages from the data segment are writable. 162.El 163.It Fa a_text 164Contains the size of the text segment in bytes. 165.It Fa a_data 166Contains the size of the data segment in bytes. 167.It Fa a_bss 168Contains the number of bytes in the 169.Sq bss segment 170and is used by the kernel to set the initial break 171.Pq Xr brk 2 172after the data segment. 173The kernel loads the program so that this amount of writable memory 174appears to follow the data segment and initially reads as zeroes. 175.Po 176.Em bss 177= block started by symbol 178.Pc 179.It Fa a_syms 180Contains the size in bytes of the symbol table section. 181.It Fa a_entry 182Contains the address in memory of the entry point 183of the program after the kernel has loaded it; 184the kernel starts the execution of the program 185from the machine instruction at this address. 186.It Fa a_trsize 187Contains the size in bytes of the text relocation table. 188.It Fa a_drsize 189Contains the size in bytes of the data relocation table. 190.El 191.Pp 192The 193.Pa a.out.h 194include file defines several macros which use an 195.Fa exec 196structure to test consistency or to locate section offsets in the binary file. 197.Bl -tag -width N_BADMAG(exec) 198.It Fn N_BADMAG exec 199Nonzero if the 200.Fa a_magic 201field does not contain a recognized value. 202.It Fn N_TXTOFF exec 203The byte offset in the binary file of the beginning of the text segment. 204.It Fn N_SYMOFF exec 205The byte offset of the beginning of the symbol table. 206.It Fn N_STROFF exec 207The byte offset of the beginning of the string table. 208.El 209.Pp 210Relocation records have a standard format which 211is described by the 212.Fa relocation_info 213structure: 214.Bd -literal -offset indent 215struct relocation_info { 216 int r_address; 217 unsigned int r_symbolnum : 24, 218 r_pcrel : 1, 219 r_length : 2, 220 r_extern : 1, 221 r_baserel : 1, 222 r_jmptable : 1, 223 r_relative : 1, 224 r_copy : 1; 225}; 226.Ed 227.Pp 228The 229.Fa relocation_info 230fields are used as follows: 231.Bl -tag -width r_symbolnum 232.It Fa r_address 233Contains the byte offset of a pointer that needs to be link-edited. 234Text relocation offsets are reckoned from the start of the text segment, 235and data relocation offsets from the start of the data segment. 236The link editor adds the value that is already stored at this offset 237into the new value that it computes using this relocation record. 238.It Fa r_symbolnum 239Contains the ordinal number of a symbol structure 240in the symbol table (it is 241.Em not 242a byte offset). 243After the link editor resolves the absolute address for this symbol, 244it adds that address to the pointer that is undergoing relocation. 245(If the 246.Fa r_extern 247bit is clear, the situation is different; see below.) 248.It Fa r_pcrel 249If this is set, 250the link editor assumes that it is updating a pointer 251that is part of a machine code instruction using pc-relative addressing. 252The address of the relocated pointer is implicitly added 253to its value when the running program uses it. 254.It Fa r_length 255Contains the log base 2 of the length of the pointer in bytes; 2560 for 1-byte displacements, 1 for 2-byte displacements, 2572 for 4-byte displacements. 258.It Fa r_extern 259Set if this relocation requires an external reference; 260the link editor must use a symbol address to update the pointer. 261When the 262.Fa r_extern 263bit is clear, the relocation is 264.Sq local ; 265the link editor updates the pointer to reflect 266changes in the load addresses of the various segments, 267rather than changes in the value of a symbol (except when 268.Fa r_baserel 269is also set (see below). 270In this case, the content of the 271.Fa r_symbolnum 272field is an 273.Fa n_type 274value (see below); 275this type field tells the link editor 276what segment the relocated pointer points into. 277.It Fa r_baserel 278If set, the symbol, as identified by the 279.Fa r_symbolnum 280field, is to be relocated to an offset into the Global Offset Table. 281At run-time, the entry in the Global Offset Table at this offset is set to 282be the address of the symbol. 283.It Fa r_jmptable 284If set, the symbol, as identified by the 285.Fa r_symbolnum 286field, is to be relocated to an offset into the Procedure Linkage Table. 287.It Fa r_relative 288If set, this relocation is relative to the (run-time) load address of the 289image this object file is going to be a part of. 290This type of relocation 291only occurs in shared objects. 292.It Fa r_copy 293If set, this relocation record identifies a symbol whose contents should 294be copied to the location given in 295.Fa r_address . 296The copying is done by the run-time link-editor from a suitable data 297item in a shared object. 298.El 299.Pp 300Symbols map names to addresses (or more generally, strings to values). 301Since the link-editor adjusts addresses, 302a symbol's name must be used to stand for its address 303until an absolute value has been assigned. 304Symbols consist of a fixed-length record in the symbol table 305and a variable-length name in the string table. 306The symbol table is an array of 307.Fa nlist 308structures: 309.Bd -literal -offset indent 310struct nlist { 311 union { 312 char *n_name; 313 long n_strx; 314 } n_un; 315 unsigned char n_type; 316 char n_other; 317 short n_desc; 318 unsigned long n_value; 319}; 320.Ed 321.Pp 322The fields are used as follows: 323.Bl -tag -width n_un.n_strx 324.It Fa n_un.n_strx 325Contains a byte offset into the string table 326for the name of this symbol. 327When a program accesses a symbol table with the 328.Xr nlist 3 329function, 330this field is replaced with the 331.Fa n_un.n_name 332field, which is a pointer to the string in memory. 333.It Fa n_type 334Used by the link editor to determine 335how to update the symbol's value. 336The 337.Fa n_type 338field is broken down into three sub-fields using bitmasks. 339The link editor treats symbols with the 340.Dv N_EXT 341type bit set as 342.Sq external 343symbols and permits references to them from other binary files. 344The 345.Dv N_TYPE 346mask selects bits of interest to the link editor: 347.Bl -tag -width N_TEXT 348.It Dv N_UNDF 349An undefined symbol. 350The link editor must locate an external symbol with the same name 351in another binary file to determine the absolute value of this symbol. 352As a special case, if the 353.Fa n_value 354field is nonzero and no binary file in the link-edit defines this symbol, 355the link-editor will resolve this symbol to an address 356in the bss segment, 357reserving an amount of bytes equal to 358.Fa n_value . 359If this symbol is undefined in more than one binary file 360and the binary files do not agree on the size, 361the link editor chooses the greatest size found across all binaries. 362.It Dv N_ABS 363An absolute symbol. 364The link editor does not update an absolute symbol. 365.It Dv N_TEXT 366A text symbol. 367This symbol's value is a text address and 368the link editor will update it when it merges binary files. 369.It Dv N_DATA 370A data symbol; similar to 371.Dv N_TEXT 372but for data addresses. 373The values for text and data symbols are not file offsets but 374addresses; to recover the file offsets, it is necessary 375to identify the loaded address of the beginning of the corresponding 376section and subtract it, then add the offset of the section. 377.It Dv N_BSS 378A bss symbol; like text or data symbols but 379has no corresponding offset in the binary file. 380.It Dv N_FN 381A filename symbol. 382The link editor inserts this symbol before 383the other symbols from a binary file when 384merging binary files. 385The name of the symbol is the filename given to the link editor, 386and its value is the first text address from that binary file. 387Filename symbols are not needed for link-editing or loading, 388but are useful for debuggers. 389.El 390.Pp 391The 392.Dv N_STAB 393mask selects bits of interest to symbolic debuggers 394such as 395.Xr gdb 1 ; 396the values are described in 397.Xr stab 5 . 398.It Fa n_other 399This field provides information on the nature of the symbol independent of 400the symbol's location in terms of segments as determined by the 401.Fa n_type 402field. 403Currently, the lower 4 bits of the 404.Fa n_other 405field hold one of two values: 406.Dv AUX_FUNC 407and 408.Dv AUX_OBJECT 409(see 410.Aq Pa link.h 411for their definitions). 412.Dv AUX_FUNC 413associates the symbol with a callable function, while 414.Dv AUX_OBJECT 415associates the symbol with data, irrespective of their locations in 416either the text or the data segment. 417This field is intended to be used by 418.Xr ld 1 419for the construction of dynamic executables. 420.It Fa n_desc 421Reserved for use by debuggers; passed untouched by the link editor. 422Different debuggers use this field for different purposes. 423.It Fa n_value 424Contains the value of the symbol. 425For text, data and bss symbols, this is an address; 426for other symbols (such as debugger symbols), 427the value may be arbitrary. 428.El 429.Pp 430The string table consists of an 431.Em unsigned long 432length followed by null-terminated symbol strings. 433The length represents the size of the entire table in bytes, 434so its minimum value (or the offset of the first string) 435is always 4 on 32-bit machines. 436.Sh SEE ALSO 437.Xr as 1 , 438.Xr gdb 1 , 439.Xr ld 1 , 440.Xr brk 2 , 441.Xr execve 2 , 442.Xr nlist 3 , 443.Xr core 5 , 444.Xr elf 5 , 445.Xr link 5 , 446.Xr stab 5 447.Sh HISTORY 448The 449.Pa a.out.h 450include file appeared in 451.At v7 . 452.Sh BUGS 453Since not all of the supported architectures use the 454.Fa a_midmag 455field, 456it can be difficult to determine what 457architecture a binary will execute on 458without examining its actual machine code. 459Even with a machine identifier, 460the byte order of the 461.Fa exec 462header is machine-dependent. 463