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. 120This flag is 121set by 122.Xr as 1 123when given the 124.Sq -k 125flag and is preserved by 126.Xr ld 1 127if necessary. 128.El 129.Pp 130If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent 131executable image (eg. a shared library), which is to be loaded into the 132process address space by the run-time link editor. 133.Pp 134The macro 135.Dv N_GETMID() 136returns the machine-id. 137This indicates which machine(s) the binary is intended to run on. 138.Pp 139.Dv N_GETMAGIC() 140specifies the magic number, which uniquely identifies binary files 141and distinguishes different loading conventions. 142The field must contain one of the following values: 143.Bl -tag -width ZMAGIC 144.It Dv OMAGIC 145The text and data segments immediately follow the header 146and are contiguous. 147The kernel loads both text and data segments into writable memory. 148.It Dv NMAGIC 149As with 150.Dv OMAGIC , 151text and data segments immediately follow the header and are contiguous. 152However, the kernel loads the text into read-only memory 153and loads the data into writable memory at the next 154page boundary after the text. 155.It Dv ZMAGIC 156The kernel loads individual pages on demand from the binary. 157The header, text segment and data segment are all 158padded by the link editor to a multiple of the page size. 159Pages that the kernel loads from the text segment are read-only, 160while pages from the data segment are writable. 161.El 162.It Fa a_text 163Contains the size of the text segment in bytes. 164.It Fa a_data 165Contains the size of the data segment in bytes. 166.It Fa a_bss 167Contains the number of bytes in the 168.Sq bss segment 169and is used by the kernel to set the initial break 170.Pq Xr brk 2 171after the data segment. 172The kernel loads the program so that this amount of writable memory 173appears to follow the data segment and initially reads as zeroes. 174.It Fa a_syms 175Contains the size in bytes of the symbol table section. 176.It Fa a_entry 177Contains the address in memory of the entry point 178of the program after the kernel has loaded it; 179the kernel starts the execution of the program 180from the machine instruction at this address. 181.It Fa a_trsize 182Contains the size in bytes of the text relocation table. 183.It Fa a_drsize 184Contains the size in bytes of the data relocation table. 185.El 186.Pp 187The 188.Pa a.out.h 189include file defines several macros which use an 190.Fa exec 191structure to test consistency or to locate section offsets in the binary file. 192.Bl -tag -width N_BADMAG(exec) 193.It Fn N_BADMAG exec 194Nonzero if the 195.Fa a_magic 196field does not contain a recognized value. 197.It Fn N_TXTOFF exec 198The byte offset in the binary file of the beginning of the text segment. 199.It Fn N_SYMOFF exec 200The byte offset of the beginning of the symbol table. 201.It Fn N_STROFF exec 202The byte offset of the beginning of the string table. 203.El 204.Pp 205Relocation records have a standard format which 206is described by the 207.Fa relocation_info 208structure: 209.Bd -literal -offset indent 210struct relocation_info { 211 int r_address; 212 unsigned int r_symbolnum : 24, 213 r_pcrel : 1, 214 r_length : 2, 215 r_extern : 1, 216 r_baserel : 1, 217 r_jmptable : 1, 218 r_relative : 1, 219 r_copy : 1; 220}; 221.Ed 222.Pp 223The 224.Fa relocation_info 225fields are used as follows: 226.Bl -tag -width r_symbolnum 227.It Fa r_address 228Contains the byte offset of a pointer that needs to be link-edited. 229Text relocation offsets are reckoned from the start of the text segment, 230and data relocation offsets from the start of the data segment. 231The link editor adds the value that is already stored at this offset 232into the new value that it computes using this relocation record. 233.It Fa r_symbolnum 234Contains the ordinal number of a symbol structure 235in the symbol table (it is 236.Em not 237a byte offset). 238After the link editor resolves the absolute address for this symbol, 239it adds that address to the pointer that is undergoing relocation. 240(If the 241.Fa r_extern 242bit is clear, the situation is different; see below.) 243.It Fa r_pcrel 244If this is set, 245the link editor assumes that it is updating a pointer 246that is part of a machine code instruction using pc-relative addressing. 247The address of the relocated pointer is implicitly added 248to its value when the running program uses it. 249.It Fa r_length 250Contains the log base 2 of the length of the pointer in bytes; 2510 for 1-byte displacements, 1 for 2-byte displacements, 2522 for 4-byte displacements. 253.It Fa r_extern 254Set if this relocation requires an external reference; 255the link editor must use a symbol address to update the pointer. 256When the 257.Fa r_extern 258bit is clear, the relocation is 259.Sq local ; 260the link editor updates the pointer to reflect 261changes in the load addresses of the various segments, 262rather than changes in the value of a symbol (except when 263.Fa r_baserel 264is also set (see below). 265In this case, the content of the 266.Fa r_symbolnum 267field is an 268.Fa n_type 269value (see below); 270this type field tells the link editor 271what segment the relocated pointer points into. 272.It Fa r_baserel 273If set, the symbol, as identified by the 274.Fa r_symbolnum 275field, is to be relocated to an offset into the Global Offset Table. 276At run-time, the entry in the Global Offset Table at this offset is set to 277be the address of the symbol. 278.It Fa r_jmptable 279If set, the symbol, as identified by the 280.Fa r_symbolnum 281field, is to be relocated to an offset into the Procedure Linkage Table. 282.It Fa r_relative 283If set, this relocation is relative to the (run-time) load address of the 284image this object file is going to be a part of. 285This type of relocation 286only occurs in shared objects. 287.It Fa r_copy 288If set, this relocation record identifies a symbol whose contents should 289be copied to the location given in 290.Fa r_address. 291The copying is done by the run-time link-editor from a suitable data 292item in a shared object. 293.El 294.Pp 295Symbols map names to addresses (or more generally, strings to values). 296Since the link-editor adjusts addresses, 297a symbol's name must be used to stand for its address 298until an absolute value has been assigned. 299Symbols consist of a fixed-length record in the symbol table 300and a variable-length name in the string table. 301The symbol table is an array of 302.Fa nlist 303structures: 304.Bd -literal -offset indent 305struct nlist { 306 union { 307 char *n_name; 308 long n_strx; 309 } n_un; 310 unsigned char n_type; 311 char n_other; 312 short n_desc; 313 unsigned long n_value; 314}; 315.Ed 316.Pp 317The fields are used as follows: 318.Bl -tag -width n_un.n_strx 319.It Fa n_un.n_strx 320Contains a byte offset into the string table 321for the name of this symbol. 322When a program accesses a symbol table with the 323.Xr nlist 3 324function, 325this field is replaced with the 326.Fa n_un.n_name 327field, which is a pointer to the string in memory. 328.It Fa n_type 329Used by the link editor to determine 330how to update the symbol's value. 331The 332.Fa n_type 333field is broken down into three sub-fields using bitmasks. 334The link editor treats symbols with the 335.Dv N_EXT 336type bit set as 337.Sq external 338symbols and permits references to them from other binary files. 339The 340.Dv N_TYPE 341mask selects bits of interest to the link editor: 342.Bl -tag -width N_TEXT 343.It Dv N_UNDF 344An undefined symbol. 345The link editor must locate an external symbol with the same name 346in another binary file to determine the absolute value of this symbol. 347As a special case, if the 348.Fa n_value 349field is nonzero and no binary file in the link-edit defines this symbol, 350the link-editor will resolve this symbol to an address 351in the bss segment, 352reserving an amount of bytes equal to 353.Fa n_value . 354If this symbol is undefined in more than one binary file 355and the binary files do not agree on the size, 356the link editor chooses the greatest size found across all binaries. 357.It Dv N_ABS 358An absolute symbol. 359The link editor does not update an absolute symbol. 360.It Dv N_TEXT 361A text symbol. 362This symbol's value is a text address and 363the link editor will update it when it merges binary files. 364.It Dv N_DATA 365A data symbol; similar to 366.Dv N_TEXT 367but for data addresses. 368The values for text and data symbols are not file offsets but 369addresses; to recover the file offsets, it is necessary 370to identify the loaded address of the beginning of the corresponding 371section and subtract it, then add the offset of the section. 372.It Dv N_BSS 373A bss symbol; like text or data symbols but 374has no corresponding offset in the binary file. 375.It Dv N_FN 376A filename symbol. 377The link editor inserts this symbol before 378the other symbols from a binary file when 379merging binary files. 380The name of the symbol is the filename given to the link editor, 381and its value is the first text address from that binary file. 382Filename symbols are not needed for link-editing or loading, 383but are useful for debuggers. 384.El 385.Pp 386The 387.Dv N_STAB 388mask selects bits of interest to symbolic debuggers 389such as 390.Xr gdb 1 ; 391the values are described in 392.Xr stab 5 . 393.It Fa n_other 394This field provides information on the nature of the symbol independent of 395the symbol's location in terms of segments as determined by the 396.Fa n_type 397field. 398Currently, the lower 4 bits of the 399.Fa n_other 400field hold one of two values: 401.Dv AUX_FUNC 402and 403.Dv AUX_OBJECT 404.Po 405see 406.Aq Pa link.h 407for their definitions 408.Pc . 409.Dv AUX_FUNC 410associates the symbol with a callable function, while 411.Dv AUX_OBJECT 412associates the symbol with data, irrespective of their locations in 413either the text or the data segment. 414This field is intended to be used by 415.Xr ld 1 416for the construction of dynamic executables. 417.It Fa n_desc 418Reserved for use by debuggers; passed untouched by the link editor. 419Different debuggers use this field for different purposes. 420.It Fa n_value 421Contains the value of the symbol. 422For text, data and bss symbols, this is an address; 423for other symbols (such as debugger symbols), 424the value may be arbitrary. 425.El 426.Pp 427The string table consists of an 428.Em unsigned long 429length followed by null-terminated symbol strings. 430The length represents the size of the entire table in bytes, 431so its minimum value (or the offset of the first string) 432is always 4 on 32-bit machines. 433.Sh SEE ALSO 434.Xr as 1 , 435.Xr gdb 1 , 436.Xr ld 1 , 437.Xr brk 2 , 438.Xr execve 2 , 439.Xr nlist 3 , 440.Xr core 5 , 441.Xr elf 5 , 442.Xr link 5 , 443.Xr stab 5 444.Sh HISTORY 445The 446.Pa a.out.h 447include file appeared in 448.At v7 . 449.Sh BUGS 450Since not all of the supported architectures use the 451.Fa a_midmag 452field, 453it can be difficult to determine what 454architecture a binary will execute on 455without examining its actual machine code. 456Even with a machine identifier, 457the byte order of the 458.Fa exec 459header is machine-dependent. 460.Pp 461Nobody seems to agree on what 462.Em bss 463stands for. 464