archive (library) file
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
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 IDs (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
CUT seconds since the Epoch. The user
and group IDs 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
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.
No archive format is currently specified by any standard. AT&T System V UNIX has historically distributed archives in a different format from all of the above.
There have been at least four
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
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 IDs (each type char), the file mode (type int) and the file size (type long). Files were padded to an even number of bytes.
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.