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.\"     @(#)ar.5.5	8.2 (Berkeley) 6/1/94
.\"
.Dd June 1, 1994
.Dt AR 5
.Os
.Sh NAME
.Nm ar
.Nd a.out archive (library) file format
.Sh SYNOPSIS
.In ar.h
.Sh DESCRIPTION
The archive command
.Nm
combines several files into one.
Archives are mainly used as libraries of object files intended to be
loaded using the link-editor
.Xr ld 1 .
.Pp
A file created with
.Nm
begins with the
.Dq magic
string
.Dq Li "!<arch>\en" .
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.
.Pp
The header is made up of six variable length
.Tn 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.
.Pp
The modification time is the file
.Fa st_mtime
field, i.e.,
.Dv CUT
seconds since
the epoch.
The user and group id's are the file
.Fa st_uid
and
.Fa st_gid
fields.
The file mode is the file
.Fa st_mode
field.
The file size is the file
.Fa st_size
field.
The two-byte trailer is the string "\`\en".
.Pp
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
.Tn 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.
.Pp
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.
.Pp
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
.Pq Dq \en
character, although the size in the header does not reflect this.
.Sh COMPATIBILITY
The current a.out archive format is not specified by any standard.
.Pp
ELF systems use the
.Nm
format specified by the
.At V.4
ABI, with the same headers but different long file name handling.
.Sh SEE ALSO
.Xr ar 1 ,
.Xr stat 2
.Sh HISTORY
There have been at least four
.Nm
formats.
The first was denoted by the leading
.Dq 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.
.Pp
The second was denoted by the leading
.Dq 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.
.Pp
Both of these historical formats may be read with
.Xr ar 1 .
.Pp
The current archive format (without support for long character names and
names with embedded spaces) was introduced in
.Bx 4.0 .
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
.Bx 4.4 .
.Sh BUGS
The
.Tn <ar.h>
header file, and the
.Nm
manual page, do not currently describe the ELF archive format.