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. ar is included as one of the GNU Binutils.
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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.
The global header is a single field containing the magic ASCII string "!<arch>" followed by a single LF control character
The common format is as follows.
| Field Offset from | Field Offset to | Field Name | Field Format |
|---|---|---|---|
| 0 | 15 | File name | ASCII |
| 16 | 27 | File modification timestamp | Decimal |
| 28 | 33 | Owner ID | Decimal |
| 34 | 39 | Group ID | Decimal |
| 40 | 47 | File mode | Octal |
| 48 | 57 | File size in bytes | Decimal |
| 58 | 59 | File magic | 0x60 0x0A |
Due to the limitations of file name length and format, both the GNU and BSD variants devised different methods of storing long filenames.
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 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.
The special file "/" is not terminated with a specific sequence; the end is assumed once the last symbol name has been read.
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
instead of:
cc main.c class1.o class2.o class3.o
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