MT(4)               UNIX Programmer's Manual                MT(4)

NAME
     mt - UNIX magtape interface

DESCRIPTION
     The files _m_t_0, ..., _m_t_1_5 refer to the UNIX magtape drives,
     which may be on the MASSBUS using the TM03 formatter _h_t(4),
     or on the UNIBUS using either the TM-11 or TS-11 formatters
     _t_m(4) or _t_s(4).  The following description applies to any of
     the transport/controller pairs.  The files _m_t_0 and _m_t_1 refer
     to drive 0 at 800 and 1600 bpi respectively, _m_t_2 and _m_t_3
     refer to drive 1 at 800 and 1600 bpi respectively, and so
     on.  The files _n_m_t_0, ..., _n_m_t_1_5 are rewound when closed; the
     others are not.  When a file open for writing is closed, two
     end-of-files are written.  If the tape is not to be rewound
     it is positioned with the head between the two tapemarks.

     A standard tape consists of a series of 1024 byte records
     terminated by an end-of-file.  To the extent possible, the
     system makes it possible, if inefficient, to treat the tape
     like any other file.  Seeks have their usual meaning and it
     is possible to read or write a byte at a time.  Writing in
     very small units is inadvisable, as only a small fraction of
     the tape will be used.

     The _m_t files discussed above are useful when it is desired
     to access the tape in a way compatible with ordinary files.
     When foreign tapes are to be dealt with, and especially when
     long records are to be read or written, the `raw' interface
     is appropriate.  The associated files are named _r_m_t_0, ...,
     _r_m_t_1_5, and _n_r_m_t_0, ..., _n_r_m_t_1_5 but the same minor-device con-
     siderations as for the regular files still apply.  A number
     of other ioctl operations are available on raw magnetic
     tape.  The following definitions are from <sys/mtio.h>:

     /*
      * Structures and definitions for mag tape io control commands
      */

     /* mag tape io control commands */
     #define MTIOCTOP    (('m'<<8)|1)   /* do a mag tape op */
     #define MTIOCGET    (('m'<<8)|2)   /* get mag tape status */

     /* structure for MTIOCTOP - mag tape op command */
     struct    mtop {
          short     mt_op;         /* operations defined below */
          daddr_t   mt_count; /* how many of them */
     };

     /* operations */
     #define MTWEOF 0    /* write an end-of-file record */
     #define MTFSF  1    /* forward space file */
     #define MTBSF  2    /* backward space file */

Printed 5/17/83                                                 1

MT(4)               UNIX Programmer's Manual                MT(4)

     #define MTFSR  3    /* forward space record */
     #define MTBSR  4    /* backward space record */
     #define MTREW  5    /* rewind */
     #define MTOFFL 6    /* rewind and put the drive offline */
     #define MTNOP  7    /* no operation, sets status only */

     /* structure for MTIOCGET - mag tape get status command */

     struct    mtget     {
          short     mt_type;       /* type of magtape device */
     /* the following two registers are grossly device dependent */
          short     mt_dsreg;      /* ``drive status'' register */
          short     mt_erreg;      /* ``error'' register */
     /* end device-dependent registers */
          short     mt_resid;      /* residual count */
     /* the following two are not yet implemented */
          daddr_t mt_fileno;  /* file number of current position */
          daddr_t mt_blkno;   /* block number of current position */
     /* end not yet implemented */
     };

     /*
      * Constants for mt_type byte
      */
     #define MT_ISTS          01
     #define MT_ISHT          02
     #define MT_ISTM          03

     Each _r_e_a_d or _w_r_i_t_e call reads or writes the next record on
     the tape.  In the write case the record has the same length
     as the buffer given.  During a read, the record size is
     passed back as the number of bytes read, provided it is no
     greater than the buffer size; if the record is long, an
     error is indicated.  In raw tape I/O seeks are ignored.  A
     zero byte count is returned when a tape mark is read, but
     another read will fetch the first record of the new tape
     file.

FILES
     /dev/mt??           block files, rewound on close
     /dev/nmt??          block files, not rewound on close
     /dev/rmt??          raw files, rewound on close
     /dev/nrmt??         raw files, not rewound on close

SEE ALSO
     mt(1), tar(1), tp(1), ht(4), tm(4), ts(4)

BUGS
     Drive density selection is dependent on capabilities of both
     hardware and software.

Printed 5/17/83                                                 2