xref: /freebsd/share/man/man5/a.out.5 (revision 1e413cf93298b5b97441a21d9a50fdcd0ee9945e)
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5.\" Donn Seeley at UUNET Technologies, Inc.
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35.\"	@(#)a.out.5	8.1 (Berkeley) 6/5/93
36.\" $FreeBSD$
37.\"
38.Dd June 5, 1993
39.Dt A.OUT 5
40.Os
41.Sh NAME
42.Nm a.out
43.Nd format of executable binary files
44.Sh SYNOPSIS
45.In a.out.h
46.Sh DESCRIPTION
47The include file
48.In a.out.h
49declares three structures and several macros.
50The structures describe the format of
51executable machine code files
52.Pq Sq binaries
53on the system.
54.Pp
55A binary file consists of up to 7 sections.
56In order, these sections are:
57.Bl -tag -width "text relocations"
58.It exec header
59Contains parameters used by the kernel
60to load a binary file into memory and execute it,
61and by the link editor
62.Xr ld 1
63to combine a binary file with other binary files.
64This section is the only mandatory one.
65.It text segment
66Contains machine code and related data
67that are loaded into memory when a program executes.
68May be loaded read-only.
69.It data segment
70Contains initialized data; always loaded into writable memory.
71.It text relocations
72Contains records used by the link editor
73to update pointers in the text segment when combining binary files.
74.It data relocations
75Like the text relocation section, but for data segment pointers.
76.It symbol table
77Contains records used by the link editor
78to cross reference the addresses of named variables and functions
79.Pq Sq symbols
80between binary files.
81.It string table
82Contains the character strings corresponding to the symbol names.
83.El
84.Pp
85Every binary file begins with an
86.Fa exec
87structure:
88.Bd -literal -offset indent
89struct exec {
90	unsigned long	a_midmag;
91	unsigned long	a_text;
92	unsigned long	a_data;
93	unsigned long	a_bss;
94	unsigned long	a_syms;
95	unsigned long	a_entry;
96	unsigned long	a_trsize;
97	unsigned long	a_drsize;
98};
99.Ed
100.Pp
101The fields have the following functions:
102.Bl -tag -width a_trsize
103.It Fa a_midmag
104This field is stored in host byte-order.
105It has a number of sub-components accessed by the macros
106.Fn N_GETFLAG ,
107.Fn N_GETMID ,
108and
109.Fn N_GETMAGIC ,
110and set by the macro
111.Fn N_SETMAGIC .
112.Pp
113The macro
114.Fn N_GETFLAG
115returns a few flags:
116.Bl -tag -width EX_DYNAMIC
117.It Dv EX_DYNAMIC
118indicates that the executable requires the services of the run-time link editor.
119.It Dv EX_PIC
120indicates that the object contains position independent code.
121This 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 independent
132executable image (e.g.\& a shared library), which is to be loaded into the
133process address space by the run-time link editor.
134.Pp
135The macro
136.Fn N_GETMID
137returns the machine-id.
138This indicates which machine(s) the binary is intended to run on.
139.Pp
140.Fn 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.Em ( bss
176= block started by symbol)
177.It Fa a_syms
178Contains the size in bytes of the symbol table section.
179.It Fa a_entry
180Contains the address in memory of the entry point
181of the program after the kernel has loaded it;
182the kernel starts the execution of the program
183from the machine instruction at this address.
184.It Fa a_trsize
185Contains the size in bytes of the text relocation table.
186.It Fa a_drsize
187Contains the size in bytes of the data relocation table.
188.El
189.Pp
190The
191.In a.out.h
192include file defines several macros which use an
193.Fa exec
194structure to test consistency or to locate section offsets in the binary file.
195.Bl -tag -width N_BADMAG(exec)
196.It Fn N_BADMAG exec
197Nonzero if the
198.Fa a_magic
199field does not contain a recognized value.
200.It Fn N_TXTOFF exec
201The byte offset in the binary file of the beginning of the text segment.
202.It Fn N_SYMOFF exec
203The byte offset of the beginning of the symbol table.
204.It Fn N_STROFF exec
205The byte offset of the beginning of the string table.
206.El
207.Pp
208Relocation records have a standard format which
209is described by the
210.Fa relocation_info
211structure:
212.Bd -literal -offset indent
213struct relocation_info {
214	int		r_address;
215	unsigned int	r_symbolnum : 24,
216			r_pcrel : 1,
217			r_length : 2,
218			r_extern : 1,
219			r_baserel : 1,
220			r_jmptable : 1,
221			r_relative : 1,
222			r_copy : 1;
223};
224.Ed
225.Pp
226The
227.Fa relocation_info
228fields are used as follows:
229.Bl -tag -width r_symbolnum
230.It Fa r_address
231Contains the byte offset of a pointer that needs to be link-edited.
232Text relocation offsets are reckoned from the start of the text segment,
233and data relocation offsets from the start of the data segment.
234The link editor adds the value that is already stored at this offset
235into the new value that it computes using this relocation record.
236.It Fa r_symbolnum
237Contains the ordinal number of a symbol structure
238in the symbol table (it is
239.Em not
240a byte offset).
241After the link editor resolves the absolute address for this symbol,
242it adds that address to the pointer that is undergoing relocation.
243(If the
244.Fa r_extern
245bit is clear, the situation is different; see below.)
246.It Fa r_pcrel
247If this is set,
248the link editor assumes that it is updating a pointer
249that is part of a machine code instruction using pc-relative addressing.
250The address of the relocated pointer is implicitly added
251to its value when the running program uses it.
252.It Fa r_length
253Contains the log base 2 of the length of the pointer in bytes;
2540 for 1-byte displacements, 1 for 2-byte displacements,
2552 for 4-byte displacements.
256.It Fa r_extern
257Set if this relocation requires an external reference;
258the link editor must use a symbol address to update the pointer.
259When the
260.Fa r_extern
261bit is clear, the relocation is
262.Sq local ;
263the link editor updates the pointer to reflect
264changes in the load addresses of the various segments,
265rather than changes in the value of a symbol (except when
266.Fa r_baserel
267is also set (see below).
268In this case, the content of the
269.Fa r_symbolnum
270field is an
271.Fa n_type
272value (see below);
273this type field tells the link editor
274what segment the relocated pointer points into.
275.It Fa r_baserel
276If set, the symbol, as identified by the
277.Fa r_symbolnum
278field, is to be relocated to an offset into the Global Offset Table.
279At run-time, the entry in the Global Offset Table at this offset is set to
280be the address of the symbol.
281.It Fa r_jmptable
282If set, the symbol, as identified by the
283.Fa r_symbolnum
284field, is to be relocated to an offset into the Procedure Linkage Table.
285.It Fa r_relative
286If set, this relocation is relative to the (run-time) load address of the
287image this object file is going to be a part of.
288This type of relocation
289only occurs in shared objects.
290.It Fa r_copy
291If set, this relocation record identifies a symbol whose contents should
292be copied to the location given in
293.Fa r_address .
294The copying is done by the run-time link-editor from a suitable data
295item in a shared object.
296.El
297.Pp
298Symbols map names to addresses (or more generally, strings to values).
299Since the link-editor adjusts addresses,
300a symbol's name must be used to stand for its address
301until an absolute value has been assigned.
302Symbols consist of a fixed-length record in the symbol table
303and a variable-length name in the string table.
304The symbol table is an array of
305.Fa nlist
306structures:
307.Bd -literal -offset indent
308struct nlist {
309	union {
310		char	*n_name;
311		long	n_strx;
312	} n_un;
313	unsigned char	n_type;
314	char		n_other;
315	short		n_desc;
316	unsigned long	n_value;
317};
318.Ed
319.Pp
320The fields are used as follows:
321.Bl -tag -width n_un.n_strx
322.It Fa n_un.n_strx
323Contains a byte offset into the string table
324for the name of this symbol.
325When a program accesses a symbol table with the
326.Xr nlist 3
327function,
328this field is replaced with the
329.Fa n_un.n_name
330field, which is a pointer to the string in memory.
331.It Fa n_type
332Used by the link editor to determine
333how to update the symbol's value.
334The
335.Fa n_type
336field is broken down into three sub-fields using bitmasks.
337The link editor treats symbols with the
338.Dv N_EXT
339type bit set as
340.Sq external
341symbols and permits references to them from other binary files.
342The
343.Dv N_TYPE
344mask selects bits of interest to the link editor:
345.Bl -tag -width N_TEXT
346.It Dv N_UNDF
347An undefined symbol.
348The link editor must locate an external symbol with the same name
349in another binary file to determine the absolute value of this symbol.
350As a special case, if the
351.Fa n_value
352field is nonzero and no binary file in the link-edit defines this symbol,
353the link-editor will resolve this symbol to an address
354in the bss segment,
355reserving an amount of bytes equal to
356.Fa n_value .
357If this symbol is undefined in more than one binary file
358and the binary files do not agree on the size,
359the link editor chooses the greatest size found across all binaries.
360.It Dv N_ABS
361An absolute symbol.
362The link editor does not update an absolute symbol.
363.It Dv N_TEXT
364A text symbol.
365This symbol's value is a text address and
366the link editor will update it when it merges binary files.
367.It Dv N_DATA
368A data symbol; similar to
369.Dv N_TEXT
370but for data addresses.
371The values for text and data symbols are not file offsets but
372addresses; to recover the file offsets, it is necessary
373to identify the loaded address of the beginning of the corresponding
374section and subtract it, then add the offset of the section.
375.It Dv N_BSS
376A bss symbol; like text or data symbols but
377has no corresponding offset in the binary file.
378.It Dv N_FN
379A filename symbol.
380The link editor inserts this symbol before
381the other symbols from a binary file when
382merging binary files.
383The name of the symbol is the filename given to the link editor,
384and its value is the first text address from that binary file.
385Filename symbols are not needed for link-editing or loading,
386but are useful for debuggers.
387.El
388.Pp
389The
390.Dv N_STAB
391mask selects bits of interest to symbolic debuggers
392such as
393.Xr gdb 1 ;
394the values are described in
395.Xr stab 5 .
396.It Fa n_other
397This field provides information on the nature of the symbol independent of
398the symbol's location in terms of segments as determined by the
399.Fa n_type
400field.
401Currently, the lower 4 bits of the
402.Fa n_other
403field hold one of two values:
404.Dv AUX_FUNC
405and
406.Dv AUX_OBJECT
407(see
408.In link.h
409for their definitions).
410.Dv AUX_FUNC
411associates the symbol with a callable function, while
412.Dv AUX_OBJECT
413associates the symbol with data, irrespective of their locations in
414either the text or the data segment.
415This field is intended to be used by
416.Xr ld 1
417for the construction of dynamic executables.
418.It Fa n_desc
419Reserved for use by debuggers; passed untouched by the link editor.
420Different debuggers use this field for different purposes.
421.It Fa n_value
422Contains the value of the symbol.
423For text, data and bss symbols, this is an address;
424for other symbols (such as debugger symbols),
425the value may be arbitrary.
426.El
427.Pp
428The string table consists of an
429.Em unsigned long
430length followed by null-terminated symbol strings.
431The length represents the size of the entire table in bytes,
432so its minimum value (or the offset of the first string)
433is always 4 on 32-bit machines.
434.Sh SEE ALSO
435.Xr as 1 ,
436.Xr gdb 1 ,
437.Xr ld 1 ,
438.Xr brk 2 ,
439.Xr execve 2 ,
440.Xr nlist 3 ,
441.Xr core 5 ,
442.Xr elf 5 ,
443.Xr link 5 ,
444.Xr stab 5
445.Sh HISTORY
446The
447.In a.out.h
448include file appeared in
449.At v7 .
450.Sh BUGS
451Since not all of the supported architectures use the
452.Fa a_midmag
453field,
454it can be difficult to determine what
455architecture a binary will execute on
456without examining its actual machine code.
457Even with a machine identifier,
458the byte order of the
459.Fa exec
460header is machine-dependent.
461