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Unix Version 7 - man page for tm (v7 section 4)

TM(4)				     Kernel Interfaces Manual				    TM(4)

       tm - TM-11/TU-10 magtape interface

       The files mt0, ..., mt7 refer to the DEC TU10/TM11 magtape.  When closed it can be rewound
       or not, see below.  If it was open for writing, two end-of-files are written.  If the tape
       is not to be rewound it is positioned with the head between the two tapemarks.

       If the 0200 bit is on in the minor device number the tape is not rewound when closed.

       A standard tape consists of a series of 512 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, however, because it tends to  cre-
       ate monstrous record gaps.

       The  mt	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 rmt0, ..., rmt7.  Each read or write 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,  pro-
       vided it is no greater than the buffer size; if the record is long, an error is indicated.
       In raw tape I/O, the buffer must begin on a word boundary and  the  count  must	be  even.
       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.

       /dev/mt?, /dev/rmt?


       If any non-data error is encountered, it refuses to do anything more until closed.  In raw
       I/O,  there should be a way to perform forward and backward record and file spacing and to
       write an EOF mark.


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