NAME

ar − common 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 the archive magic string. 

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

Each archive which contains common 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 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 stored 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.  Conversion tools such as convert(1) exist to aid in the transportation of non-common format archives to this format.

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 file are:

• The number of symbols.  Length: 4 bytes. 

• The array of offsets into the archive file.  Length: 4 bytes ∗ “the number of symbols”. 

• The name string table.  Length: ar_size − (4 bytes ∗ (“the number of symbols” + 1)). 

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.

SEE ALSO

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

WARNINGS

strip(1) removes 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).
 

September 29, 2021