TAR(5)								File Formats Manual							    TAR(5)

NAME

       tar - tape archive file format

DESCRIPTION

       Tar, (the tape archive command) dumps several files into one, in a medium suitable for transportation.

       A  ``tar  tape''  or  file is a series of blocks.  Each block is of size TBLOCK.  A file on the tape is represented by a header block which
       describes the file, followed by zero or more blocks which give the contents of the file.  At the end of the tape are two blocks filled with
       binary zeros, as an end-of-file indicator.

       The  blocks  are grouped for physical I/O operations.  Each group of n blocks (where n is set by the b keyletter on the tar(1) command line
       -- default is 20 blocks) is written with a single system call; on nine-track tapes, the result of this write is a single tape record.   The
       last  group  is	always	written  at  the full size, so blocks after the two zero blocks contain random data.  On reading, the specified or
       default group size is used for the first read, but if that read returns less than a full tape block, the reduced block  size  is  used  for
       further reads.

       The header block looks like:

	      #define TBLOCK 512
	      #define NAMSIZ 100

	      union hblock {
		   char dummy[TBLOCK];
		   struct header {
			char name[NAMSIZ];
			char mode[8];
			char uid[8];
			char gid[8];
			char size[12];
			char mtime[12];
			char chksum[8];
			char linkflag;
			char linkname[NAMSIZ];
		   } dbuf;
	      };

       Name is a null-terminated string.  The other fields are zero-filled octal numbers in ASCII.  Each field (of width w) contains w-2 digits, a
       space, and a null, except size and mtime, which do not contain the trailing null and chksum which has a null followed by a space.  Name	is
       the  name  of the file, as specified on the tar command line.  Files dumped because they were in a directory which was named in the command
       line have the directory name as prefix and /filename as suffix.	Mode is the file mode, with the top bit masked off.  Uid and gid  are  the
       user  and  group  numbers  which own the file.  Size is the size of the file in bytes.  Links and symbolic links are dumped with this field
       specified as zero.  Mtime is the modification time of the file at the time it was dumped.  Chksum is an octal ASCII value which	represents
       the  sum of all the bytes in the header block.  When calculating the checksum, the chksum field is treated as if it were all blanks.  Link-
       flag is NULL if the file is ``normal'' or a special file, ASCII `1' if it is an hard link, and ASCII `2' if it is  a  symbolic  link.   The
       name linked-to, if any, is in linkname, with a trailing null.  Unused fields of the header are binary zeros (and are included in the check-
       sum).

       The first time a given i-node number is dumped, it is dumped as a regular file.	The second and subsequent times, it is dumped  as  a  link
       instead.  Upon retrieval, if a link entry is retrieved, but not the file it was linked to, an error message is printed and the tape must be
       manually re-scanned to retrieve the linked-to file.

       The encoding of the header is designed to be portable across machines.

SEE ALSO

       tar(1)

BUGS

       Names or linknames longer than NAMSIZ produce error reports and cannot be dumped.

4.2 Berkeley Distribution					 November 7, 1985							    TAR(5)