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 ar_hdr /* file member header */ { char ar_name[16]; /* '/' terminated file member name */ char ar_date[12]; /* file member date */ char ar_uid[6] /* file member user identification */ char ar_gid[6] /* file member group identification */ char ar_mode[8] /* file member mode (octal) */ char ar_size[10]; /* file member size */ char ar_fmag[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.

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, ar_name[0] is '/'), ar_name[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: 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. 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 name2.

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 |    |
         |____|____|____|____|

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
_____________________|______________|___________________________            

ar(1), ld(1), strip(1), a.out(4)

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 editor ld.