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@(#)mtop.4 4.1 (Berkeley) 05/15/85
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 */ /* mag tape io control commands */ #define MTIOCTOP (('m'<<8)|1) /* do a mag tape op */ #define MTIOCGET (('m'<<8)|2) /* get mag tape status */ /* 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 01 #define MT_ISHT 02 #define MT_ISTM 03 #define MT_ISMT 04 #define MT_ISUT 05
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.