NAME
ar —
a.out archive (library) file
format
SYNOPSIS
#include <ar.h>
DESCRIPTION
The archive command
ar combines several files into one.
Archives are mainly used as libraries of object files intended to be loaded
using the link-editor
ld(1).
A file created with
ar begins with the “magic”
string “
!<arch>\n
”. The rest of the
archive is made up of objects, each of which is composed of a header for a
file, a possible file name, and the file contents. The header is portable
between machine architectures, and, if the file contents are printable, the
archive is itself printable.
The header is made up of six variable length ASCII fields, followed by a two
character trailer. The fields are the object name (16 characters), the file
last modification time (12 characters), the user and group id's (each 6
characters), the file mode (8 characters) and the file size (10 characters).
All numeric fields are in decimal, except for the file mode which is in octal.
The modification time is the file
st_mtime field, i.e.,
CUT
seconds since the epoch. The user and group id's
are the file
st_uid and
st_gid
fields. The file mode is the file
st_mode field. The
file size is the file
st_size field. The two-byte
trailer is the string "`\n".
Only the name field has any provision for overflow. If any file name is more
than 16 characters in length or contains an embedded space, the string
"#1/" followed by the ASCII length of the name is written in the
name field. The file size (stored in the archive header) is incremented by the
length of the name. The name is then written immediately following the archive
header.
Any unused characters in any of these fields are written as space characters. If
any fields are their particular maximum number of characters in length, there
will be no separation between the fields.
Objects in the archive are always an even number of bytes long; files which are
an odd number of bytes long are padded with a newline (“\n”)
character, although the size in the header does not reflect this.
COMPATIBILITY
The current a.out archive format is not specified by any standard.
ELF systems use the
ar format specified by the
AT&T System V Release 4 UNIX ABI, with
the same headers but different long file name handling.
SEE ALSO
ar(1),
stat(2)
HISTORY
There have been at least four
ar formats. The first was
denoted by the leading “magic” number 0177555 (stored as type
int). These archives were almost certainly created on a 16-bit machine, and
contain headers made up of five fields. The fields are the object name (8
characters), the file last modification time (type long), the user id (type
char), the file mode (type char) and the file size (type unsigned int). Files
were padded to an even number of bytes.
The second was denoted by the leading “magic” number 0177545 (stored
as type int). These archives may have been created on either 16 or 32-bit
machines, and contain headers made up of six fields. The fields are the object
name (14 characters), the file last modification time (type long), the user
and group id's (each type char), the file mode (type int), and the file size
(type long). Files were padded to an even number of bytes.
Both of these historical formats may be read with
ar(1).
The current archive format (without support for long character names and names
with embedded spaces) was introduced in
4.0BSD. The
headers were the same as the current format, with the exception that names
longer than 16 characters were truncated, and names with embedded spaces (and
often trailing spaces) were not supported. It has been extended for these
reasons, as described above. This format first appeared in
4.4BSD.
BUGS
The <ar.h> header file, and the
ar manual page, do not
currently describe the ELF archive format.