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.Dv N_GETFLAG() , 107.Dv N_GETMID() , and 108.Dv N_GETMAGIC() , 109and set by the macro 110.Dv N_SETMAGIC(). 111.Pp 112The macro 113.Dv N_GETFLAG() 114returns a few flags: 115.Bl -tag -width EX_DYNAMIC 116.It Dv EX_DYNAMIC 117indicates that the executable requires the services of the run-time link editor. 118.It Dv EX_PIC 119indicates that the object contains position independent code. This flag is 120set by 121.Xr as 1 122when given the 123.Sq -k 124flag and is preserved by 125.Xr ld 1 126if necessary. 127.El 128.Pp 129If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent 130executable image (eg. a shared library), which is to be loaded into the 131process address space by the run-time link editor. 132.Pp 133The macro 134.Dv N_GETMID() 135returns the machine-id. 136This indicates which machine(s) the binary is intended to run on. 137.Pp 138.Dv N_GETMAGIC() 139specifies the magic number, which uniquely identifies binary files 140and distinguishes different loading conventions. 141The field must contain one of the following values: 142.Bl -tag -width ZMAGIC 143.It Dv OMAGIC 144The text and data segments immediately follow the header 145and are contiguous. 146The kernel loads both text and data segments into writable memory. 147.It Dv NMAGIC 148As with 149.Dv OMAGIC , 150text and data segments immediately follow the header and are contiguous. 151However, the kernel loads the text into read-only memory 152and loads the data into writable memory at the next 153page boundary after the text. 154.It Dv ZMAGIC 155The kernel loads individual pages on demand from the binary. 156The header, text segment and data segment are all 157padded by the link editor to a multiple of the page size. 158Pages that the kernel loads from the text segment are read-only, 159while pages from the data segment are writable. 160.El 161.It Fa a_text 162Contains the size of the text segment in bytes. 163.It Fa a_data 164Contains the size of the data segment in bytes. 165.It Fa a_bss 166Contains the number of bytes in the 167.Sq bss segment 168and is used by the kernel to set the initial break 169.Pq Xr brk 2 170after the data segment. 171The kernel loads the program so that this amount of writable memory 172appears to follow the data segment and initially reads as zeroes. 173.It Fa a_syms 174Contains the size in bytes of the symbol table section. 175.It Fa a_entry 176Contains the address in memory of the entry point 177of the program after the kernel has loaded it; 178the kernel starts the execution of the program 179from the machine instruction at this address. 180.It Fa a_trsize 181Contains the size in bytes of the text relocation table. 182.It Fa a_drsize 183Contains the size in bytes of the data relocation table. 184.El 185.Pp 186The 187.Pa a.out.h 188include file defines several macros which use an 189.Fa exec 190structure to test consistency or to locate section offsets in the binary file. 191.Bl -tag -width N_BADMAG(exec) 192.It Fn N_BADMAG exec 193Nonzero if the 194.Fa a_magic 195field does not contain a recognized value. 196.It Fn N_TXTOFF exec 197The byte offset in the binary file of the beginning of the text segment. 198.It Fn N_SYMOFF exec 199The byte offset of the beginning of the symbol table. 200.It Fn N_STROFF exec 201The byte offset of the beginning of the string table. 202.El 203.Pp 204Relocation records have a standard format which 205is described by the 206.Fa relocation_info 207structure: 208.Bd -literal -offset indent 209struct relocation_info { 210 int r_address; 211 unsigned int r_symbolnum : 24, 212 r_pcrel : 1, 213 r_length : 2, 214 r_extern : 1, 215 r_baserel : 1, 216 r_jmptable : 1, 217 r_relative : 1, 218 r_copy : 1; 219}; 220.Ed 221.Pp 222The 223.Fa relocation_info 224fields are used as follows: 225.Bl -tag -width r_symbolnum 226.It Fa r_address 227Contains the byte offset of a pointer that needs to be link-edited. 228Text relocation offsets are reckoned from the start of the text segment, 229and data relocation offsets from the start of the data segment. 230The link editor adds the value that is already stored at this offset 231into the new value that it computes using this relocation record. 232.It Fa r_symbolnum 233Contains the ordinal number of a symbol structure 234in the symbol table (it is 235.Em not 236a byte offset). 237After the link editor resolves the absolute address for this symbol, 238it adds that address to the pointer that is undergoing relocation. 239(If the 240.Fa r_extern 241bit is clear, the situation is different; see below.) 242.It Fa r_pcrel 243If this is set, 244the link editor assumes that it is updating a pointer 245that is part of a machine code instruction using pc-relative addressing. 246The address of the relocated pointer is implicitly added 247to its value when the running program uses it. 248.It Fa r_length 249Contains the log base 2 of the length of the pointer in bytes; 2500 for 1-byte displacements, 1 for 2-byte displacements, 2512 for 4-byte displacements. 252.It Fa r_extern 253Set if this relocation requires an external reference; 254the link editor must use a symbol address to update the pointer. 255When the 256.Fa r_extern 257bit is clear, the relocation is 258.Sq local ; 259the link editor updates the pointer to reflect 260changes in the load addresses of the various segments, 261rather than changes in the value of a symbol (except when 262.Fa r_baserel 263is also set (see below). 264In this case, the content of the 265.Fa r_symbolnum 266field is an 267.Fa n_type 268value (see below); 269this type field tells the link editor 270what segment the relocated pointer points into. 271.It Fa r_baserel 272If set, the symbol, as identified by the 273.Fa r_symbolnum 274field, is to be relocated to an offset into the Global Offset Table. 275At run-time, the entry in the Global Offset Table at this offset is set to 276be the address of the symbol. 277.It Fa r_jmptable 278If set, the symbol, as identified by the 279.Fa r_symbolnum 280field, is to be relocated to an offset into the Procedure Linkage Table. 281.It Fa r_relative 282If set, this relocation is relative to the (run-time) load address of the 283image this object file is going to be a part of. This type of relocation 284only occurs in shared objects. 285.It Fa r_copy 286If set, this relocation record identifies a symbol whose contents should 287be copied to the location given in 288.Fa r_address. 289The copying is done by the run-time link-editor from a suitable data 290item in a shared object. 291.El 292.Pp 293Symbols map names to addresses (or more generally, strings to values). 294Since the link-editor adjusts addresses, 295a symbol's name must be used to stand for its address 296until an absolute value has been assigned. 297Symbols consist of a fixed-length record in the symbol table 298and a variable-length name in the string table. 299The symbol table is an array of 300.Fa nlist 301structures: 302.Bd -literal -offset indent 303struct nlist { 304 union { 305 char *n_name; 306 long n_strx; 307 } n_un; 308 unsigned char n_type; 309 char n_other; 310 short n_desc; 311 unsigned long n_value; 312}; 313.Ed 314.Pp 315The fields are used as follows: 316.Bl -tag -width n_un.n_strx 317.It Fa n_un.n_strx 318Contains a byte offset into the string table 319for the name of this symbol. 320When a program accesses a symbol table with the 321.Xr nlist 3 322function, 323this field is replaced with the 324.Fa n_un.n_name 325field, which is a pointer to the string in memory. 326.It Fa n_type 327Used by the link editor to determine 328how to update the symbol's value. 329The 330.Fa n_type 331field is broken down into three sub-fields using bitmasks. 332The link editor treats symbols with the 333.Dv N_EXT 334type bit set as 335.Sq external 336symbols and permits references to them from other binary files. 337The 338.Dv N_TYPE 339mask selects bits of interest to the link editor: 340.Bl -tag -width N_TEXT 341.It Dv N_UNDF 342An undefined symbol. 343The link editor must locate an external symbol with the same name 344in another binary file to determine the absolute value of this symbol. 345As a special case, if the 346.Fa n_value 347field is nonzero and no binary file in the link-edit defines this symbol, 348the link-editor will resolve this symbol to an address 349in the bss segment, 350reserving an amount of bytes equal to 351.Fa n_value . 352If this symbol is undefined in more than one binary file 353and the binary files do not agree on the size, 354the link editor chooses the greatest size found across all binaries. 355.It Dv N_ABS 356An absolute symbol. 357The link editor does not update an absolute symbol. 358.It Dv N_TEXT 359A text symbol. 360This symbol's value is a text address and 361the link editor will update it when it merges binary files. 362.It Dv N_DATA 363A data symbol; similar to 364.Dv N_TEXT 365but for data addresses. 366The values for text and data symbols are not file offsets but 367addresses; to recover the file offsets, it is necessary 368to identify the loaded address of the beginning of the corresponding 369section and subtract it, then add the offset of the section. 370.It Dv N_BSS 371A bss symbol; like text or data symbols but 372has no corresponding offset in the binary file. 373.It Dv N_FN 374A filename symbol. 375The link editor inserts this symbol before 376the other symbols from a binary file when 377merging binary files. 378The name of the symbol is the filename given to the link editor, 379and its value is the first text address from that binary file. 380Filename symbols are not needed for link-editing or loading, 381but are useful for debuggers. 382.El 383.Pp 384The 385.Dv N_STAB 386mask selects bits of interest to symbolic debuggers 387such as 388.Xr gdb 1 ; 389the values are described in 390.Xr stab 5 . 391.It Fa n_other 392This field provides information on the nature of the symbol independent of 393the symbol's location in terms of segments as determined by the 394.Fa n_type 395field. Currently, the lower 4 bits of the 396.Fa n_other 397field hold one of two values: 398.Dv AUX_FUNC 399and 400.Dv AUX_OBJECT 401.Po 402see 403.Aq Pa link.h 404for their definitions 405.Pc . 406.Dv AUX_FUNC 407associates the symbol with a callable function, while 408.Dv AUX_OBJECT 409associates the symbol with data, irrespective of their locations in 410either the text or the data segment. 411This field is intended to be used by 412.Xr ld 1 413for the construction of dynamic executables. 414.It Fa n_desc 415Reserved for use by debuggers; passed untouched by the link editor. 416Different debuggers use this field for different purposes. 417.It Fa n_value 418Contains the value of the symbol. 419For text, data and bss symbols, this is an address; 420for other symbols (such as debugger symbols), 421the value may be arbitrary. 422.El 423.Pp 424The string table consists of an 425.Em unsigned long 426length followed by null-terminated symbol strings. 427The length represents the size of the entire table in bytes, 428so its minimum value (or the offset of the first string) 429is always 4 on 32-bit machines. 430.Sh SEE ALSO 431.Xr as 1 , 432.Xr gdb 1 , 433.Xr ld 1 , 434.Xr brk 2 , 435.Xr execve 2 , 436.Xr nlist 3 , 437.Xr core 5 , 438.Xr link 5 , 439.Xr stab 5 440.Sh HISTORY 441The 442.Pa a.out.h 443include file appeared in 444.At v7 . 445.Sh BUGS 446Since not all of the supported architectures use the 447.Fa a_midmag 448field, 449it can be difficult to determine what 450architecture a binary will execute on 451without examining its actual machine code. 452Even with a machine identifier, 453the byte order of the 454.Fa exec 455header is machine-dependent. 456.Pp 457Nobody seems to agree on what 458.Em bss 459stands for. 460.Pp 461New binary file formats may be supported in the future, 462and they probably will not be compatible at any level 463with this ancient format. 464