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     MT(4)                                                       MT(4)

     NAME
          mt - magtape interface

     SYNOPSIS
          #include <sys/types.h>
          #include <sys/mtio.h>

     DESCRIPTION
          The rmt files refer to magnetic tape drives.  Filenames
          beginning with `rmt' are rewound when closed; those begin-
          ning with `nrmt' are not.  When a file open for writing is
          closed, two file marks are written.  If the tape is not to
          be rewound, it is positioned with the head between the two
          file marks.

          Conventionally rmt0 is 800, rmt1 is 1600, and rmt2 is 6250
          bpi.

          Each read(2) or write call reads or writes the next record
          on the tape.  Read returns at most a single record; the
          return value is the record size.  If the next record is
          larger than the read buffer, an error is returned.  A file
          mark causes read to return 0; the next read will return the
          next record.  Seeks are ignored.

          An ioctl(2) call performs special operations:

          MTIOCTOP   perform a suboperation encoded as below in the
                     mt_op field of a structure whose address is
                     passed as the third argument of ioctl.
                     struct mtop {
                            short   mt_op;     /* operation */
                            daddr_t mt_count;  /* repeat count */
                     };
                     MTWEOF  write an end-of-file record
                     MTFSF   forward space file
                     MTBSF   backward space file
                     MTFSR   forward space record
                     MTBSR   backward space record
                     MTREW   rewind
                     MTOFFL  rewind and put the drive offline

          The files described above provide a `raw' interface.  There
          is also a `block' interface which attempts to treat the tape
          like an ordinary file as much as possible.  Block tapes are
          accessed through files with names beginning with `mt' or
          `nmt'.  Such a tape contains a single file, consisting of a
          series of 1024-byte records followed by a file mark.  Seeks
          have their usual meaning, and it is possible to read and
          write a byte at a time, though writing in very small units

     MT(4)                                                       MT(4)

          may create enormous record gaps.  The file always ends at
          the most recently written byte.

          Conventions for minor device numbers vary among different
          hardware drivers:

               For the TU78, the drive unit number is encoded in the
               two low-order bits.  Adding 4 prevents the tape from
               rewinding at close time.  Adding 8 selects 6250 bpi for
               writing; the default is 1600.  The tape drive senses
               density automatically when reading.

               The TE16 is like the TU78, except that the default den-
               sity is 800 bpi, and adding 8 to the device number
               selects 1600.

               For TMSCP tape drives such as the TU81 and the TK50,
               the unit number is encoded in the three low-order bits.
               Adding 128 prevents the tape from rewinding on close.
               Density is selected by octal bits 070; the eight possi-
               ble values represent eight different device-dependent
               tape formats.  For 9-track tape drives, add 0 for 800
               bpi, 8 for 1600, 16 for 6250.  For TK50 cartridge
               drives, add 24 (old-style block format).  For TK70
               drives, add 8.  The tape drives sense density automati-
               cally on reading, but if a drive doesn't support a par-
               ticular density, the hardware may complain when the
               device is opened.

     FILES
     SEE ALSO
          tape(1)

     BUGS
          If any non-data error is encountered, the tape drivers gen-
          erally refuse to do anything more until closed.
          The naming convention behaves poorly with multiple tape
          drives.
          Block tape has probably outlived its usefulness.