ar(4)

ar -- archive file format

Synopsis

   #include <ar.h>

Description

The archive command ar(1) is used to combine several files into one. Archives are used mainly as libraries to be searched by the link editor ld(1).

Each archive begins with a unique string identifier called an 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]==' ', 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

	0	1	2	3
`0x01020304`	01	02	03	04

The contents of the symbol table are as follows:

1.: The number of symbols. Length: 4 bytes.
2.: The array of offsets, one per symbol, 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

28 e c t \0

32 f u n c

36 t i o n

40 \0 n a m

44 e \0

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 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[0]=='/', followed by one trailing slash (ar_name[1]=='/'), followed by blanks (ar_name[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

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

References

a.out(4), ar(1), ld(1), sputl(3X), strip(1)

Notices

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.