ar(4) ar(4)
NAME
ar - archive file format
SYNOPSIS
#include <ar.h>
DESCRIPTION
The archive command ar is used to combine several files into one.
Archives are used mainly as libraries to be searched by the link edi-
tor ld(1).
Each archive begins with the archive magic string.
#define ARMAG "!<arch>\n" /* magic string *
#define SARMAG 8 /* length of magic string */
Following the archive magic string are the archive file members. Each
file member is preceded by a file member header which is of the fol-
lowing format:
#define ARFMAG "\n" /* header trailer string *
struct arhdr /* file member header */
{
char arname[16]; /* '/' terminated file member name */
char ardate[12]; /* file member date */
char aruid[6]; /* file member user identification */
char argid[6]; /* file member group identification */
char armode[8]; /* file member mode (octal) */
char arsize[10]; /* file member size */
char arfmag[2]; /* header trailer string */
};
All data items in the file member headers are printable ASCII code.
The numeric information contained in the headers is stored as decimal
numbers (except for armode which is in octal). Thus, if the archive
contains printable files, it is also printable.
If the file member name fits, the arname field contains the name
directly, and is terminated by a slash (/) and padded with blanks on
the right. If the member's name does not fit, arname contains a slash
(/) followed by a decimal representation of the name's offset in the
archive string table described below.
The ardate field is the modification date of the file at the time of
its insertion into the archive. Common format archives can be moved
from system to system as long as the portable archive command ar is
used.
Each archive file member begins on an even byte boundary; a newline is
inserted between files if necessary. Nevertheless, the size given
reflects the actual size of the file exclusive of padding.
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ar(4) ar(4)
Notice there is no provision for empty areas in an archive file.
Each archive that contains object files [see a.out(4)] includes an
archive symbol table. This symbol table is used by the link editor ld
to determine which archive members must be loaded during the link edit
process. The archive symbol table (if it exists) is always the first
file in the archive (but is never listed) and is automatically created
and/or updated by ar.
The archive symbol table has a zero length name (i.e., arname[0] is
"/"), arname[1]==' ', etc.). All "words" in this symbol table have
four bytes, using the machine-independent encoding shown below. All
machines use the encoding described here for the symbol table, even if
the machine's "natural" byte order is different.
________________________________________________
|0 |1 |2 |3 |
| | | | |
0x01020304 | 01 | 02 | 03 | 04 |
| | | | |
|__________|___________|___________|___________|
The contents of this file are as follows:
1. The number of symbols. Length: 4 bytes.
2. The array of offsets into the archive file. Length: 4 bytes * "the
number of symbols".
3. The name string table. Length: arsize - 4 bytes * ("the number of
symbols" + 1).
As an example, the following symbol table defines 4 symbols. The
archive member at file offset 114 defines name and object. The archive
member at file offset 426 defines function and a second version of
name.
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ar(4) ar(4)
Offset +0 +1 +2 +3
_________________________________
0 | 4 | 4 offset entries
|_______________________________|
4 | 114 | name
|_______________________________|
8 | 114 | object
|_______________________________|
12 | 426 | function
|_______________________________|
16 | 426 | name
|_______________________________|
20 | n | a | m | e |
|_______|_______|_______|_______|
24 | \0 | o | b | j |
|_______|_______|_______|_______|
28 | e | c | t | \0 |
|_______|_______|_______|_______|
32 | f | u | n | c |
|_______|_______|_______|_______|
36 | t | i | o | n |
|_______|_______|_______|_______|
40 | \0 | n | a | m |
|_______|_______|_______|_______|
44 | e | \0 | | |
|_______|_______|_______|_______|
The number of symbols and the array of offsets are managed with
sgetl() and sputl(). The string table contains exactly as many null
terminated strings as there are elements in the offsets array. Each
offset from the array is associated with the corresponding name from
the string table (in order). The names in the string table are all the
defined global symbols found in the common object files in the
archive. Each offset is the location of the archive header for the
associated symbol.
If some archive member's name is more than 15 bytes long, a special
archive member contains a table of filenames, each followed by a slash
and a newline. This string table member, if present, will precede all
"normal" archive members. The special archive symbol table is not a
"normal" member, and must be first if it exists. The arname entry of
the string table's member header holds a zero length name
arname[0]=='/', followed by one trailing slash (arname[1]=='/'),
followed by blanks (arname[2]==' ', etc.). Offsets into the string
table begin at zero. Example arname values for short and long
filenames appear below.
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ar(4) ar(4)
Offset +0 +1 +2 +3 +4 +5 +6 +7 +8 +9
______________________________________________________________
0 | f | i | l | e | _ | n | a | m | e | _ |
|_____|______|_____|_____|_____|_____|_____|_____|_____|_____|
10 | s | a | m | p | l | e | / | \n | l | o |
|_____|______|_____|_____|_____|_____|_____|_____|_____|_____|
20 | n | g | e | r | f | i | l | e | n | a |
|_____|______|_____|_____|_____|_____|_____|_____|_____|_____|
30 | m | e | x | a | m | p | l | e | / | \n |
|_____|______|_____|_____|_____|_____|_____|_____|_____|_____|
______________________________________________________________________
| Name | ar_name | Note |
|________________________|______________|_____________________________|
| short-name | short-name | Not in string table |
|________________________|______________|_____________________________|
| filenamesample | /0 | Offset 0 in string table |
|________________________|______________|_____________________________|
| longerfilenamexample | /18 | Offset 18 in string table |
|________________________|______________|_____________________________|
NOTES
strip() will remove all archive symbol entries from the header. The
archive symbol entries must be restored via the -ts options of the ar
command before the archive can be used with the link editor ld.
SEE ALSO
ar(1), ld(1), strip(1), sputl(3X), a.out(4).
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