ar.h, ar - archive file format
The archive command ar is used to combine several files into one. Ar‐
chives are used mainly as libraries to be searched by the link editor
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 follow‐
#define ARFMAG "`\n" /* header trailer string */
struct ar_hdr /* file member header */
char ar_name; /* '/' terminated file member name */
char ar_date; /* file member date */
char ar_uid /* file member user identification */
char ar_gid /* file member group identification */
char ar_mode /* file member mode (octal) */
char ar_size; /* file member size */
char ar_fmag; /* 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 num‐
bers (except for ar_mode which is in octal). Thus, if the archive con‐
tains 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
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 ar‐
chive 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, ar_name
is '/'), ar_name==' ', 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: ar_size - 4 bytes * (``the
number of symbols'' + 1).
As an example, the following symbol table defines 4 symbols. The ar‐
chive member at file offset 114 defines name. The archive member at
file offset 122 defines object. The archive member at file offset 426
defines function and the archive member at file offset 434 defines
Example Symbol Table
Offset +0 +1 +2 +3
0 | 4 | 4 offset entries
4 | 114 | name
8 | 122 | object
12 | 426 | function
16 | 434 | name2
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 | 2 | \0 | |
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 sym‐
bols 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 ar‐
chive 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
ar_name=='/', followed by one trailing slash (ar_name=='/'), fol‐
lowed by blanks (ar_name==' ', etc.). Offsets into the string table
begin at zero. Example ar_name values for short and long file names
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
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 ALSOar(1), ld(1), strip(1), a.out(4)NOTES
The strip utility will remove all archive symbol entries from the
header. The archive symbol entries must be restored with the -ts
options of the ar command before the archive can be used with the link
Jul 1, 1998 AR.H(3HEAD)