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AR(5)			    BSD File Formats Manual			 AR(5)

NAME
     ar — format of archives managed by ar(1) and ranlib(1)

SYNOPSIS
     #include <ar.h>

DESCRIPTION
     An archive managed by the ar(1) and ranlib(1) utilities is a single file
     that stores the individual members of the archive along with metadata for
     each member.  There are two major variants of the ar(1) archive format,
     the BSD variant and the SVR4/GNU variant.	Both variants are described by
     this manual page.

     The header file <ar.h> defines constants and structures used to describe
     the layout of these archives.

   Archive Layout
     ar(1) archives start with a string of magic bytes "!<arch>\n" (constant
     ARMAG in header <ar.h>).  The content of the archive follows the magic
     bytes.  Each member stored in the archive is preceded by a fixed size ar‐
     chive header that stores file permissions, last modification time, the
     owner, and the group of the archived file.

     Archive headers start at an even byte offset in the archive file.	If the
     length of the preceding archive member was odd, then an extra newline
     character “\n” is used as padding.

     The archive header comprises six fixed-size ASCII strings followed by a
     two character trailer (see struct ar_hdr in header file <ar.h>):

     struct ar_hdr {
	     char ar_name[16];		     /* name */
	     char ar_date[12];		     /* modification time */
	     char ar_uid[6];		     /* user id */
	     char ar_gid[6];		     /* group id */
	     char ar_mode[8];		     /* octal file permissions */
	     char ar_size[10];		     /* size in bytes */
	     char ar_fmag[2];		     /* consistency check */
     };

     Unused characters in the header are filled with space (ASCII 20H) charac‐
     ters.  Each field of the header abuts the next without additional pad‐
     ding.

     The members of the archive header are as follows:
     ar_date	 This field holds the decimal representation of the modifica‐
		 tion time, in seconds since the epoch, of the archive member.
     ar_fmag	 This trailer field holds the two characters "`\n" (constant
		 ARFMAG defined in header file <ar.h>), and is used for con‐
		 sistency checks.
     ar_gid	 This field holds the decimal representation of the numeric
		 user id of the creator of the member.
     ar_mode	 This field holds octal representation of the file permissions
		 for the member.
     ar_name	 This field holds the name of an archive member.  The usage of
		 this field depends on the format variant:
		 BSD	   In the BSD variant, names that are shorter than 16
			   characters and without embedded spaces are stored
			   directly in this field.  If a name has an embedded
			   space, or if it is longer than 16 characters, then
			   the string "#1/" followed by the decimal represen‐
			   tation of the length of the file name is placed in
			   this field.	The actual file name is stored immedi‐
			   ately after the archive header.  The content of the
			   archive member follows the file name.  The ar_size
			   field of the header (see below) will then hold the
			   sum of the size of the file name and the size of
			   the member.
		 SVR4/GNU  In the SVR4/GNU variant, names up to 15 characters
			   in length are stored directly in this field, and
			   are terminated by a "/" (ASCII 2FH) character.
			   Names larger than 15 characters in length are
			   stored in a special archive string table member
			   (see Archive String Table below), and the ar_name
			   field holds the string "/" followed by the decimal
			   representation of the offset in the archive string
			   table of the actual name.
     ar_size	 In the SVR4/GNU variant, this field holds the decimal repre‐
		 sentation of actual size in bytes of the archived file.  In
		 the BSD variant, for member names that use the ar_name field
		 directly, this field holds the decimal representation of the
		 actual size in bytes of the archived member.  For member
		 names that use the extension mechanism described above, the
		 field will hold the sum of the sizes, in bytes, of the file‐
		 name and the archive member.
     ar_uid	 This field holds the decimal representation of the numeric
		 group id of the creator of the member.

   Archive Symbol Table
     An archive may additionally contain an archive symbol table used by the
     link editor, ld(1).  This symbol table has the member name "__.SYMDEF" in
     the BSD variant of the archive format, and the name "/" in the SVR4/GNU
     variant.

     The format of the symbol table depends on the format variant:
     BSD       In the BSD variant, the symbol table has 4 parts encoded in a
	       machine dependent manner:
	       1.   The first part is a binary value containing size in bytes
		    of the second part encoded as a C “long”.
	       2.   The second part is a list of struct ranlib structures (see
		    <ranlib.h>). Each ranlib structure describes one symbol
		    and comprises of two C “long” values.  The first “long” is
		    a zero-based offset into the string table in the fourth
		    part for the symbol's name.	 The second “long” is an off‐
		    set from the beginning of the archive to the start of the
		    archive header for the member that defines the symbol.
	       3.   The third part is a binary value denoting the length of
		    the string table contained in the fourth part.
	       4.   The fourth part is a string table containing NUL-termi‐
		    nated strings.
     SVR4/GNU  In the SVR4/GNU variant, the symbol table comprises of three
	       parts which follow each other without padding:
	       1.   The first part comprises of a count of entries in the sym‐
		    bol table, stored a 4 byte binary value in MSB first
		    order.
	       2.   The next part is an array of 4 byte file offsets within
		    the archive to archive header for members that define the
		    symbol in question.	 Each offset in stored in MSB first
		    order.
	       3.   The third part is a string table, that contains NUL-termi‐
		    nated strings for the symbols in the symbol table.

   Archive String Table
     In the SVR4/GNU variant of the ar(1) archive format, long file names are
     stored in a separate archive string table and referenced from the archive
     header for each member.  Each file name is terminated by the string
     "/\n".  The string table itself has a name of "//".

SEE ALSO
     ar(1), ranlib(1), archive(3), elf(3), gelf(3), elf(5)

BSD			      September 07, 2007			   BSD
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