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1 .\" $OpenBSD: a.out.5,v 1.15 2007/05/31 19:19:58 jmc Exp $
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34 .\" @(#)a.out.5 8.1 (Berkeley) 6/5/93
35 .\"
36 .Dd $Mdocdate: May 31 2007 $
37 .Dt A.OUT 5
38 .Os
39 .Sh NAME
40 .Nm a.out
41 .Nd format of executable binary files
42 .Sh SYNOPSIS
43 .Fd #include <sys/types.h>
44 .Fd #include <a.out.h>
45 .Sh DESCRIPTION
46 The include file
47 .Aq Pa a.out.h
48 declares three structures and several macros.
49 The structures describe the format of executable machine code files
50 .Pq Dq binaries
51 on the system.
52 .Pp
53 A binary file consists of up to 7 sections.
54 In order, these sections are:
55 .Bl -tag -width "text relocations"
56 .It exec header
57 Contains parameters used by the kernel to load a binary file into memory
58 and execute it, and by the link editor
59 .Xr ld 1
60 to combine a binary file with other binary files.
61 This section is the only mandatory one.
62 .It text segment
63 Contains machine code and related data
64 that are loaded into memory when a program executes.
65 May be loaded read-only.
66 .It data segment
67 Contains initialized data; always loaded into writable memory.
68 .It text relocations
69 Contains records used by the link editor
70 to update pointers in the text segment when combining binary files.
71 .It data relocations
72 Like the text relocation section, but for data segment pointers.
73 .It symbol table
74 Contains records used by the link editor
75 to cross reference the addresses of named variables and functions
76 .Pq Dq symbols
77 between binary files.
78 .It string table
79 Contains the character strings corresponding to the symbol names.
80 .El
81 .Pp
82 Every binary file begins with an
83 .Fa exec
84 structure:
85 .Bd -literal -offset indent
86 struct exec {
87 u_int32_t a_midmag;
88 u_int32_t a_text;
89 u_int32_t a_data;
90 u_int32_t a_bss;
91 u_int32_t a_syms;
92 u_int32_t a_entry;
93 u_int32_t a_trsize;
94 u_int32_t a_drsize;
95 };
96 .Ed
97 .Pp
98 The fields have the following functions:
99 .Bl -tag -width a_trsize
100 .It Fa a_midmag
101 This field is stored in network byte-order so that binaries for
102 machines with alternate byte orders can be distinguished.
103 It has a number of sub-components accessed by the macros
104 .Fn N_GETFLAG ,
105 .Fn N_GETMID ,
106 and
107 .Fn N_GETMAGIC ,
108 and set by the macro
109 .Fn N_SETMAGIC .
110 .Pp
111 The macro
112 .Fn N_GETFLAG
113 returns a few flags:
114 .Bl -tag -width EX_DYNAMIC
115 .It Dv EX_DYNAMIC
116 Indicates that the executable requires the services of the run-time link editor.
117 .It Dv EX_PIC
118 Indicates that the object contains position independent code.
119 This flag is set by
120 .Xr as 1
121 when given the
122 .Fl k
123 flag and is preserved by
124 .Xr ld 1
125 if necessary.
126 .El
127 .Pp
128 If both
129 .Dv EX_DYNAMIC
130 and
131 .Dv EX_PIC
132 are set, the object file is a position independent
133 executable image (e.g., a shared library), which is to be loaded into the
134 process address space by the run-time link editor.
135 .Pp
136 The macro
137 .Fn N_GETMID
138 returns the machine ID.
139 This indicates which machine(s) the binary is intended to run on.
140 .Pp
141 .Fn N_GETMAGIC
142 specifies the magic number, which uniquely identifies binary files
143 and distinguishes different loading conventions.
144 The field must contain one of the following values:
145 .Bl -tag -width ZMAGIC
146 .It Dv OMAGIC
147 The text and data segments immediately follow the header and are contiguous.
148 The kernel loads both text and data segments into writable memory.
149 .It Dv NMAGIC
150 As with
151 .Dv OMAGIC ,
152 text and data segments immediately follow the header and are contiguous.
153 However, the kernel loads the text into read-only memory and loads the data
154 into writable memory at the next page boundary after the text.
155 .It Dv ZMAGIC
156 The kernel loads individual pages on demand from the binary.
157 The header, text segment and data segment are all
158 padded by the link editor to a multiple of the page size.
159 Pages that the kernel loads from the text segment are read-only,
160 while pages from the data segment are writable.
161 .El
162 .It Fa a_text
163 Contains the size of the text segment in bytes.
164 .It Fa a_data
165 Contains the size of the data segment in bytes.
166 .It Fa a_bss
167 Contains the number of bytes in the
168 .Dq BSS segment
169 and is used by the kernel to set the initial break
170 .Pq Xr brk 2
171 after the data segment.
172 The kernel loads the program so that this amount of writable memory
173 appears to follow the data segment and initially reads as zeroes.
174 .It Fa a_syms
175 Contains the size in bytes of the symbol table section.
176 .It Fa a_entry
177 Contains the address in memory of the entry point
178 of the program after the kernel has loaded it;
179 the kernel starts the execution of the program
180 from the machine instruction at this address.
181 .It Fa a_trsize
182 Contains the size in bytes of the text relocation table.
183 .It Fa a_drsize
184 Contains the size in bytes of the data relocation table.
185 .El
186 .Pp
187 The
188 .Pa a.out.h
189 include file defines several macros which use an
190 .Fa exec
191 structure to test consistency or to locate section offsets in the binary file.
192 .Bl -tag -width N_TRELOFF(exec)
193 .It Fn N_BADMAG exec
194 Non-zero if the
195 .Fa a_magic
196 field does not contain a recognized value.
197 .It Fn N_TXTOFF exec
198 The byte offset of the beginning of the text segment.
199 .It Fn N_DATOFF exec
200 The byte offset of the beginning of the data segment.
201 .It Fn N_DRELOFF exec
202 The byte offset of the beginning of the data relocation table.
203 .It Fn N_TRELOFF exec
204 The byte offset of the beginning of the text relocation table.
205 .It Fn N_SYMOFF exec
206 The byte offset of the beginning of the symbol table.
207 .It Fn N_STROFF exec
208 The byte offset of the beginning of the string table.
209 .El
210 .Pp
211 Relocation records have a standard format which is described by the
212 .Fa relocation_info
213 structure:
214 .Bd -literal -offset indent
215 struct relocation_info {
216 int r_address;
217 unsigned int r_symbolnum : 24,
218 r_pcrel : 1,
219 r_length : 2,
220 r_extern : 1,
221 r_baserel : 1,
222 r_jmptable : 1,
223 r_relative : 1,
224 r_copy : 1;
225 };
226 .Ed
227 .Pp
228 The
229 .Fa relocation_info
230 fields are used as follows:
231 .Bl -tag -width r_symbolnum
232 .It Fa r_address
233 Contains the byte offset of a pointer that needs to be link-edited.
234 Text relocation offsets are reckoned from the start of the text segment,
235 and data relocation offsets from the start of the data segment.
236 The link editor adds the value that is already stored at this offset
237 into the new value that it computes using this relocation record.
238 .It Fa r_symbolnum
239 Contains the ordinal number of a symbol structure in the symbol table (it is
240 .Em not
241 a byte offset).
242 After the link editor resolves the absolute address for this symbol,
243 it adds that address to the pointer that is undergoing relocation.
244 (If the
245 .Fa r_extern
246 bit is clear, the situation is different; see below.)
247 .It Fa r_pcrel
248 If this is set, the link editor assumes that it is updating a pointer
249 that is part of a machine code instruction using pc-relative addressing.
250 The address of the relocated pointer is implicitly added
251 to its value when the running program uses it.
252 .It Fa r_length
253 Contains the log base 2 of the length of the pointer in bytes;
254 0 for 1-byte displacements, 1 for 2-byte displacements,
255 2 for 4-byte displacements.
256 .It Fa r_extern
257 Set if this relocation requires an external reference;
258 the link editor must use a symbol address to update the pointer.
259 When the
260 .Fa r_extern
261 bit is clear, the relocation is
262 .Dq local ;
263 the link editor updates the pointer to reflect
264 changes in the load addresses of the various segments,
265 rather than changes in the value of a symbol (except when
266 .Fa r_baserel
267 is also set, see below).
268 In this case, the content of the
269 .Fa r_symbolnum
270 field is an
271 .Fa n_type
272 value (see below);
273 this type field tells the link editor
274 what segment the relocated pointer points into.
275 .It Fa r_baserel
276 If set, the symbol, as identified by the
277 .Fa r_symbolnum
278 field, is to be relocated to an offset into the Global Offset Table.
279 At run-time, the entry in the Global Offset Table at this offset is set to
280 be the address of the symbol.
281 .It Fa r_jmptable
282 If set, the symbol, as identified by the
283 .Fa r_symbolnum
284 field, is to be relocated to an offset into the Procedure Linkage Table.
285 .It Fa r_relative
286 If set, this relocation is relative to the (run-time) load address of the
287 image this object file is going to be a part of.
288 This type of relocation only occurs in shared objects.
289 .It Fa r_copy
290 If set, this relocation record identifies a symbol whose contents should
291 be copied to the location given in
292 .Fa r_address .
293 The copying is done by the run-time link editor from a suitable data
294 item in a shared object.
295 .El
296 .Pp
297 Symbols map names to addresses (or more generally, strings to values).
298 Since the link editor adjusts addresses,
299 a symbol's name must be used to stand for its address
300 until an absolute value has been assigned.
301 Symbols consist of a fixed-length record in the symbol table
302 and a variable-length name in the string table.
303 The symbol table is an array of
304 .Fa nlist
305 structures:
306 .Bd -literal -offset indent
307 struct nlist {
308 union {
309 char *n_name;
310 long n_strx;
311 } n_un;
312 unsigned char n_type;
313 char n_other;
314 short n_desc;
315 unsigned long n_value;
316 };
317 .Ed
318 .Pp
319 The fields are used as follows:
320 .Bl -tag -width n_un.n_strx
321 .It Fa n_un.n_strx
322 Contains a byte offset into the string table for the name of this symbol.
323 When a program accesses a symbol table with the
324 .Xr nlist 3
325 function, this field is replaced with the
326 .Fa n_un.n_name
327 field, which is a pointer to the string in memory.
328 .It Fa n_type
329 Used by the link editor to determine how to update the symbol's value.
330 The
331 .Fa n_type
332 field is broken down into three sub-fields using bitmasks.
333 The link editor treats symbols with the
334 .Dv N_EXT
335 type bit set as
336 .Dq external
337 symbols and permits references to them from other binary files.
338 The
339 .Dv N_TYPE
340 mask selects bits of interest to the link editor:
341 .Bl -tag -width N_TEXT
342 .It Dv N_UNDF
343 An undefined symbol.
344 The link editor must locate an external symbol with the same name
345 in another binary file to determine the absolute value of this symbol.
346 As a special case, if the
347 .Fa n_value
348 field is non-zero and no binary file in the link-edit defines this symbol,
349 the link editor will resolve this symbol to an address
350 in the BSS segment, reserving an amount of bytes equal to
351 .Fa n_value .
352 If this symbol is undefined in more than one binary file
353 and the binary files do not agree on the size,
354 the link editor chooses the greatest size found across all binaries.
355 .It Dv N_ABS
356 An absolute symbol.
357 The link editor does not update an absolute symbol.
358 .It Dv N_TEXT
359 A text symbol.
360 This symbol's value is a text address and
361 the link editor will update it when it merges binary files.
362 .It Dv N_DATA
363 A data symbol; similar to
364 .Dv N_TEXT
365 but for data addresses.
366 The values for text and data symbols are not file offsets but
367 addresses; to recover the file offsets, it is necessary
368 to identify the loaded address of the beginning of the corresponding
369 section and subtract it, then add the offset of the section.
370 .It Dv N_BSS
371 A BSS symbol; like text or data symbols but
372 has no corresponding offset in the binary file.
373 .It Dv N_FN
374 A filename symbol.
375 The link editor inserts this symbol before
376 the other symbols from a binary file when
377 merging binary files.
378 The name of the symbol is the filename given to the link editor,
379 and its value is the first text address from that binary file.
380 Filename symbols are not needed for link editing or loading,
381 but are useful for debuggers.
382 .El
383 .Pp
384 The
385 .Dv N_STAB
386 mask selects bits of interest to symbolic debuggers
387 such as
388 .Xr gdb 1 ;
389 the values are described in
390 .Xr stab 5 .
391 .It Fa n_other
392 This field provides information on the nature of the symbol independent of
393 the symbol's location in terms of segments as determined by the
394 .Fa n_type
395 field.
396 Currently, the lower 4 bits of the
397 .Fa n_other
398 field hold one of two values:
399 .Dv AUX_FUNC
400 and
401 .Dv AUX_OBJECT
402 .Po
403 see
404 .Aq Pa link.h
405 for their definitions
406 .Pc .
407 .Dv AUX_FUNC
408 associates the symbol with a callable function, while
409 .Dv AUX_OBJECT
410 associates the symbol with data, irrespective of their locations in
411 either the text or the data segment.
412 This field is intended to be used by
413 .Xr ld 1
414 for the construction of dynamic executables.
415 .It Fa n_desc
416 Reserved for use by debuggers; passed untouched by the link editor.
417 Different debuggers use this field for different purposes.
418 .It Fa n_value
419 Contains the value of the symbol.
420 For text, data and BSS symbols, this is an address;
421 for other symbols (such as debugger symbols),
422 the value may be arbitrary.
423 .El
424 .Pp
425 The string table consists of an
426 .Em u_int32_t
427 length followed by null-terminated symbol strings.
428 The length represents the size of the entire table in bytes,
429 so its minimum value (or the offset of the first string)
430 is always 4 on 32-bit machines.
431 .Sh SEE ALSO
432 .Xr as 1 ,
433 .Xr gdb 1 ,
434 .Xr ld 1 ,
435 .Xr brk 2 ,
436 .Xr execve 2 ,
437 .Xr nlist 3 ,
438 .Xr core 5 ,
439 .Xr link 5 ,
440 .Xr stab 5
441 .Sh HISTORY
442 The
443 .Pa a.out.h
444 include file appeared in
445 .At v3 .
446 .Sh BUGS
447 Nobody seems to agree on what
448 .Em BSS
449 stands for.
450 .Pp
451 New binary file formats may be supported in the future,
452 and they probably will not be compatible at any level
453 with this ancient format.