.TH AR 6
.SH NAME
ar \- archive (library) file format
.SH SYNOPSIS
.B #include <ar.h>
.SH DESCRIPTION
The archive command
.IR ar (1)
is used to combine several files into
one.
Archives are used mainly as libraries to be searched
by the loaders
.IR 8l (1)
.I et al.
.PP
A file produced by
.I 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:
.IP
.EX
.ta \w'#define 'u +\w'SAR_HDR 'u
.ec %
#define	ARMAG	"!<arch>\n"
#define	SARMAG	8

#define	ARFMAG	"`\n"

struct ar_hdr {
	char	name[16];
	char	date[12];
	char	uid[6];
	char	gid[6];
	char	mode[8];
	char	size[10];
	char	fmag[2];
};
#define	SAR_HDR	60
.ec \
.EE
.LP
The
.B name
is a blank-padded string.
The
.L fmag
field contains
.L ARFMAG
to help verify the presence of a header.
The other fields are left-adjusted, blank-padded numbers.
They are decimal except for
.LR mode ,
which is octal.
The date is the modification date of the file (see
.IR stat (2))
at the time of its insertion into the archive.
The mode is the low 9 bits of the file permission mode.
The length of the header is
.LR SAR_HDR .
Because the
.L ar_hdr
structure is padded in an architecture-dependent manner,
the structure should never be read or written as a unit;
instead, each field should be read or written independently.
.PP
Each file begins on an even (0 mod 2) boundary;
a newline is inserted between files if necessary.
Nevertheless 
.B size
reflects the
actual size of the file exclusive of padding.
.PP
When all members of an archive are object files of
the same architecture,
.B ar
automatically adds an extra file, named
.BR __.SYMDEF ,
as the first member of the archive.  This file
contains an index used by the loaders to locate all
externally defined text and data symbols in the archive.
.PP
There is no provision for empty areas in an archive
file.
.SH "SEE ALSO"
.IR ar (1), 
.IR 8l (1), 
.IR nm (1),
.IR stat (2)
.SH BUGS
The
.B uid
and
.B gid
fields are unused in Plan 9.
They provide compatibility with Unix
.I ar
format.