ar(4) UNIX System V 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 editor
ld.
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 following
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 information in the file member headers is in printable ASCII. The
numeric information contained in the headers is stored as decimal numbers
(except for ar_mode which is in octal). Thus, if the archive contains
printable files, the archive itself is printable.
If the file member name fits, the ar_name 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, ar_name contains a slash (/)
followed by a decimal representation of the name's offset in the archive
string table described below.
The ar_date 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.
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ar(4) UNIX System V ar(4)
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.
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 (that is, arname[0] is
'/'), arname[1]==' ', and so on). 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: ar_size - 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.
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
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| ___|____|____|____|
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 file names, each followed by a slash
and a new-line. 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 ar_name 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]==' ', and so on). Offsets into the string
table begin at zero. Example ar_name values for short and long file
names appear below.
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 |
| ______|______|______|_______|______|______|_______|______|______|_______|
Member Name ar_name Note
____________________________|_____________________|______________________________
short-name | short-name/ | Not in string table
file_name_sample | /0 | Offset 0 in string table
longerfilenamexample | /18 | Offset 18 in string table
____________________________|_____________________|______________________________
SEE ALSO
ar(1), ld(1), strip(1), sputl(3X), a.out(4)
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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.
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