term% cat index.txt AR(5) File Formats Manual AR(5)
NAME
ar - archive (library) file format
SYNOPSIS
#include <ar.h>
DESCRIPTION
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.
A file produced by ar has a magic string at the start, followed by the
constituent files, each preceded by a file header. The magic number
and header layout as described in the include file are:
/* Header describing ‘ar' archive file format.
\h'\n[lss]u'Copyright (C) 1996-2025 Free Software Foundation, Inc.
\h'\n[lss]u'This file is part of the GNU C Library.
\h'\n[lss]u'The GNU C Library is free software; you can redistribute it and/or
\h'\n[lss]u'modify it under the terms of the GNU Lesser General Public
\h'\n[lss]u'License as published by the Free Software Foundation; either
\h'\n[lss]u'version 2.1 of the License, or (at your option) any later version.
\h'\n[lss]u'The GNU C Library is distributed in the hope that it will be useful,
\h'\n[lss]u'but WITHOUT ANY WARRANTY; without even the implied warranty of
\h'\n[lss]u'MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
\h'\n[lss]u'Lesser General Public License for more details.
\h'\n[lss]u'You should have received a copy of the GNU Lesser General Public
\h'\n[lss]u'License along with the GNU C Library; if not, see
\h'\n[lss]u'<https://www.gnu.org/licenses/>. */
#ifndef _AR_H
#define _AR_H 1
#include <sys/cdefs.h>
/* Archive files start with the ARMAG identifying string. Then follows a
\h'\n[lss]u'‘struct ar_hdr', and as many bytes of member file data as its ‘ar_size'
\h'\n[lss]u'member indicates, for each member file. */
#define ARMAG "!<arch>\n"/* String that begins an archive file. */
#define SARMAG 8/* Size of that string. */
#define ARFMAG "‘\n"/* String in ar_fmag at end of each header. */
__BEGIN_DECLS
struct ar_hdr
\h'\n[lss]u'{
\h'\n[lss]u'char ar_name[16];/* Member file name, sometimes / terminated. */
\h'\n[lss]u'char ar_date[12];/* File date, decimal seconds since Epoch. */
\h'\n[lss]u'char ar_uid[6], ar_gid[6];/* User and group IDs, in ASCII decimal. */
\h'\n[lss]u'char ar_mode[8];/* File mode, in ASCII octal. */
\h'\n[lss]u'char ar_size[10];/* File size, in ASCII decimal. */
\h'\n[lss]u'char ar_fmag[2];/* Always contains ARFMAG. */
\h'\n[lss]u'};
__END_DECLS
#endif /* ar.h */
The name is a blank-padded string. The ar_fmag field contains ARFMAG
to help verify the presence of a header. The other fields are left-ad‐
justed, blank-padded numbers. They are decimal except for ar_mode,
which is octal. The date is the modification date of the file at the
time of its insertion into the archive.
Each file begins on a even (0 mod 2) boundary; a new-line is inserted
between files if necessary. Nevertheless the size given reflects the
actual size of the file exclusive of padding.
There is no provision for empty areas in an archive file.
The encoding of the header is portable across machines. If an archive
contains printable files, the archive itself is printable.
SEE ALSO
ar(1), ld(1), nm(1)
BUGS
File names lose trailing blanks. Most software dealing with archives
takes even an included blank as a name terminator.
AR(5)