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researchv10 Dan Cross

.TH MT 4
.CT 2 comm_mach
.SH NAME
mt \- magtape interface
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/mtio.h>
.SH DESCRIPTION
The 
.I rmt
files refer to magnetic tape drives.
Filenames beginning with
.L rmt
are rewound when closed;
those beginning with
.L 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.
.PP
Conventionally 
.B rmt0
is 800,
.B rmt1
is 1600,
and
.B rmt2
is 6250 bpi.
.PP
Each
.IR read (2)
or
.IR write
call reads or writes the next record on the tape.
.I 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
.I read
to return 0;
the next
.I read
will return the next record.
Seeks are ignored.
.PP
An
.IR ioctl (2)
call performs special operations:
.TF MTIOCEEOT
.TP
.B MTIOCTOP
perform a suboperation encoded as below in the
.B mt_op
field of a structure whose address is passed
as the third argument of
.I ioctl.
.IP
.EX
.ta \w'struct 'u +\w'daddr_t 'u +\w'mt_count;  'u
struct	mtop {
	short	mt_op;		/* operation */
	daddr_t	mt_count;	/* repeat count */
};
.EE
.PD0
.RS
.TF MTOFFL
.TP
.B MTWEOF
write an end-of-file record
.TP
.B MTFSF
forward space file
.TP
.B MTBSF
backward space file
.TP
.B MTFSR
forward space record
.TP
.B MTBSR
backward space record
.TP
.B MTREW
rewind
.TP
.B MTOFFL
rewind and put the drive offline
.RE
.PD
.PP
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
.L mt
or
.LR 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 may create enormous record gaps.
The file always ends at the most recently written byte.
.PP
Conventions for minor device numbers
vary among different hardware drivers:
.IP ""
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.
.IP
The TE16 is like the TU78,
except that the default density is 800 bpi,
and adding 8 to the device number selects 1600.
.IP
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 possible 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 automatically on reading,
but if a drive
doesn't support a particular density,
the hardware may complain when the device is opened.
.SH FILES
.F /dev/mt?
.br
.F /dev/rmt?
.br
.F /dev/nmt?
.br
.F /dev/nrmt?
.SH "SEE ALSO"
.IR tape (1)
.SH BUGS
If any non-data error is encountered,
the tape drivers generally refuse to do anything
more until closed.
.br
The naming convention behaves poorly with multiple tape drives.
.br
Block tape has probably outlived its usefulness.

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