ar (Unix)

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The archiver, also known simply as ar, is a Unix utility that maintains groups of files as a single archive file. Today, ar is generally used only to create and update static library files that the link editor or linker uses; it can be used to create archives for any purpose, but has been largely replaced by tar for purposes other than static libraries.[1] An implementation of ar is included as one of the GNU Binutils.[2]

In the Linux Standard Base, ar has been deprecated and is expected to disappear in a future release of that Standard. The rationale provided was that "the LSB does not include software development utilities nor does it specify .o and .a file formats."[3]

File format details[edit]

The ar format has never been standardized; modern archives are based on a common format with two known variants, BSD and GNU.

Historically there have been other variants including AIX (small), AIX (big) and Coherent, which all vary significantly from the common format.

Debian ".deb" archives use the common format.

An ar file begins with a global header, followed by a header and data section for each file stored within the ar file.

The data section is 2 byte aligned. If it would end on an odd offset, a '\n' is used as filler.

Global header[edit]

The global header is a single field containing the magic ASCII string "!<arch>" followed by a single LF control character.

File header[edit]

The common format is as follows. Numeric values are encoded in ASCII and all values right-padded with ASCII spaces (0x20).

Offset Length Name Format
0 16 File name ASCII
16 12 File modification timestamp Decimal
28 6 Owner ID Decimal
34 6 Group ID Decimal
40 8 File mode Octal
48 10 File size in bytes Decimal
58 2 File magic 0x60 0x0A

As the headers only include printable ASCII characters and line feeds, an archive containing only text files therefore still appears to be a text file itself.

The members are aligned to even byte boundaries. "Each archive file member begins on an even byte boundary; a newline is inserted between files if necessary. Nevertheless, the size given reflects the actual size of the file exclusive of padding."[4]

Due to the limitations of file name length and format, both the GNU and BSD variants devised different methods of storing long filenames. Both formats currently suffer from the year 2038 problem and will need to be modified in the future to provide more room for the timestamp.

BSD variant[edit]

BSD ar stores extended filenames by placing the string "#1/" followed by the file name length in the file name field, and appending the real filename to the file header.

GNU variant[edit]

GNU ar stores multiple extended filenames in the data section of a file with the name "//", this record is referred to by future headers. A header references an extended filename by storing a "/" followed by a decimal offset to the start of the filename in the extended filename data section. The format of this "//" file itself is simply a list of the long filenames, each separated by one or more LF characters. Note that the decimal offsets are number of characters, not line or string number within the "//" file.

GNU ar uses a '/' to mark the end of the filename; this allows for the use of spaces without the use of an extended filename.

GNU ar uses the special filename "/" to denote that the following data entry contains a symbol lookup table, which is used in ar libraries to speed up access. This symbol table is built in three parts which are recorded together as contiguous data.

  1. A 32-bit big endian integer, giving the number of entries in the table.
  2. A set of 32-bit big endian integers. One for each symbol, recording the position within the archive of the header for the file containing this symbol.
  3. A set of Zero-terminated strings. Each is a symbol name, and occurs in the same order as the list of positions in part 2.

The special file "/" is not terminated with a specific sequence; the end is assumed once the last symbol name has been read.

Example usage[edit]

For example, to create an archive from files class1.o, class2.o, class3.o, the following command would be used:

ar rcs libclass.a class1.o class2.o class3.o

to compile a program that depends on class1.o, class2.o, and class3.o one could do:

cc main.c libclass.a

or (if libclass.a is placed in standard library path, like /usr/local/lib)

cc main.c -lclass

or (during linking)

ld ... main.o -lclass ...

instead of:

cc main.c class1.o class2.o class3.o

See also[edit]

External links[edit]

References[edit]

  1. ^ "Static Libraries". TLDP. Retrieved 3 October 2013. 
  2. ^ "ar(1) - Linux man page". Retrieved 3 October 2013. 
  3. ^ Linux Standard Base Core Specification, version 4.1, Chapter 15. Commands and Utilities > ar
  4. ^ "ar.h". www.unix.com. The UNIX and Linux Forums.