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.\" $DragonFly: src/share/man/man5/a.out.5,v 1.3 2006/05/26 19:39:40 swildner Exp $ 38.\" 39.Dd June 5, 1993 40.Dt A.OUT 5 41.Os 42.Sh NAME 43.Nm a.out 44.Nd format of executable binary files 45.Sh SYNOPSIS 46.In a.out.h 47.Sh DESCRIPTION 48The include file 49.In a.out.h 50declares three structures and several macros. 51The structures describe the format of 52executable machine code files 53.Pq Sq binaries 54on the system. 55.Pp 56A binary file consists of up to 7 sections. 57In order, these sections are: 58.Bl -tag -width "text relocations" 59.It exec header 60Contains parameters used by the kernel 61to load a binary file into memory and execute it, 62and by the link editor 63.Xr ld 1 64to combine a binary file with other binary files. 65This section is the only mandatory one. 66.It text segment 67Contains machine code and related data 68that are loaded into memory when a program executes. 69May be loaded read-only. 70.It data segment 71Contains initialized data; always loaded into writable memory. 72.It text relocations 73Contains records used by the link editor 74to update pointers in the text segment when combining binary files. 75.It data relocations 76Like the text relocation section, but for data segment pointers. 77.It symbol table 78Contains records used by the link editor 79to cross reference the addresses of named variables and functions 80.Pq Sq symbols 81between binary files. 82.It string table 83Contains the character strings corresponding to the symbol names. 84.El 85.Pp 86Every binary file begins with an 87.Fa exec 88structure: 89.Bd -literal -offset indent 90struct exec { 91 unsigned long a_midmag; 92 unsigned long a_text; 93 unsigned long a_data; 94 unsigned long a_bss; 95 unsigned long a_syms; 96 unsigned long a_entry; 97 unsigned long a_trsize; 98 unsigned long a_drsize; 99}; 100.Ed 101.Pp 102The fields have the following functions: 103.Bl -tag -width a_trsize 104.It Fa a_midmag 105This field is stored in host byte-order. 106It has a number of sub-components accessed by the macros 107.Fn N_GETFLAG , 108.Fn N_GETMID , 109and 110.Fn N_GETMAGIC , 111and set by the macro 112.Fn N_SETMAGIC . 113.Pp 114The macro 115.Fn N_GETFLAG 116returns a few flags: 117.Bl -tag -width EX_DYNAMIC 118.It Dv EX_DYNAMIC 119indicates that the executable requires the services of the run-time link editor. 120.It Dv EX_PIC 121indicates that the object contains position independent code. 122This flag is 123set by 124.Xr as 1 125when given the 126.Sq -k 127flag and is preserved by 128.Xr ld 1 129if necessary. 130.El 131.Pp 132If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent 133executable image (eg. a shared library), which is to be loaded into the 134process address space by the run-time link editor. 135.Pp 136The macro 137.Fn N_GETMID 138returns the machine-id. 139This indicates which machine(s) the binary is intended to run on. 140.Pp 141.Fn N_GETMAGIC 142specifies the magic number, which uniquely identifies binary files 143and distinguishes different loading conventions. 144The field must contain one of the following values: 145.Bl -tag -width ZMAGIC 146.It Dv OMAGIC 147The text and data segments immediately follow the header 148and are contiguous. 149The kernel loads both text and data segments into writable memory. 150.It Dv NMAGIC 151As with 152.Dv OMAGIC , 153text and data segments immediately follow the header and are contiguous. 154However, the kernel loads the text into read-only memory 155and loads the data into writable memory at the next 156page boundary after the text. 157.It Dv ZMAGIC 158The kernel loads individual pages on demand from the binary. 159The header, text segment and data segment are all 160padded by the link editor to a multiple of the page size. 161Pages that the kernel loads from the text segment are read-only, 162while pages from the data segment are writable. 163.El 164.It Fa a_text 165Contains the size of the text segment in bytes. 166.It Fa a_data 167Contains the size of the data segment in bytes. 168.It Fa a_bss 169Contains the number of bytes in the 170.Sq bss segment 171and is used by the kernel to set the initial break 172.Pq Xr brk 2 173after the data segment. 174The kernel loads the program so that this amount of writable memory 175appears to follow the data segment and initially reads as zeroes. 176.Po 177.Em bss 178= block started by symbol 179.Pc 180.It Fa a_syms 181Contains the size in bytes of the symbol table section. 182.It Fa a_entry 183Contains the address in memory of the entry point 184of the program after the kernel has loaded it; 185the kernel starts the execution of the program 186from the machine instruction at this address. 187.It Fa a_trsize 188Contains the size in bytes of the text relocation table. 189.It Fa a_drsize 190Contains the size in bytes of the data relocation table. 191.El 192.Pp 193The 194.In a.out.h 195include file defines several macros which use an 196.Fa exec 197structure to test consistency or to locate section offsets in the binary file. 198.Bl -tag -width N_BADMAG(exec) 199.It Fn N_BADMAG exec 200Nonzero if the 201.Fa a_magic 202field does not contain a recognized value. 203.It Fn N_TXTOFF exec 204The byte offset in the binary file of the beginning of the text segment. 205.It Fn N_SYMOFF exec 206The byte offset of the beginning of the symbol table. 207.It Fn N_STROFF exec 208The byte offset of the beginning of the string table. 209.El 210.Pp 211Relocation records have a standard format which 212is described by the 213.Fa relocation_info 214structure: 215.Bd -literal -offset indent 216struct relocation_info { 217 int r_address; 218 unsigned int r_symbolnum : 24, 219 r_pcrel : 1, 220 r_length : 2, 221 r_extern : 1, 222 r_baserel : 1, 223 r_jmptable : 1, 224 r_relative : 1, 225 r_copy : 1; 226}; 227.Ed 228.Pp 229The 230.Fa relocation_info 231fields are used as follows: 232.Bl -tag -width r_symbolnum 233.It Fa r_address 234Contains the byte offset of a pointer that needs to be link-edited. 235Text relocation offsets are reckoned from the start of the text segment, 236and data relocation offsets from the start of the data segment. 237The link editor adds the value that is already stored at this offset 238into the new value that it computes using this relocation record. 239.It Fa r_symbolnum 240Contains the ordinal number of a symbol structure 241in the symbol table (it is 242.Em not 243a byte offset). 244After the link editor resolves the absolute address for this symbol, 245it adds that address to the pointer that is undergoing relocation. 246(If the 247.Fa r_extern 248bit is clear, the situation is different; see below.) 249.It Fa r_pcrel 250If this is set, 251the link editor assumes that it is updating a pointer 252that is part of a machine code instruction using pc-relative addressing. 253The address of the relocated pointer is implicitly added 254to its value when the running program uses it. 255.It Fa r_length 256Contains the log base 2 of the length of the pointer in bytes; 2570 for 1-byte displacements, 1 for 2-byte displacements, 2582 for 4-byte displacements. 259.It Fa r_extern 260Set if this relocation requires an external reference; 261the link editor must use a symbol address to update the pointer. 262When the 263.Fa r_extern 264bit is clear, the relocation is 265.Sq local ; 266the link editor updates the pointer to reflect 267changes in the load addresses of the various segments, 268rather than changes in the value of a symbol (except when 269.Fa r_baserel 270is also set (see below). 271In this case, the content of the 272.Fa r_symbolnum 273field is an 274.Fa n_type 275value (see below); 276this type field tells the link editor 277what segment the relocated pointer points into. 278.It Fa r_baserel 279If set, the symbol, as identified by the 280.Fa r_symbolnum 281field, is to be relocated to an offset into the Global Offset Table. 282At run-time, the entry in the Global Offset Table at this offset is set to 283be the address of the symbol. 284.It Fa r_jmptable 285If set, the symbol, as identified by the 286.Fa r_symbolnum 287field, is to be relocated to an offset into the Procedure Linkage Table. 288.It Fa r_relative 289If set, this relocation is relative to the (run-time) load address of the 290image this object file is going to be a part of. 291This type of relocation 292only occurs in shared objects. 293.It Fa r_copy 294If set, this relocation record identifies a symbol whose contents should 295be copied to the location given in 296.Fa r_address . 297The copying is done by the run-time link-editor from a suitable data 298item in a shared object. 299.El 300.Pp 301Symbols map names to addresses (or more generally, strings to values). 302Since the link-editor adjusts addresses, 303a symbol's name must be used to stand for its address 304until an absolute value has been assigned. 305Symbols consist of a fixed-length record in the symbol table 306and a variable-length name in the string table. 307The symbol table is an array of 308.Fa nlist 309structures: 310.Bd -literal -offset indent 311struct nlist { 312 union { 313 char *n_name; 314 long n_strx; 315 } n_un; 316 unsigned char n_type; 317 char n_other; 318 short n_desc; 319 unsigned long n_value; 320}; 321.Ed 322.Pp 323The fields are used as follows: 324.Bl -tag -width n_un.n_strx 325.It Fa n_un.n_strx 326Contains a byte offset into the string table 327for the name of this symbol. 328When a program accesses a symbol table with the 329.Xr nlist 3 330function, 331this field is replaced with the 332.Fa n_un.n_name 333field, which is a pointer to the string in memory. 334.It Fa n_type 335Used by the link editor to determine 336how to update the symbol's value. 337The 338.Fa n_type 339field is broken down into three sub-fields using bitmasks. 340The link editor treats symbols with the 341.Dv N_EXT 342type bit set as 343.Sq external 344symbols and permits references to them from other binary files. 345The 346.Dv N_TYPE 347mask selects bits of interest to the link editor: 348.Bl -tag -width N_TEXT 349.It Dv N_UNDF 350An undefined symbol. 351The link editor must locate an external symbol with the same name 352in another binary file to determine the absolute value of this symbol. 353As a special case, if the 354.Fa n_value 355field is nonzero and no binary file in the link-edit defines this symbol, 356the link-editor will resolve this symbol to an address 357in the bss segment, 358reserving an amount of bytes equal to 359.Fa n_value . 360If this symbol is undefined in more than one binary file 361and the binary files do not agree on the size, 362the link editor chooses the greatest size found across all binaries. 363.It Dv N_ABS 364An absolute symbol. 365The link editor does not update an absolute symbol. 366.It Dv N_TEXT 367A text symbol. 368This symbol's value is a text address and 369the link editor will update it when it merges binary files. 370.It Dv N_DATA 371A data symbol; similar to 372.Dv N_TEXT 373but for data addresses. 374The values for text and data symbols are not file offsets but 375addresses; to recover the file offsets, it is necessary 376to identify the loaded address of the beginning of the corresponding 377section and subtract it, then add the offset of the section. 378.It Dv N_BSS 379A bss symbol; like text or data symbols but 380has no corresponding offset in the binary file. 381.It Dv N_FN 382A filename symbol. 383The link editor inserts this symbol before 384the other symbols from a binary file when 385merging binary files. 386The name of the symbol is the filename given to the link editor, 387and its value is the first text address from that binary file. 388Filename symbols are not needed for link-editing or loading, 389but are useful for debuggers. 390.El 391.Pp 392The 393.Dv N_STAB 394mask selects bits of interest to symbolic debuggers 395such as 396.Xr gdb 1 ; 397the values are described in 398.Xr stab 5 . 399.It Fa n_other 400This field provides information on the nature of the symbol independent of 401the symbol's location in terms of segments as determined by the 402.Fa n_type 403field. 404Currently, the lower 4 bits of the 405.Fa n_other 406field hold one of two values: 407.Dv AUX_FUNC 408and 409.Dv AUX_OBJECT 410(see 411.In link.h 412for their definitions). 413.Dv AUX_FUNC 414associates the symbol with a callable function, while 415.Dv AUX_OBJECT 416associates the symbol with data, irrespective of their locations in 417either the text or the data segment. 418This field is intended to be used by 419.Xr ld 1 420for the construction of dynamic executables. 421.It Fa n_desc 422Reserved for use by debuggers; passed untouched by the link editor. 423Different debuggers use this field for different purposes. 424.It Fa n_value 425Contains the value of the symbol. 426For text, data and bss symbols, this is an address; 427for other symbols (such as debugger symbols), 428the value may be arbitrary. 429.El 430.Pp 431The string table consists of an 432.Em unsigned long 433length followed by null-terminated symbol strings. 434The length represents the size of the entire table in bytes, 435so its minimum value (or the offset of the first string) 436is always 4 on 32-bit machines. 437.Sh SEE ALSO 438.Xr as 1 , 439.Xr gdb 1 , 440.Xr ld 1 , 441.Xr brk 2 , 442.Xr execve 2 , 443.Xr nlist 3 , 444.Xr core 5 , 445.Xr elf 5 , 446.Xr link 5 , 447.Xr stab 5 448.Sh HISTORY 449The 450.In a.out.h 451include file appeared in 452.At v7 . 453.Sh BUGS 454Since not all of the supported architectures use the 455.Fa a_midmag 456field, 457it can be difficult to determine what 458architecture a binary will execute on 459without examining its actual machine code. 460Even with a machine identifier, 461the byte order of the 462.Fa exec 463header is machine-dependent. 464