tm.4

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.th TM IV 2/21/74
.sh NAME
tm \*- TM-11/TU-10 magtape interface
.sh DESCRIPTION
The files
.it "mt0, ..., mt7"
refer to the DEC TU10/TM11 magtape.
When opened for reading or writing,
the tape is rewound.
When closed, it is rewound;
if it was open for writing, an end-of-file is written first.
.s3
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 create monstrous record
gaps.
.s3
The
.it 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
.it "rmt0, ..., rmt7."
Each
.it read
or
.it 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, provided 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.
An error is returned when a tape mark is read,
but another read will fetch the first record of the
new tape file.
.sh FILES
/dev/mt?,
/dev/rmt?
.sh "SEE ALSO"
tp (I)
.sh BUGS
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.