Interdata_v6/usr/doc/man/man5/archive.5

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.th ARCHIVE V
.sh NAME
archive - ar (library) file format
.sh DESCRIPTION
The archive command
.it ar
is used to combine several files into one.
Archives are used mainly as libraries to be searched by the link-editor
.it ld.

A file produced by
.it ar
has a magic number 0177545(8) in the first word, followed by the
constituent files, each preceded by a file header.
The header of each file has the following format:

.dt
.nf
struct {
	char	name[14];	/* file name, null padded */
	char	uid;		/* user id of file's owner */
	char	gid;		/* group id of file's owner */
	long	date;		/* time of last modification */
	long	size;		/* size of file */
	int	mode;		/* access modes */
};

Each file begins on an even address; a null byte 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.
.sh "SEE ALSO"
ar(I), ld(I)