mdtar man page on Ultrix

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tar(5)									tar(5)

Name
       tar, mdtar - tape archive file format

Description
       The  tape archive command dumps several files, including special files,
       into one, in a medium suitable for transportation.

       A 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 option on the command line, and the
       default is 20 blocks) is written with a single system call; on  9-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 following is an example of a header block:
       #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];
		       char rdev[6]
	       } dbuf;
       };

       Digital extensions to handle multivolume archives are located in

       The  name  field	 is  a	null-terminated	 string.  The other fields are
       0-filled octal numbers in ASCII.	 Each field (of width  w)  contains  w
       minus  2	 digits, a space, and a null, except size and mtime , which do
       not contain the trailing null.  The name field specifies	 the  name  of
       the  file, as specified on the command line.  Files dumped because they
       were in a directory that was named in the command line have the	direc‐
       tory  name  as prefix and /filename as suffix.  The field specifies the
       file mode, with the top bit masked off.	The uid and gid fields specify
       the user and group numbers that own the file.  The size field specifies
       the size of the file in bytes.  Links and  symbolic  links  are	dumped
       with this field specified as zero.  The mtime field specifies the modi‐
       fication time of the file at the time it was dumped.  The chksum	 field
       is  a decimal 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.  The linkflag field is ASCII 0 if the
       file is normal or a special file and ASCII 1 if it is a hard link,  and
       ASCII  2	 if it is a symbolic link.  The name to which it is linked, if
       any, is in linkname, with a trailing null.  Unused fields of the header
       are  binary  zeros  and	are  included in the checksum.	The rdev field
       encodes the ASCII representation of a device special file's  major  and
       minor device numbers.

       The first time a given i-node number is dumped, it is dumped as a regu‐
       lar 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 rescanned to retrieve the linked file.

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

Restrictions
       Names or link names longer than NAMSIZ produce error reports and cannot
       be dumped.

       The BSD format does not handle multi-volume  conditions.	  The  Digital
       archive extension for multi-volume is described in

See Also
       tar(1)

									tar(5)
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