1.\" Copyright (c) 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This manual 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.\" $FreeBSD$ 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.In 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 (e.g.\& 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.Em ( bss 176= block started by symbol) 177.It Fa a_syms 178Contains the size in bytes of the symbol table section. 179.It Fa a_entry 180Contains the address in memory of the entry point 181of the program after the kernel has loaded it; 182the kernel starts the execution of the program 183from the machine instruction at this address. 184.It Fa a_trsize 185Contains the size in bytes of the text relocation table. 186.It Fa a_drsize 187Contains the size in bytes of the data relocation table. 188.El 189.Pp 190The 191.In a.out.h 192include file defines several macros which use an 193.Fa exec 194structure to test consistency or to locate section offsets in the binary file. 195.Bl -tag -width N_BADMAG(exec) 196.It Fn N_BADMAG exec 197Nonzero if the 198.Fa a_magic 199field does not contain a recognized value. 200.It Fn N_TXTOFF exec 201The byte offset in the binary file of the beginning of the text segment. 202.It Fn N_SYMOFF exec 203The byte offset of the beginning of the symbol table. 204.It Fn N_STROFF exec 205The byte offset of the beginning of the string table. 206.El 207.Pp 208Relocation records have a standard format which 209is described by the 210.Fa relocation_info 211structure: 212.Bd -literal -offset indent 213struct relocation_info { 214 int r_address; 215 unsigned int r_symbolnum : 24, 216 r_pcrel : 1, 217 r_length : 2, 218 r_extern : 1, 219 r_baserel : 1, 220 r_jmptable : 1, 221 r_relative : 1, 222 r_copy : 1; 223}; 224.Ed 225.Pp 226The 227.Fa relocation_info 228fields are used as follows: 229.Bl -tag -width r_symbolnum 230.It Fa r_address 231Contains the byte offset of a pointer that needs to be link-edited. 232Text relocation offsets are reckoned from the start of the text segment, 233and data relocation offsets from the start of the data segment. 234The link editor adds the value that is already stored at this offset 235into the new value that it computes using this relocation record. 236.It Fa r_symbolnum 237Contains the ordinal number of a symbol structure 238in the symbol table (it is 239.Em not 240a byte offset). 241After the link editor resolves the absolute address for this symbol, 242it adds that address to the pointer that is undergoing relocation. 243(If the 244.Fa r_extern 245bit is clear, the situation is different; see below.) 246.It Fa r_pcrel 247If this is set, 248the link editor assumes that it is updating a pointer 249that is part of a machine code instruction using pc-relative addressing. 250The address of the relocated pointer is implicitly added 251to its value when the running program uses it. 252.It Fa r_length 253Contains the log base 2 of the length of the pointer in bytes; 2540 for 1-byte displacements, 1 for 2-byte displacements, 2552 for 4-byte displacements. 256.It Fa r_extern 257Set if this relocation requires an external reference; 258the link editor must use a symbol address to update the pointer. 259When the 260.Fa r_extern 261bit is clear, the relocation is 262.Sq local ; 263the link editor updates the pointer to reflect 264changes in the load addresses of the various segments, 265rather than changes in the value of a symbol (except when 266.Fa r_baserel 267is also set (see below). 268In this case, the content of the 269.Fa r_symbolnum 270field is an 271.Fa n_type 272value (see below); 273this type field tells the link editor 274what segment the relocated pointer points into. 275.It Fa r_baserel 276If set, the symbol, as identified by the 277.Fa r_symbolnum 278field, is to be relocated to an offset into the Global Offset Table. 279At run-time, the entry in the Global Offset Table at this offset is set to 280be the address of the symbol. 281.It Fa r_jmptable 282If set, the symbol, as identified by the 283.Fa r_symbolnum 284field, is to be relocated to an offset into the Procedure Linkage Table. 285.It Fa r_relative 286If set, this relocation is relative to the (run-time) load address of the 287image this object file is going to be a part of. 288This type of relocation 289only occurs in shared objects. 290.It Fa r_copy 291If set, this relocation record identifies a symbol whose contents should 292be copied to the location given in 293.Fa r_address . 294The copying is done by the run-time link-editor from a suitable data 295item in a shared object. 296.El 297.Pp 298Symbols map names to addresses (or more generally, strings to values). 299Since the link-editor adjusts addresses, 300a symbol's name must be used to stand for its address 301until an absolute value has been assigned. 302Symbols consist of a fixed-length record in the symbol table 303and a variable-length name in the string table. 304The symbol table is an array of 305.Fa nlist 306structures: 307.Bd -literal -offset indent 308struct nlist { 309 union { 310 char *n_name; 311 long n_strx; 312 } n_un; 313 unsigned char n_type; 314 char n_other; 315 short n_desc; 316 unsigned long n_value; 317}; 318.Ed 319.Pp 320The fields are used as follows: 321.Bl -tag -width n_un.n_strx 322.It Fa n_un.n_strx 323Contains a byte offset into the string table 324for the name of this symbol. 325When a program accesses a symbol table with the 326.Xr nlist 3 327function, 328this field is replaced with the 329.Fa n_un.n_name 330field, which is a pointer to the string in memory. 331.It Fa n_type 332Used by the link editor to determine 333how to update the symbol's value. 334The 335.Fa n_type 336field is broken down into three sub-fields using bitmasks. 337The link editor treats symbols with the 338.Dv N_EXT 339type bit set as 340.Sq external 341symbols and permits references to them from other binary files. 342The 343.Dv N_TYPE 344mask selects bits of interest to the link editor: 345.Bl -tag -width N_TEXT 346.It Dv N_UNDF 347An undefined symbol. 348The link editor must locate an external symbol with the same name 349in another binary file to determine the absolute value of this symbol. 350As a special case, if the 351.Fa n_value 352field is nonzero and no binary file in the link-edit defines this symbol, 353the link-editor will resolve this symbol to an address 354in the bss segment, 355reserving an amount of bytes equal to 356.Fa n_value . 357If this symbol is undefined in more than one binary file 358and the binary files do not agree on the size, 359the link editor chooses the greatest size found across all binaries. 360.It Dv N_ABS 361An absolute symbol. 362The link editor does not update an absolute symbol. 363.It Dv N_TEXT 364A text symbol. 365This symbol's value is a text address and 366the link editor will update it when it merges binary files. 367.It Dv N_DATA 368A data symbol; similar to 369.Dv N_TEXT 370but for data addresses. 371The values for text and data symbols are not file offsets but 372addresses; to recover the file offsets, it is necessary 373to identify the loaded address of the beginning of the corresponding 374section and subtract it, then add the offset of the section. 375.It Dv N_BSS 376A bss symbol; like text or data symbols but 377has no corresponding offset in the binary file. 378.It Dv N_FN 379A filename symbol. 380The link editor inserts this symbol before 381the other symbols from a binary file when 382merging binary files. 383The name of the symbol is the filename given to the link editor, 384and its value is the first text address from that binary file. 385Filename symbols are not needed for link-editing or loading, 386but are useful for debuggers. 387.El 388.Pp 389The 390.Dv N_STAB 391mask selects bits of interest to symbolic debuggers 392such as 393.Xr gdb 1 ; 394the values are described in 395.Xr stab 5 . 396.It Fa n_other 397This field provides information on the nature of the symbol independent of 398the symbol's location in terms of segments as determined by the 399.Fa n_type 400field. 401Currently, the lower 4 bits of the 402.Fa n_other 403field hold one of two values: 404.Dv AUX_FUNC 405and 406.Dv AUX_OBJECT 407(see 408.In link.h 409for their definitions). 410.Dv AUX_FUNC 411associates the symbol with a callable function, while 412.Dv AUX_OBJECT 413associates the symbol with data, irrespective of their locations in 414either the text or the data segment. 415This field is intended to be used by 416.Xr ld 1 417for the construction of dynamic executables. 418.It Fa n_desc 419Reserved for use by debuggers; passed untouched by the link editor. 420Different debuggers use this field for different purposes. 421.It Fa n_value 422Contains the value of the symbol. 423For text, data and bss symbols, this is an address; 424for other symbols (such as debugger symbols), 425the value may be arbitrary. 426.El 427.Pp 428The string table consists of an 429.Em unsigned long 430length followed by null-terminated symbol strings. 431The length represents the size of the entire table in bytes, 432so its minimum value (or the offset of the first string) 433is always 4 on 32-bit machines. 434.Sh SEE ALSO 435.Xr as 1 , 436.Xr gdb 1 , 437.Xr ld 1 , 438.Xr brk 2 , 439.Xr execve 2 , 440.Xr nlist 3 , 441.Xr core 5 , 442.Xr elf 5 , 443.Xr link 5 , 444.Xr stab 5 445.Sh HISTORY 446The 447.In a.out.h 448include file appeared in 449.At v7 . 450.Sh BUGS 451Since not all of the supported architectures use the 452.Fa a_midmag 453field, 454it can be difficult to determine what 455architecture a binary will execute on 456without examining its actual machine code. 457Even with a machine identifier, 458the byte order of the 459.Fa exec 460header is machine-dependent. 461