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31 .\" @(#)a.out.5 8.1 (Berkeley) 6/5/93
39 .Nd format of executable binary files
45 declares three structures and several macros.
46 The structures describe the format of
47 executable machine code files
51 A binary file consists of up to 7 sections.
52 In order, these sections are:
53 .Bl -tag -width "text relocations"
55 Contains parameters used by the kernel
56 to load a binary file into memory and execute it,
57 and by the link editor
59 to combine a binary file with other binary files.
60 This section is the only mandatory one.
62 Contains machine code and related data
63 that are loaded into memory when a program executes.
64 May be loaded read-only.
66 Contains initialized data; always loaded into writable memory.
68 Contains records used by the link editor
69 to update pointers in the text segment when combining binary files.
71 Like the text relocation section, but for data segment pointers.
73 Contains records used by the link editor
74 to cross reference the addresses of named variables and functions
78 Contains the character strings corresponding to the symbol names.
81 Every binary file begins with an
84 .Bd -literal -offset indent
86 unsigned long a_midmag;
91 unsigned long a_entry;
92 unsigned long a_trsize;
93 unsigned long a_drsize;
97 The fields have the following functions:
98 .Bl -tag -width a_trsize
100 This field is stored in host byte-order.
101 It has a number of sub-components accessed by the macros
112 .Bl -tag -width EX_DYNAMIC
114 indicates that the executable requires the services of the run-time link editor.
116 indicates that the object contains position independent code.
122 flag and is preserved by
127 If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent
128 executable image (e.g.\& a shared library), which is to be loaded into the
129 process address space by the run-time link editor.
133 returns the machine-id.
134 This indicates which machine(s) the binary is intended to run on.
137 specifies the magic number, which uniquely identifies binary files
138 and distinguishes different loading conventions.
139 The field must contain one of the following values:
140 .Bl -tag -width ZMAGIC
142 The text and data segments immediately follow the header
144 The kernel loads both text and data segments into writable memory.
148 text and data segments immediately follow the header and are contiguous.
149 However, the kernel loads the text into read-only memory
150 and loads the data into writable memory at the next
151 page boundary after the text.
153 The kernel loads individual pages on demand from the binary.
154 The header, text segment and data segment are all
155 padded by the link editor to a multiple of the page size.
156 Pages that the kernel loads from the text segment are read-only,
157 while pages from the data segment are writable.
160 Contains the size of the text segment in bytes.
162 Contains the size of the data segment in bytes.
164 Contains the number of bytes in the
166 and is used by the kernel to set the initial break
168 after the data segment.
169 The kernel loads the program so that this amount of writable memory
170 appears to follow the data segment and initially reads as zeroes.
172 = block started by symbol)
174 Contains the size in bytes of the symbol table section.
176 Contains the address in memory of the entry point
177 of the program after the kernel has loaded it;
178 the kernel starts the execution of the program
179 from the machine instruction at this address.
181 Contains the size in bytes of the text relocation table.
183 Contains the size in bytes of the data relocation table.
188 include file defines several macros which use an
190 structure to test consistency or to locate section offsets in the binary file.
191 .Bl -tag -width N_BADMAG(exec)
195 field does not contain a recognized value.
197 The byte offset in the binary file of the beginning of the text segment.
199 The byte offset of the beginning of the symbol table.
201 The byte offset of the beginning of the string table.
204 Relocation records have a standard format which
208 .Bd -literal -offset indent
209 struct relocation_info {
211 unsigned int r_symbolnum : 24,
224 fields are used as follows:
225 .Bl -tag -width r_symbolnum
227 Contains the byte offset of a pointer that needs to be link-edited.
228 Text relocation offsets are reckoned from the start of the text segment,
229 and data relocation offsets from the start of the data segment.
230 The link editor adds the value that is already stored at this offset
231 into the new value that it computes using this relocation record.
233 Contains the ordinal number of a symbol structure
234 in the symbol table (it is
237 After the link editor resolves the absolute address for this symbol,
238 it adds that address to the pointer that is undergoing relocation.
241 bit is clear, the situation is different; see below.)
244 the link editor assumes that it is updating a pointer
245 that is part of a machine code instruction using pc-relative addressing.
246 The address of the relocated pointer is implicitly added
247 to its value when the running program uses it.
249 Contains the log base 2 of the length of the pointer in bytes;
250 0 for 1-byte displacements, 1 for 2-byte displacements,
251 2 for 4-byte displacements.
253 Set if this relocation requires an external reference;
254 the link editor must use a symbol address to update the pointer.
257 bit is clear, the relocation is
259 the link editor updates the pointer to reflect
260 changes in the load addresses of the various segments,
261 rather than changes in the value of a symbol (except when
263 is also set (see below).
264 In this case, the content of the
269 this type field tells the link editor
270 what segment the relocated pointer points into.
272 If set, the symbol, as identified by the
274 field, is to be relocated to an offset into the Global Offset Table.
275 At run-time, the entry in the Global Offset Table at this offset is set to
276 be the address of the symbol.
278 If set, the symbol, as identified by the
280 field, is to be relocated to an offset into the Procedure Linkage Table.
282 If set, this relocation is relative to the (run-time) load address of the
283 image this object file is going to be a part of.
284 This type of relocation
285 only occurs in shared objects.
287 If set, this relocation record identifies a symbol whose contents should
288 be copied to the location given in
290 The copying is done by the run-time link-editor from a suitable data
291 item in a shared object.
294 Symbols map names to addresses (or more generally, strings to values).
295 Since the link-editor adjusts addresses,
296 a symbol's name must be used to stand for its address
297 until an absolute value has been assigned.
298 Symbols consist of a fixed-length record in the symbol table
299 and a variable-length name in the string table.
300 The symbol table is an array of
303 .Bd -literal -offset indent
309 unsigned char n_type;
312 unsigned long n_value;
316 The fields are used as follows:
317 .Bl -tag -width n_un.n_strx
319 Contains a byte offset into the string table
320 for the name of this symbol.
321 When a program accesses a symbol table with the
324 this field is replaced with the
326 field, which is a pointer to the string in memory.
328 Used by the link editor to determine
329 how to update the symbol's value.
332 field is broken down into three sub-fields using bitmasks.
333 The link editor treats symbols with the
337 symbols and permits references to them from other binary files.
340 mask selects bits of interest to the link editor:
341 .Bl -tag -width N_TEXT
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
348 field is nonzero and no binary file in the link-edit defines this symbol,
349 the link-editor will resolve this symbol to an address
351 reserving an amount of bytes equal to
353 If this symbol is undefined in more than one binary file
354 and the binary files do not agree on the size,
355 the link editor chooses the greatest size found across all binaries.
358 The link editor does not update an absolute symbol.
361 This symbol's value is a text address and
362 the link editor will update it when it merges binary files.
364 A data symbol; similar to
366 but for data addresses.
367 The values for text and data symbols are not file offsets but
368 addresses; to recover the file offsets, it is necessary
369 to identify the loaded address of the beginning of the corresponding
370 section and subtract it, then add the offset of the section.
372 A bss symbol; like text or data symbols but
373 has no corresponding offset in the binary file.
376 The link editor inserts this symbol before
377 the other symbols from a binary file when
378 merging binary files.
379 The name of the symbol is the filename given to the link editor,
380 and its value is the first text address from that binary file.
381 Filename symbols are not needed for link-editing or loading,
382 but are useful for debuggers.
387 mask selects bits of interest to symbolic debuggers
390 the values are described in
393 This field provides information on the nature of the symbol independent of
394 the symbol's location in terms of segments as determined by the
397 Currently, the lower 4 bits of the
399 field hold one of two values:
405 for their definitions).
407 associates the symbol with a callable function, while
409 associates the symbol with data, irrespective of their locations in
410 either the text or the data segment.
411 This field is intended to be used by
413 for the construction of dynamic executables.
415 Reserved for use by debuggers; passed untouched by the link editor.
416 Different debuggers use this field for different purposes.
418 Contains the value of the symbol.
419 For text, data and bss symbols, this is an address;
420 for other symbols (such as debugger symbols),
421 the value may be arbitrary.
424 The string table consists of an
426 length followed by null-terminated symbol strings.
427 The length represents the size of the entire table in bytes,
428 so its minimum value (or the offset of the first string)
429 is always 4 on 32-bit machines.
444 include file appeared in
447 Since not all of the supported architectures use the
450 it can be difficult to determine what
451 architecture a binary will execute on
452 without examining its actual machine code.
453 Even with a machine identifier,
454 the byte order of the
456 header is machine-dependent.