NAME

     rmt — remote magtape protocol module

SYNOPSIS

     rmt

DESCRIPTION

     Rmt is a program used by tar, cpio, mt, and the remote dump and restore programs in
     manipulating a magnetic tape drive through an interprocess communication connection.  Rmt is
     normally started up with an rexec(3) or rcmd(3) call or the rsh(1) command.

     The rmt program accepts requests specific to the manipulation of magnetic tapes, performs
     the commands, then responds with a status indication.  All responses are in ASCII and in one
     of two forms.  Successful commands have responses of:

           Anumber\n

     Number is an ASCII representation of a decimal number.  Unsuccessful commands are responded
     to with:

           Eerror-number\nerror-message\n

     Error-number is one of the possible error numbers described in intro(2) and error-message is
     the corresponding error string as printed from a call to perror(3).  The protocol is
     comprised of the following commands, which are sent as indicated - no spaces are supplied
     between the command and its arguments, or between its arguments, and ‘\n’ indicates that a
     newline should be supplied:

     Odevice\nmode\n
             Open the specified deviceusing the indicated mode.Deviceis a full pathname and
             modeis an ASCIIrepresentation of a decimal number suitable for passing to open(2).If
             a device had already been opened, it is closed before a new open is performed.

     Cdevice\n
             Close the currently open device.  The devicespecified is ignored.

     Loffset\nwhence\n
             Perform an lseek(2) operation using the specified parameters.  The response value is
             that returned from the lseek call.

     Wcount\n
             Write data onto the open device.  Rmt reads count bytes from the connection,
             aborting if a premature end-of-file is encountered.  The response value is that
             returned from the write(2) call.

     Rcount\n
             Read count bytes of data from the open device.  If count exceeds the size of the
             data buffer (10 kilobytes), it is truncated to the data buffer size.  rmt then
             performs the requested read(2) and responds with Acount-read\n if the read was
             successful; otherwise an error in the standard format is returned.  If the read was
             successful, the data read is then sent.

     Ioperation\ncount\n
             Perform a MTIOCOP ioctl(2) command using the specified parameters.  The parameters
             are interpreted as the ASCII representations of the decimal values to place in the
             mt_op and mt_count fields of the structure used in the ioctl call.  The return value
             is the count parameter when the operation is successful.

     S       Return the status of the open device, as obtained with a MTIOCGET ioctl call.  If
             the operation was successful, an ``ack'' is sent with the size of the status buffer,
             then the status buffer is sent (in binary).

     Any other command causes rmt to exit.

DIAGNOSTICS

     All responses are of the form described above.

SEE ALSO

     tar(1), cpio(1), mt(1), rsh(1), rcmd(3), rexec(3), mtio(4), rdump(8), rrestore(8)

BUGS

     People should be discouraged from using this for a remote file access protocol.

HISTORY

     The rmt command appeared in 4.2BSD.