MTIO(4)

NAME
mtio - UNIX magtape interface

DESCRIPTION
The files mt0, ..., mt7 are 800 bpi tape drives, and the files mt8, ..., mt15 are 1600 bpi or 6250 bpi tape drives (as selected by a switch on the drive). The files mt0, ..., mt3 and mt8, ..., mt11 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 tends to create 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, ..., rmt15, 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 MTERASE	8	/* erase from current position to EOT */
#define MTRET	9	/* retension the (streamer) tape */
#define MTSEOD	10	/* skip to end of (streamer) tape data */
#define MTNORET	11	/* don't retention the tape this time */
/* 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
#define	MT_ISXT		0x08
#define	MT_ISST		0x09
#define	MT_ISTB		0x0A
/* 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

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.

FILES
/dev/mt0, ..., /dev/mt15
/dev/rmt0, ..., /dev/rmt15

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

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