tar - tape (or other media) archive file format

     A ``tar tape'' or  file  contains  a  series  of  records.   Each  record
     contains  TRECORDSIZE  bytes  (see  below).   Although this format may be
     thought of as being on magnetic tape, other media are often  used.   Each
     file archived is represented by a header record which describes the file,
     followed by zero or more records which give the contents of the file.  At
     the  end  of  the  archive  file there may be a record filled with binary
     zeros as an end-of-file indicator.  A reasonable system  should  write  a
     record  of  zeros  at  the  end,  but must not assume that an end-of-file
     record exists when reading an archive.

     The records may be blocked for physical I/O operations.  Each block of  N
     records (where N is set by the -b option to tar) is written with a single
     write() operation.  On magnetic tapes, the result of such a  write  is  a
     single  tape  record.  When writing an archive, the last block of records
     should be written at the full size, with records after  the  zero  record
     containing  all  zeroes.   When  reading  an archive, a reasonable system
     should properly handle an archive whose last block is  shorter  than  the
     rest, or which contains garbage records after a zero record.

     The header record is defined in the header file <tar.h> as follows:

      * Standard Archive Format - Standard TAR - USTAR
     #define RECORDSIZE 512
     #define NAMSIZ  100
     #define TUNMLEN 32
     #define TGNMLEN 32

     union record {
             char            charptr[RECORDSIZE];
             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    magic[8];
                     char    uname[TUNMLEN];
                     char    gname[TGNMLEN];
                     char    devmajor[8];
                     char    devminor[8];

             } header;

     /* The checksum field is filled with this while the checksum is computed.
     #define CHKBLANKS "        "    /* 8 blanks, no null */

     /* The magic field is filled with this if uname and gname are valid. */
     #define TMAGIC  "ustar  "       /* 7 chars and a null */

     /* The linkflag defines the type of file */
     #define LF_OLDNORMAL '\0' /* Normal disk file, Unix compatible */
     #define LF_NORMAL '0'           /* Normal disk file */
     #define LF_LINK '1'             /* Link to previously dumped file */
     #define LF_SYMLINK '2'          /* Symbolic link */
     #define LF_CHR  '3'             /* Character special file */
     #define LF_BLK  '4'             /* Block special file */
     #define LF_DIR          '5'             /* Directory */
     #define LF_FIFO '6'             /* FIFO special file */
     #define LF_CONTIG '7'           /* Contiguous file */
     /* Further link types may be defined later. */

     /* Bits used in the mode field - values in octal */
     #define TSUID           04000           /* Set UID on execution */
     #define TSGID           02000           /* Set GID on execution */
     #define TSVTX           01000           /* Save text (sticky bit) */

     /* File permissions */
     #define TUREAD  00400           /* read by owner */
     #define TUWRITE 00200           /* write by owner */
     #define TUEXEC  00100           /* execute/search by owner */
     #define TGREAD  00040           /* read by group */
     #define TGWRITE 00020           /* write by group */
     #define TGEXEC  00010           /* execute/search by group */
     #define TOREAD  00004           /* read by other */
     #define TOWRITE 00002           /* write by other */
     #define TOEXEC  00001           /* execute/search by other */

     All characters in header records are represented using  8-bit  characters
     in  the  local  variant  of  ASCII.   Each  field within the structure is
     contiguous; that is, there is no padding used within the structure.  Each
     character on the archive medium is stored contiguously.

     Bytes representing the contents of files (after the header record of each
     file)  are not translated in any way and are not constrained to represent
     characters or to be in any character set.  The  tar(5)  format  does  not
     distinguish  text  files  from  binary  files, and no translation of file
     contents should be performed.

     The fields name, linkname, magic, uname, and  gname  are  null-terminated
     character strings.  All other fields are  zero-filled  octal  numbers  in
     ASCII.  Each numeric field (of width w) contains w-2 digits, a space, and
     a null, except size and mtime, which do not contain the trailing null.

     The name field is the pathname of the file, with directory names (if any)
     preceding the file name, separated by slashes.

     The mode field provides nine bits specifying file permissions  and  three
     bits to specify the Set UID, Set GID and Save Text (TSVTX) modes.  Values
     for these bits are defined above.  When special permissions are  required
     to create a file with a given mode, and the user restoring files from the
     archive does not hold such permissions, the mode bit(s) specifying  those
     special  permissions  are  ignored.  Modes which are not supported by the
     operating system restoring  files  from  the  archive  will  be  ignored.
     Unsupported  modes should be faked up when creating an archive; e.g.  the
     group permission could be copied from the `other' permission.

     The uid and gid fields are the user and group  ID  of  the  file  owners,

     The size field is the size  of  the  file  in  bytes;  linked  files  are
     archived with this field specified as zero.

     The mtime field is the modification time of the file at the time  it  was
     archived.   It is the ASCII representation of the octal value of the last
     time the file was modified, represented as in integer number  of  seconds
     since January 1, 1970, 00:00 Coordinated Universal Time.

     The chksum field is the ASCII representaion of the  octal  value  of  the
     simple  sum  of  all  bytes in the header record.  Each 8-bit byte in the
     header is treated as an unsigned value.  These values  are  added  to  an
     unsigned integer, initialized to zero, the precision of which shall be no
     less than seventeen bits.  When  calculating  the  checksum,  the  chksum
     field is treated as if it were all blanks.

     The typeflag field specifies the type of file archived.  If a  particular
     implementation  does not recognize or permit the specified type, the file
     will be extracted as if it were a regular file.  As this  action  occurs,
     tar issues a warning to the standard error.

          represents a regular file.  For backward compatibility,  a  typeflag
          value  of  LF_OLDNORMAL  should  be silently recognized as a regular
          file.  New archives should be created using  LF_NORMAL.   Also,  for
          backward  compatability,  tar  treats a regular file whose name ends
          with a slash as a directory.

          represents a file linked to another file, of  any  type,  previously
          archived.  Such files are identified in Unix by each file having the
          same device and inode number.  The linked-to name  is  specified  in
          the linkname field with a trailing null.

          represents a symbolic link to another file.  The linked-to  name  is
          specified in the linkname field with a trailing null.

     LF_CHR or LF_BLK
          represent  character  special  files   and   block   special   files
          respectively.   In  this  case the devmajor and devminor fields will
          contain the major and minor device numbers respectively.   Operating
          systems  may  map  the  device  specifications  to  their  own local
          specification, or may ignore the entry.

          specifies a directory or sub-directory.  The directory name  in  the
          name  field  should  end  with  a  slash.   On  systems  where  disk
          allocation is performed on a directory basis  the  size  field  will
          contain  the  maximum  number  of bytes (which may be rounded to the
          nearest disk block allocation unit) which the directory may hold.  A
          size field of zero indicates no such limiting.  Systems which do not
          support limiting in this manner should ignore the size field.

          specifies a FIFO special file.  Note that the archiving  of  a  FIFO
          file archives the existence of this file and not its contents.

          specifies a contiguous file, which is the  same  as  a  normal  file
          except that, in operating systems which support it, all its space is
          allocated contiguously on the disk.  Operating systems which do  not
          allow  contiguous  allocation  should  silently treat this type as a
          normal file.

     `A' - `Z'
          are reserved for custom implementations.   None  are  used  by  this
          version of the tar program.

          values are reserved for specification in  future  revisions  of  the
          P1003 standard, and should not be used by any tar program.

     The magic field indicates that this  archive  was  output  in  the  P1003
     archive  format.  If this field contains TMAGIC, then the uname and gname
     fields will contain the ASCII representation of the owner  and  group  of
     the  file  respectively.   If found, the user and group ID represented by
     these names will be used rather than the values contained within the  uid
     and  gid  fields.  User names longer than TUNMLEN-1 or group names longer
     than TGNMLEN-1 characters will be truncated.

     tar(1), ar(5), cpio(5), dump(8), restor(8), restore(8)

     Names or link names longer than NAMSIZ-1 characters cannot be archived.

     This format does not yet address multi-volume archives.

     This manual page was adapted by John Gilmore from Draft 6  of  the  P1003