4.2BSD/usr/man/man4/mtio.4

.TH MTIO 4 "27 July 1983"
.UC 4
.SH NAME
mtio \- UNIX magtape interface
.SH DESCRIPTION
The files
.I "mt0, ..., mt15"
refer to the UNIX magtape drives,
which may be on the MASSBUS using the TM03 formatter
.IR ht (4),
or TM78 formatter,
.IR mt (4),
or on the UNIBUS using either the TM11 or TS11 formatters
.IR tm (4),
TU45 compatible formatters,
.IR ut (4),
or
.IR ts (4).
The following description applies to any of the transport/controller pairs.
The files
.I "mt0, ..., mt7"
are 800bpi, 
.I "mt8, ..., mt15"
are 1600bpi, and
.I "mt16, ..., mt23"
are 6250bpi.
(But note that only 1600 bpi is available with the TS11.)
The files
.IR "mt0, ..., mt3" ,
.IR "mt8, ..., mt11" ,
and
.I "mt16, ..., mt19"
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.
.PP
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,
however, because it tends to create monstrous record
gaps.
.PP
The
.I 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
.I "rmt0, ..., rmt23,"
but the same minor-device considerations as for the regular files still apply.
A number of other ioctl operations are available
on raw magnetic tape.
The following definitions are from
.RI < sys/mtio.h >:
.PP
.nf
/*
 * Structures and definitions for mag tape io control commands
 */

/* 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 */
#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		0x01
#define	MT_ISHT		0x02
#define	MT_ISTM		0x03
#define	MT_ISMT		0x04
#define	MT_ISUT		0x05
#define	MT_ISCPC	0x06
#define	MT_ISAR		0x07

/* mag tape io control commands */
#define	MTIOCTOP	_IOW(m, 1, struct mtop)		/* do a mag tape op */
#define	MTIOCGET	_IOR(m, 2, struct mtget)	/* get tape status */

#ifndef KERNEL
#define	DEFTAPE	"/dev/rmt12"
#endif
.fi
.ft R
.PP
Each
.I read
or
.I 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 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.
.SH FILES
/dev/mt?
.br
/dev/rmt?
.SH "SEE ALSO"
mt(1),
tar(1),
tp(1),
ht(4),
tm(4),
ts(4),
mt(4),
ut(4)
.SH BUGS
The status should be returned in a device independent format.