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31 .\" @(#)a.out.5 8.1 (Berkeley) 6/5/93
32 .\" $FreeBSD: src/share/man/man5/a.out.5,v 1.10.2.4 2002/04/16 14:50:18 trhodes Exp $
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40 .Nd format of executable binary files
46 declares three structures and several macros.
47 The structures describe the format of
48 executable machine code files
52 A binary file consists of up to 7 sections.
53 In order, these sections are:
54 .Bl -tag -width "text relocations"
56 Contains parameters used by the kernel
57 to load a binary file into memory and execute it,
58 and by the link editor
60 to combine a binary file with other binary files.
61 This section is the only mandatory one.
63 Contains machine code and related data
64 that are loaded into memory when a program executes.
65 May be loaded read-only.
67 Contains initialized data; always loaded into writable memory.
69 Contains records used by the link editor
70 to update pointers in the text segment when combining binary files.
72 Like the text relocation section, but for data segment pointers.
74 Contains records used by the link editor
75 to cross reference the addresses of named variables and functions
79 Contains the character strings corresponding to the symbol names.
82 Every binary file begins with an
85 .Bd -literal -offset indent
87 unsigned long a_midmag;
92 unsigned long a_entry;
93 unsigned long a_trsize;
94 unsigned long a_drsize;
98 The fields have the following functions:
99 .Bl -tag -width a_trsize
101 This field is stored in host byte-order.
102 It has a number of sub-components accessed by the macros
113 .Bl -tag -width EX_DYNAMIC
115 indicates that the executable requires the services of the run-time link editor.
117 indicates that the object contains position independent code.
123 flag and is preserved by
128 If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent
129 executable image (eg. a shared library), which is to be loaded into the
130 process address space by the run-time link editor.
134 returns the machine-id.
135 This indicates which machine(s) the binary is intended to run on.
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
143 The text and data segments immediately follow the header
145 The kernel loads both text and data segments into writable memory.
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.
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.
161 Contains the size of the text segment in bytes.
163 Contains the size of the data segment in bytes.
165 Contains the number of bytes in the
167 and is used by the kernel to set the initial break
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.
174 = block started by symbol
177 Contains the size in bytes of the symbol table section.
179 Contains the address in memory of the entry point
180 of the program after the kernel has loaded it;
181 the kernel starts the execution of the program
182 from the machine instruction at this address.
184 Contains the size in bytes of the text relocation table.
186 Contains the size in bytes of the data relocation table.
191 include file defines several macros which use an
193 structure to test consistency or to locate section offsets in the binary file.
194 .Bl -tag -width N_BADMAG(exec)
198 field does not contain a recognized value.
200 The byte offset in the binary file of the beginning of the text segment.
202 The byte offset of the beginning of the symbol table.
204 The byte offset of the beginning of the string table.
207 Relocation records have a standard format which
211 .Bd -literal -offset indent
212 struct relocation_info {
214 unsigned int r_symbolnum : 24,
227 fields are used as follows:
228 .Bl -tag -width r_symbolnum
230 Contains the byte offset of a pointer that needs to be link-edited.
231 Text relocation offsets are reckoned from the start of the text segment,
232 and data relocation offsets from the start of the data segment.
233 The link editor adds the value that is already stored at this offset
234 into the new value that it computes using this relocation record.
236 Contains the ordinal number of a symbol structure
237 in the symbol table (it is
240 After the link editor resolves the absolute address for this symbol,
241 it adds that address to the pointer that is undergoing relocation.
244 bit is clear, the situation is different; see below.)
247 the link editor assumes that it is updating a pointer
248 that is part of a machine code instruction using pc-relative addressing.
249 The address of the relocated pointer is implicitly added
250 to its value when the running program uses it.
252 Contains the log base 2 of the length of the pointer in bytes;
253 0 for 1-byte displacements, 1 for 2-byte displacements,
254 2 for 4-byte displacements.
256 Set if this relocation requires an external reference;
257 the link editor must use a symbol address to update the pointer.
260 bit is clear, the relocation is
262 the link editor updates the pointer to reflect
263 changes in the load addresses of the various segments,
264 rather than changes in the value of a symbol (except when
266 is also set (see below).
267 In this case, the content of the
272 this type field tells the link editor
273 what segment the relocated pointer points into.
275 If set, the symbol, as identified by the
277 field, is to be relocated to an offset into the Global Offset Table.
278 At run-time, the entry in the Global Offset Table at this offset is set to
279 be the address of the symbol.
281 If set, the symbol, as identified by the
283 field, is to be relocated to an offset into the Procedure Linkage Table.
285 If set, this relocation is relative to the (run-time) load address of the
286 image this object file is going to be a part of.
287 This type of relocation
288 only occurs in shared objects.
290 If set, this relocation record identifies a symbol whose contents should
291 be copied to the location given in
293 The copying is done by the run-time link-editor from a suitable data
294 item in a shared object.
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
306 .Bd -literal -offset indent
312 unsigned char n_type;
315 unsigned long n_value;
319 The fields are used as follows:
320 .Bl -tag -width n_un.n_strx
322 Contains a byte offset into the string table
323 for the name of this symbol.
324 When a program accesses a symbol table with the
327 this field is replaced with the
329 field, which is a pointer to the string in memory.
331 Used by the link editor to determine
332 how to update the symbol's value.
335 field is broken down into three sub-fields using bitmasks.
336 The link editor treats symbols with the
340 symbols and permits references to them from other binary files.
343 mask selects bits of interest to the link editor:
344 .Bl -tag -width N_TEXT
347 The link editor must locate an external symbol with the same name
348 in another binary file to determine the absolute value of this symbol.
349 As a special case, if the
351 field is nonzero and no binary file in the link-edit defines this symbol,
352 the link-editor will resolve this symbol to an address
354 reserving an amount of bytes equal to
356 If this symbol is undefined in more than one binary file
357 and the binary files do not agree on the size,
358 the link editor chooses the greatest size found across all binaries.
361 The link editor does not update an absolute symbol.
364 This symbol's value is a text address and
365 the link editor will update it when it merges binary files.
367 A data symbol; similar to
369 but for data addresses.
370 The values for text and data symbols are not file offsets but
371 addresses; to recover the file offsets, it is necessary
372 to identify the loaded address of the beginning of the corresponding
373 section and subtract it, then add the offset of the section.
375 A bss symbol; like text or data symbols but
376 has no corresponding offset in the binary file.
379 The link editor inserts this symbol before
380 the other symbols from a binary file when
381 merging binary files.
382 The name of the symbol is the filename given to the link editor,
383 and its value is the first text address from that binary file.
384 Filename symbols are not needed for link-editing or loading,
385 but are useful for debuggers.
390 mask selects bits of interest to symbolic debuggers
393 the values are described in
396 This field provides information on the nature of the symbol independent of
397 the symbol's location in terms of segments as determined by the
400 Currently, the lower 4 bits of the
402 field hold one of two values:
408 for their definitions).
410 associates the symbol with a callable function, while
412 associates the symbol with data, irrespective of their locations in
413 either the text or the data segment.
414 This field is intended to be used by
416 for the construction of dynamic executables.
418 Reserved for use by debuggers; passed untouched by the link editor.
419 Different debuggers use this field for different purposes.
421 Contains the value of the symbol.
422 For text, data and bss symbols, this is an address;
423 for other symbols (such as debugger symbols),
424 the value may be arbitrary.
427 The string table consists of an
429 length followed by null-terminated symbol strings.
430 The length represents the size of the entire table in bytes,
431 so its minimum value (or the offset of the first string)
432 is always 4 on 32-bit machines.
447 include file appeared in
450 Since not all of the supported architectures use the
453 it can be difficult to determine what
454 architecture a binary will execute on
455 without examining its actual machine code.
456 Even with a machine identifier,
457 the byte order of the
459 header is machine-dependent.