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.
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.
ELF systems use the
ar
format specified by the
AT&T
System V.4 UNIX
ABI, with the same headers but different long file name handling.