MTIO(4)

NAME
mtio - UNIX magtape interface

DESCRIPTION
The files mt0, ..., mt15 refer to the UNIX magtape drives, which may be on the MASSBUS using the TM03 formatter ht(4), or on the UNIBUS using either the TM11 or TS11 formatters tm(4) or ts(4). The following description applies to any of the transport/controller pairs. The files mt0, ..., mt7 are 800bpi (or the transport's lowest density), mt8, ..., mt15 are 1600bpi (or the transport's second density), and mt16, ..., mt23 are 6250bpi (or the transport's third density). (But note that only 1600 bpi is available with the TS11.) The files mt0, ..., mt3, mt8, ..., mt11, and 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.

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 uses most of the tape in 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, ..., 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 <sys/mtio.h>:

/*
 * 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 */
#define MTCACHE	8	/* enable controller cache */
#define MTNOCACHE	9	/* disable controller cache */
/* 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.  These are the same
 * for other controllers compatible with the types listed.
 */
#define	MT_ISTS		0x01		/* TS-11 */
#define	MT_ISHT		0x02		/* TM03 Massbus: TE16, TU45, TU77 */
#define	MT_ISTM		0x03		/* TM11/TE10 Unibus */
#define	MT_ISMT		0x04		/* TM78/TU78 Massbus */
#define	MT_ISUT		0x05		/* SI TU-45 emulation on Unibus */
#define	MT_ISCPC	0x06		/* SUN */
#define	MT_ISAR		0x07		/* SUN */
#define	MT_ISTMSCP	0x08		/* DEC TMSCP protocol (TU81, TK50) */
/* 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 */
#define MTIOCIEOT	_IO(m, 3)			/* ignore EOT error */
#define MTIOCEEOT	_IO(m, 4)			/* enable EOT error */
#ifndef KERNEL
#define	DEFTAPE	"/dev/rmt12"
#endif

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, 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.

The MTCACHE and MTNOCACHE commands do not apply to all tape drivers. At present only the TMSCP driver supports those commands and then only for drives such as the TU81+.

FILES
/dev/mt?
/dev/rmt?

SEE ALSO
mt(1), tar(1), tp(1), ht(4), tm(4), ts(4)

BUGS
The status should be returned in a device independent format.

The special file naming should be redone in a more consistent and understandable manner.