1.\" $NetBSD: a.out.5,v 1.8 1994/11/30 19:31:09 jtc Exp $ 2.\" 3.\" Copyright (c) 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This man page is derived from documentation contributed to Berkeley by 7.\" Donn Seeley at UUNET Technologies, Inc. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the University of 20.\" California, Berkeley and its contributors. 21.\" 4. Neither the name of the University nor the names of its contributors 22.\" may be used to endorse or promote products derived from this software 23.\" without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 26.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 27.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 28.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 29.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 30.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 31.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 32.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 33.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 34.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 35.\" SUCH DAMAGE. 36.\" 37.\" @(#)a.out.5 8.1 (Berkeley) 6/5/93 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.Fd #include <a.out.h> 47.Sh DESCRIPTION 48The include file 49.Aq Pa 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 network byte-order so that binaries for 106for machines with alternate byte orders can be distinguished. 107It has a number of sub-components accessed by the macros 108.Dv N_GETFLAG() , 109.Dv N_GETMID() , and 110.Dv N_GETMAGIC() , 111and set by the macro 112.Dv N_SETMAGIC(). 113.Pp 114The macro 115.Dv 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. This 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 indendent 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.Dv N_GETMID() 137returns the machine-id. 138This indicates which machine(s) the binary is intended to run on. 139.Pp 140.Dv 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. This 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. Currently, the lower 4 bits of the 398.Fa n_other 399field hold one of two values: 400.Dv AUX_FUNC 401and 402.Dv AUX_OBJECT 403.Po 404see 405.Aq Pa link.h 406for their definitions 407.Pc . 408.Dv AUX_FUNC 409associates the symbol with a callable function, while 410.Dv AUX_OBJECT 411associates the symbol with data, irrespective of their locations in 412either the text or the data segment. 413This field is intended to be used by 414.Xr ld 1 415for the construction of dynamic executables. 416.It Fa n_desc 417Reserved for use by debuggers; passed untouched by the link editor. 418Different debuggers use this field for different purposes. 419.It Fa n_value 420Contains the value of the symbol. 421For text, data and bss symbols, this is an address; 422for other symbols (such as debugger symbols), 423the value may be arbitrary. 424.El 425.Pp 426The string table consists of an 427.Em unsigned long 428length followed by null-terminated symbol strings. 429The length represents the size of the entire table in bytes, 430so its minimum value (or the offset of the first string) 431is always 4 on 32-bit machines. 432.Sh SEE ALSO 433.Xr as 1 , 434.Xr gdb 1 , 435.Xr ld 1 , 436.Xr brk 2 , 437.Xr execve 2 , 438.Xr nlist 3 , 439.Xr core 5 , 440.Xr dbx 5 , 441.Xr stab 5 , 442.Xr link 5 443.Sh HISTORY 444The 445.Pa a.out.h 446include file appeared in 447.At v7 . 448.Sh BUGS 449Nobody seems to agree on what 450.Em bss 451stands for. 452.Pp 453New binary file formats may be supported in the future, 454and they probably will not be compatible at any level 455with this ancient format. 456