4.3BSD-UWisc/man/cat5/tar.5

TAR(5)              UNIX Programmer's Manual               TAR(5)



NAME
     tar - tape archive file format

DESCRIPTION
     _T_a_r, (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
     _t_a_r(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;
          };

     _N_a_m_e 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 _s_i_z_e and
     _m_t_i_m_e, which do not contain the trailing null and _c_h_k_s_u_m
     which has a null followed by a space.  _N_a_m_e is the name of
     the file, as specified on the _t_a_r command line.  Files
     dumped because they were in a directory which was named in



Printed 12/27/86        November 7, 1985                        1






TAR(5)              UNIX Programmer's Manual               TAR(5)



     the command line have the directory name as prefix and
     /_f_i_l_e_n_a_m_e as suffix.  _M_o_d_e is the file mode, with the top
     bit masked off.  _U_i_d and _g_i_d are the user and group numbers
     which own the file.  _S_i_z_e is the size of the file in bytes.
     Links and symbolic links are dumped with this field speci-
     fied as zero.  _M_t_i_m_e is the modification time of the file at
     the time it was dumped.  _C_h_k_s_u_m is an octal ASCII value
     which represents the sum of all the bytes in the header
     block.  When calculating the checksum, the _c_h_k_s_u_m field is
     treated as if it were all blanks.  _L_i_n_k_f_l_a_g 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 _l_i_n_k_n_a_m_e, with a trailing null.
     Unused fields of the header are binary zeros (and are
     included in the checksum).

     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.























Printed 12/27/86        November 7, 1985                        2