ar - archive (library) file format
The archive command ar combines several files into one. Archives are
used mainly as libraries to be searched by the link-editor ld.
A file produced by ar has a magic string at the start, followed by the
constituent files, each preceded by a file header. The magic number and
header layout as described in the include file are:
#define ARMAG "!<arch>\n"
#define SARMAG 8
#define ARFMAG "`\n"
typedef struct ar_hdr ARHDR;
The name is terminated with a "/" character by default. Any remaining
room in the name field is blank_filled.
File names can be any length: they re not limited to 15 characters. For
additional details, see the ABI book mentioned below and the ar(1) man
The ar_fmag field contains ARFMAG to help verify the presence of a
header. The other fields are left-adjusted, blank-padded numbers. They
are decimal except for ar_mode, which is octal. The date is the
modification date of the file at the time of its insertion into the
Each file begins on a even (0 mod 2) boundary. With compiler releases
7.0 and lower, a new-line is inserted between files if necessary. The
size given reflects the actual size of the file exclusive of padding.
With compiler releases 7.1 and higher, in order to guarantee that each
ELF format file is 16-byte aligned in the archive, additional padding has
been added to the end of each file to achieve that, using the character
'\0'. This enables the loader (ld) to have faster access to members in
the archive while performing static linking. This results in the change
of size of files permanently, normally increased by 1 to 15 bytes. With
non-ELF files, this padding would not occur.
There is no provision for empty areas in an archive file.
The encoding of the header is portable across machines. If an archive
contains printable files, the archive itself is printable.
ar(1), ld(1), nm(1),
System V Application Binary Interface, ISBN 0-13-877598-2, Prentice Hall
System V ABI MIPS Processor Supplement, ISBN 0-13-880170-3, Prentice Hall
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