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.\" $FreeBSD$ 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.Fd #include <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.It Fa a_syms 176Contains the size in bytes of the symbol table section. 177.It Fa a_entry 178Contains the address in memory of the entry point 179of the program after the kernel has loaded it; 180the kernel starts the execution of the program 181from the machine instruction at this address. 182.It Fa a_trsize 183Contains the size in bytes of the text relocation table. 184.It Fa a_drsize 185Contains the size in bytes of the data relocation table. 186.El 187.Pp 188The 189.Pa a.out.h 190include file defines several macros which use an 191.Fa exec 192structure to test consistency or to locate section offsets in the binary file. 193.Bl -tag -width N_BADMAG(exec) 194.It Fn N_BADMAG exec 195Nonzero if the 196.Fa a_magic 197field does not contain a recognized value. 198.It Fn N_TXTOFF exec 199The byte offset in the binary file of the beginning of the text segment. 200.It Fn N_SYMOFF exec 201The byte offset of the beginning of the symbol table. 202.It Fn N_STROFF exec 203The byte offset of the beginning of the string table. 204.El 205.Pp 206Relocation records have a standard format which 207is described by the 208.Fa relocation_info 209structure: 210.Bd -literal -offset indent 211struct relocation_info { 212 int r_address; 213 unsigned int r_symbolnum : 24, 214 r_pcrel : 1, 215 r_length : 2, 216 r_extern : 1, 217 r_baserel : 1, 218 r_jmptable : 1, 219 r_relative : 1, 220 r_copy : 1; 221}; 222.Ed 223.Pp 224The 225.Fa relocation_info 226fields are used as follows: 227.Bl -tag -width r_symbolnum 228.It Fa r_address 229Contains the byte offset of a pointer that needs to be link-edited. 230Text relocation offsets are reckoned from the start of the text segment, 231and data relocation offsets from the start of the data segment. 232The link editor adds the value that is already stored at this offset 233into the new value that it computes using this relocation record. 234.It Fa r_symbolnum 235Contains the ordinal number of a symbol structure 236in the symbol table (it is 237.Em not 238a byte offset). 239After the link editor resolves the absolute address for this symbol, 240it adds that address to the pointer that is undergoing relocation. 241(If the 242.Fa r_extern 243bit is clear, the situation is different; see below.) 244.It Fa r_pcrel 245If this is set, 246the link editor assumes that it is updating a pointer 247that is part of a machine code instruction using pc-relative addressing. 248The address of the relocated pointer is implicitly added 249to its value when the running program uses it. 250.It Fa r_length 251Contains the log base 2 of the length of the pointer in bytes; 2520 for 1-byte displacements, 1 for 2-byte displacements, 2532 for 4-byte displacements. 254.It Fa r_extern 255Set if this relocation requires an external reference; 256the link editor must use a symbol address to update the pointer. 257When the 258.Fa r_extern 259bit is clear, the relocation is 260.Sq local ; 261the link editor updates the pointer to reflect 262changes in the load addresses of the various segments, 263rather than changes in the value of a symbol (except when 264.Fa r_baserel 265is also set (see below). 266In this case, the content of the 267.Fa r_symbolnum 268field is an 269.Fa n_type 270value (see below); 271this type field tells the link editor 272what segment the relocated pointer points into. 273.It Fa r_baserel 274If set, the symbol, as identified by the 275.Fa r_symbolnum 276field, is to be relocated to an offset into the Global Offset Table. 277At run-time, the entry in the Global Offset Table at this offset is set to 278be the address of the symbol. 279.It Fa r_jmptable 280If set, the symbol, as identified by the 281.Fa r_symbolnum 282field, is to be relocated to an offset into the Procedure Linkage Table. 283.It Fa r_relative 284If set, this relocation is relative to the (run-time) load address of the 285image this object file is going to be a part of. 286This type of relocation 287only occurs in shared objects. 288.It Fa r_copy 289If set, this relocation record identifies a symbol whose contents should 290be copied to the location given in 291.Fa r_address . 292The copying is done by the run-time link-editor from a suitable data 293item in a shared object. 294.El 295.Pp 296Symbols map names to addresses (or more generally, strings to values). 297Since the link-editor adjusts addresses, 298a symbol's name must be used to stand for its address 299until an absolute value has been assigned. 300Symbols consist of a fixed-length record in the symbol table 301and a variable-length name in the string table. 302The symbol table is an array of 303.Fa nlist 304structures: 305.Bd -literal -offset indent 306struct nlist { 307 union { 308 char *n_name; 309 long n_strx; 310 } n_un; 311 unsigned char n_type; 312 char n_other; 313 short n_desc; 314 unsigned long n_value; 315}; 316.Ed 317.Pp 318The fields are used as follows: 319.Bl -tag -width n_un.n_strx 320.It Fa n_un.n_strx 321Contains a byte offset into the string table 322for the name of this symbol. 323When a program accesses a symbol table with the 324.Xr nlist 3 325function, 326this field is replaced with the 327.Fa n_un.n_name 328field, which is a pointer to the string in memory. 329.It Fa n_type 330Used by the link editor to determine 331how to update the symbol's value. 332The 333.Fa n_type 334field is broken down into three sub-fields using bitmasks. 335The link editor treats symbols with the 336.Dv N_EXT 337type bit set as 338.Sq external 339symbols and permits references to them from other binary files. 340The 341.Dv N_TYPE 342mask selects bits of interest to the link editor: 343.Bl -tag -width N_TEXT 344.It Dv N_UNDF 345An undefined symbol. 346The link editor must locate an external symbol with the same name 347in another binary file to determine the absolute value of this symbol. 348As a special case, if the 349.Fa n_value 350field is nonzero and no binary file in the link-edit defines this symbol, 351the link-editor will resolve this symbol to an address 352in the bss segment, 353reserving an amount of bytes equal to 354.Fa n_value . 355If this symbol is undefined in more than one binary file 356and the binary files do not agree on the size, 357the link editor chooses the greatest size found across all binaries. 358.It Dv N_ABS 359An absolute symbol. 360The link editor does not update an absolute symbol. 361.It Dv N_TEXT 362A text symbol. 363This symbol's value is a text address and 364the link editor will update it when it merges binary files. 365.It Dv N_DATA 366A data symbol; similar to 367.Dv N_TEXT 368but for data addresses. 369The values for text and data symbols are not file offsets but 370addresses; to recover the file offsets, it is necessary 371to identify the loaded address of the beginning of the corresponding 372section and subtract it, then add the offset of the section. 373.It Dv N_BSS 374A bss symbol; like text or data symbols but 375has no corresponding offset in the binary file. 376.It Dv N_FN 377A filename symbol. 378The link editor inserts this symbol before 379the other symbols from a binary file when 380merging binary files. 381The name of the symbol is the filename given to the link editor, 382and its value is the first text address from that binary file. 383Filename symbols are not needed for link-editing or loading, 384but are useful for debuggers. 385.El 386.Pp 387The 388.Dv N_STAB 389mask selects bits of interest to symbolic debuggers 390such as 391.Xr gdb 1 ; 392the values are described in 393.Xr stab 5 . 394.It Fa n_other 395This field provides information on the nature of the symbol independent of 396the symbol's location in terms of segments as determined by the 397.Fa n_type 398field. 399Currently, the lower 4 bits of the 400.Fa n_other 401field hold one of two values: 402.Dv AUX_FUNC 403and 404.Dv AUX_OBJECT 405.Po 406see 407.Aq Pa link.h 408for their definitions 409.Pc . 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.Pa 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.Pp 462Nobody seems to agree on what 463.Em bss 464stands for. 465