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6 .\" This man page is derived from documentation contributed to Berkeley by
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33 .\" @(#)a.out.5 8.1 (Berkeley) 6/5/93
34 .\"
35 .Dd June 5, 1993
36 .Dt A.OUT 5
37 .Os
38 .Sh NAME
39 .Nm a.out
40 .Nd format of executable binary files
41 .Sh SYNOPSIS
42 .In sys/types.h
43 .In a.out.h
44 .Sh DESCRIPTION
45 The include file
46 .Aq Pa a.out.h
47 declares three structures and several macros.
48 The structures describe the format of
49 executable machine code files
50 .Pq Sq 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
58 to load a binary file into memory and execute it,
59 and by the link editor
60 .Xr ld 1
61 to combine a binary file with other binary files.
62 This section is the only mandatory one.
63 .It text segment
64 Contains machine code and related data
65 that are loaded into memory when a program executes.
66 May be loaded read-only.
67 .It data segment
68 Contains initialized data; always loaded into writable memory.
69 .It text relocations
70 Contains records used by the link editor
71 to update pointers in the text segment when combining binary files.
72 .It data relocations
73 Like the text relocation section, but for data segment pointers.
74 .It symbol table
75 Contains records used by the link editor
76 to cross reference the addresses of named variables and functions
77 .Pq Sq symbols
78 between binary files.
79 .It string table
80 Contains the character strings corresponding to the symbol names.
81 .El
82 .Pp
83 Every binary file begins with an
84 .Fa exec
85 structure:
86 .Bd -literal -offset indent
87 struct exec {
88 unsigned long a_midmag;
89 unsigned long a_text;
90 unsigned long a_data;
91 unsigned long a_bss;
92 unsigned long a_syms;
93 unsigned long a_entry;
94 unsigned long a_trsize;
95 unsigned long a_drsize;
96 };
97 .Ed
98 .Pp
99 The fields have the following functions:
100 .Bl -tag -width a_trsize
101 .It Fa a_midmag
102 This field is stored in network byte-order so that binaries for
103 machines with alternative byte orders can be distinguished.
104 It has a number of sub-components accessed by the macros
105 .Dv N_GETFLAG() ,
106 .Dv N_GETMID() , and
107 .Dv N_GETMAGIC() ,
108 and set by the macro
109 .Dv N_SETMAGIC() .
110 .Pp
111 The macro
112 .Dv 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. This flag is
119 set by
120 .Xr as 1
121 when given the
122 .Sq -k
123 flag and is preserved by
124 .Xr ld 1
125 if necessary.
126 .El
127 .Pp
128 If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent
129 executable image (e.g. a shared library), which is to be loaded into the
130 process address space by the run-time link editor.
131 .Pp
132 The macro
133 .Dv N_GETMID()
134 returns the machine-id.
135 This indicates which machine(s) the binary is intended to run on.
136 .Pp
137 .Dv N_GETMAGIC()
138 specifies the magic number, which uniquely identifies binary files
139 and distinguishes different loading conventions.
140 The field must contain one of the following values:
141 .Bl -tag -width ZMAGIC
142 .It Dv OMAGIC
143 The text and data segments immediately follow the header
144 and are contiguous.
145 The kernel loads both text and data segments into writable memory.
146 .It Dv NMAGIC
147 As with
148 .Dv OMAGIC ,
149 text and data segments immediately follow the header and are contiguous.
150 However, the kernel loads the text into read-only memory
151 and loads the data into writable memory at the next
152 page boundary after the text.
153 .It Dv ZMAGIC
154 The kernel loads individual pages on demand from the binary.
155 The header, text segment and data segment are all
156 padded by the link editor to a multiple of the page size.
157 Pages that the kernel loads from the text segment are read-only,
158 while pages from the data segment are writable.
159 .El
160 .It Fa a_text
161 Contains the size of the text segment in bytes.
162 .It Fa a_data
163 Contains the size of the data segment in bytes.
164 .It Fa a_bss
165 Contains the number of bytes in the
166 .Sq bss segment
167 and is used by the kernel to set the initial break
168 .Pq Xr brk 2
169 after the data segment.
170 The kernel loads the program so that this amount of writable memory
171 appears to follow the data segment and initially reads as zeroes.
172 .It Fa a_syms
173 Contains the size in bytes of the symbol table section.
174 .It Fa a_entry
175 Contains the address in memory of the entry point
176 of the program after the kernel has loaded it;
177 the kernel starts the execution of the program
178 from the machine instruction at this address.
179 .It Fa a_trsize
180 Contains the size in bytes of the text relocation table.
181 .It Fa a_drsize
182 Contains the size in bytes of the data relocation table.
183 .El
184 .Pp
185 The
186 .Pa a.out.h
187 include file defines several macros which use an
188 .Fa exec
189 structure to test consistency or to locate section offsets in the binary file.
190 .Bl -tag -width N_BADMAG(exec)
191 .It Fn N_BADMAG exec
192 Nonzero if the
193 .Fa a_magic
194 field does not contain a recognized value.
195 .It Fn N_TXTOFF exec
196 The byte offset in the binary file of the beginning of the text segment.
197 .It Fn N_SYMOFF exec
198 The byte offset of the beginning of the symbol table.
199 .It Fn N_STROFF exec
200 The byte offset of the beginning of the string table.
201 .El
202 .Pp
203 Relocation records have a standard format which
204 is described by the
205 .Fa relocation_info
206 structure:
207 .Bd -literal -offset indent
208 struct relocation_info {
209 int r_address;
210 unsigned int r_symbolnum : 24,
211 r_pcrel : 1,
212 r_length : 2,
213 r_extern : 1,
214 r_baserel : 1,
215 r_jmptable : 1,
216 r_relative : 1,
217 r_copy : 1;
218 };
219 .Ed
220 .Pp
221 The
222 .Fa relocation_info
223 fields are used as follows:
224 .Bl -tag -width r_symbolnum
225 .It Fa r_address
226 Contains the byte offset of a pointer that needs to be link-edited.
227 Text relocation offsets are reckoned from the start of the text segment,
228 and data relocation offsets from the start of the data segment.
229 The link editor adds the value that is already stored at this offset
230 into the new value that it computes using this relocation record.
231 .It Fa r_symbolnum
232 Contains the ordinal number of a symbol structure
233 in the symbol table (it is
234 .Em not
235 a byte offset).
236 After the link editor resolves the absolute address for this symbol,
237 it adds that address to the pointer that is undergoing relocation.
238 (If the
239 .Fa r_extern
240 bit is clear, the situation is different; see below.)
241 .It Fa r_pcrel
242 If this is set,
243 the link editor assumes that it is updating a pointer
244 that is part of a machine code instruction using pc-relative addressing.
245 The address of the relocated pointer is implicitly added
246 to its value when the running program uses it.
247 .It Fa r_length
248 Contains the log base 2 of the length of the pointer in bytes;
249 0 for 1-byte displacements, 1 for 2-byte displacements,
250 2 for 4-byte displacements.
251 .It Fa r_extern
252 Set if this relocation requires an external reference;
253 the link editor must use a symbol address to update the pointer.
254 When the
255 .Fa r_extern
256 bit is clear, the relocation is
257 .Sq local ;
258 the link editor updates the pointer to reflect
259 changes in the load addresses of the various segments,
260 rather than changes in the value of a symbol (except when
261 .Fa r_baserel
262 is also set, see below).
263 In this case, the content of the
264 .Fa r_symbolnum
265 field is an
266 .Fa n_type
267 value (see below);
268 this type field tells the link editor
269 what segment the relocated pointer points into.
270 .It Fa r_baserel
271 If set, the symbol, as identified by the
272 .Fa r_symbolnum
273 field, is to be relocated to an offset into the Global Offset Table.
274 At run-time, the entry in the Global Offset Table at this offset is set to
275 be the address of the symbol.
276 .It Fa r_jmptable
277 If set, the symbol, as identified by the
278 .Fa r_symbolnum
279 field, is to be relocated to an offset into the Procedure Linkage Table.
280 .It Fa r_relative
281 If set, this relocation is relative to the (run-time) load address of the
282 image this object file is going to be a part of. This type of relocation
283 only occurs in shared objects.
284 .It Fa r_copy
285 If set, this relocation record identifies a symbol whose contents should
286 be copied to the location given in
287 .Fa r_address .
288 The copying is done by the run-time link-editor from a suitable data
289 item in a shared object.
290 .El
291 .Pp
292 Symbols map names to addresses (or more generally, strings to values).
293 Since the link-editor adjusts addresses,
294 a symbol's name must be used to stand for its address
295 until an absolute value has been assigned.
296 Symbols consist of a fixed-length record in the symbol table
297 and a variable-length name in the string table.
298 The symbol table is an array of
299 .Fa nlist
300 structures:
301 .Bd -literal -offset indent
302 struct nlist {
303 union {
304 char *n_name;
305 long n_strx;
306 } n_un;
307 unsigned char n_type;
308 char n_other;
309 short n_desc;
310 unsigned long n_value;
311 };
312 .Ed
313 .Pp
314 The fields are used as follows:
315 .Bl -tag -width n_un.n_strx
316 .It Fa n_un.n_strx
317 Contains a byte offset into the string table
318 for the name of this symbol.
319 When a program accesses a symbol table with the
320 .Xr nlist 3
321 function,
322 this field is replaced with the
323 .Fa n_un.n_name
324 field, which is a pointer to the string in memory.
325 .It Fa n_type
326 Used by the link editor to determine
327 how to update the symbol's value.
328 The
329 .Fa n_type
330 field is broken down into three sub-fields using bitmasks.
331 The link editor treats symbols with the
332 .Dv N_EXT
333 type bit set as
334 .Sq external
335 symbols and permits references to them from other binary files.
336 The
337 .Dv N_TYPE
338 mask selects bits of interest to the link editor:
339 .Bl -tag -width N_TEXT
340 .It Dv N_UNDF
341 An undefined symbol.
342 The link editor must locate an external symbol with the same name
343 in another binary file to determine the absolute value of this symbol.
344 As a special case, if the
345 .Fa n_value
346 field is nonzero and no binary file in the link-edit defines this symbol,
347 the link-editor will resolve this symbol to an address
348 in the bss segment,
349 reserving an amount of bytes equal to
350 .Fa n_value .
351 If this symbol is undefined in more than one binary file
352 and the binary files do not agree on the size,
353 the link editor chooses the greatest size found across all binaries.
354 .It Dv N_ABS
355 An absolute symbol.
356 The link editor does not update an absolute symbol.
357 .It Dv N_TEXT
358 A text symbol.
359 This symbol's value is a text address and
360 the link editor will update it when it merges binary files.
361 .It Dv N_DATA
362 A data symbol; similar to
363 .Dv N_TEXT
364 but for data addresses.
365 The values for text and data symbols are not file offsets but
366 addresses; to recover the file offsets, it is necessary
367 to identify the loaded address of the beginning of the corresponding
368 section and subtract it, then add the offset of the section.
369 .It Dv N_BSS
370 A bss symbol; like text or data symbols but
371 has no corresponding offset in the binary file.
372 .It Dv N_FN
373 A filename symbol.
374 The link editor inserts this symbol before
375 the other symbols from a binary file when
376 merging binary files.
377 The name of the symbol is the filename given to the link editor,
378 and its value is the first text address from that binary file.
379 Filename symbols are not needed for link-editing or loading,
380 but are useful for debuggers.
381 .El
382 .Pp
383 The
384 .Dv N_STAB
385 mask selects bits of interest to symbolic debuggers
386 such as
387 .Xr gdb 1 ;
388 the values are described in
389 .Xr stab 5 .
390 .It Fa n_other
391 This field provides information on the nature of the symbol independent of
392 the symbol's location in terms of segments as determined by the
393 .Fa n_type
394 field. Currently, the lower 4 bits of the
395 .Fa n_other
396 field hold one of two values:
397 .Dv AUX_FUNC
398 and
399 .Dv AUX_OBJECT
400 .Po
401 see
402 .Aq Pa link.h
403 for their definitions
404 .Pc .
405 .Dv AUX_FUNC
406 associates the symbol with a callable function, while
407 .Dv AUX_OBJECT
408 associates the symbol with data, irrespective of their locations in
409 either the text or the data segment.
410 This field is intended to be used by
411 .Xr ld 1
412 for the construction of dynamic executables.
413 .It Fa n_desc
414 Reserved for use by debuggers; passed untouched by the link editor.
415 Different debuggers use this field for different purposes.
416 .It Fa n_value
417 Contains the value of the symbol.
418 For text, data and bss symbols, this is an address;
419 for other symbols (such as debugger symbols),
420 the value may be arbitrary.
421 .El
422 .Pp
423 The string table consists of an
424 .Em unsigned long
425 length followed by null-terminated symbol strings.
426 The length represents the size of the entire table in bytes,
427 so its minimum value (or the offset of the first string)
428 is always 4 on 32-bit machines.
429 .Sh SEE ALSO
430 .Xr as 1 ,
431 .Xr gdb 1 ,
432 .Xr ld 1 ,
433 .Xr brk 2 ,
434 .Xr execve 2 ,
435 .Xr nlist 3 ,
436 .Xr core 5 ,
437 .Xr elf 5 ,
438 .Xr link 5 ,
439 .Xr stab 5
440 .Sh HISTORY
441 The
442 .Pa a.out.h
443 include file appeared in
444 .At v7 .
445 .Sh BUGS
446 Nobody seems to agree on what
447 .Em bss
448 stands for.
449 .Pp
450 New binary file formats may be supported in the future,
451 and they probably will not be compatible at any level
452 with this ancient format.