NAME

ar − common archive file format

SYNOPSIS

#include <ar.h>

DESCRIPTION

Ar(1) is used to concatenate several files into an archival file.  Archives are used mainly as libraries to be searched by the link editor ld(1).

Each archive begins with the archive magic string. 

 #define  ARMAG   "!<arch>\n"/∗ magic string ∗/
#define  SARMAG  8/∗ length of magic string ∗/
 

Each archive which contains object files (see a.out(4)) includes an archive symbol table. This symbol table is used by the link editor ld(1) 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.

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. 

The ar_name field is blank-padded and slash (/) terminated.  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(1) is used. Note that older versions or ar(1) did not use the common archive format, and those archives cannot be read or written by the common archiver.

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.  If the archive symbol table exists, the first file in the archive has a zero length name (i.e., ar_name[0] == ’/’).  The contents of this archive member are machine dependent.  Further details can be found in the a.out(4) manual page for each machine.

SEE ALSO

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

CAVEATS

Strip(1) will remove all archive symbol entries from the header.  The archive symbol entries must be restored via the ts option of the ar(1) command before the archive can be used with the link editor ld(1).

Hewlett-Packard Company  —  May 11, 2021