.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 2l (1)
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 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, in octal.
The length of the header is
.LR SAR_HDR .
Because
.L struct ar_hdr
may be padded on some machines,
.L SAR_HDR
should be used in preference to
.L sizeof(struct ar_hdr)
when reading and writing file headers.
.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
There is no provision for empty areas in an archive
file.
.SH "SEE ALSO"
.IR ar (1), 
.IR 2l (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.