Manpages

NAME

mtio − general magnetic tape interface

SYNOPSIS

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/mtio.h>

DESCRIPTION

1/2", 1/4", 4mm, and 8mm magnetic tape drives all share the same general character device interface.

There are two types of tape records: data records and end-of-file (EOF) records. EOF records are also known as tape marks and file marks. A record is separated by interrecord (or tape) gaps on a tape.

End-of-recorded-media (EOM) is indicated by two EOF marks on 1/2" tape; by one EOF mark on 1/4", 4mm, and 8mm cartridge tapes.

1/2 Reel Tape"
Data bytes are recorded in parallel onto the 9-track tape. Since it is a variable-length tape device, the number of bytes in a physical record may vary.

The recording formats available (check specific tape drive) are 800 BPI, 1600 BPI, 6250 BPI, and data compression. Actual storage capacity is a function of the recording format and the length of the tape reel. For example, using a 2400 foot tape, 20 Mbyte can be stored using 800 BPI, 40 Mbyte using 1600 BPI, 140 Mbyte using 6250 BPI, or up to 700 Mbyte using data compression.

1/4 Cartridge Tape"
Data is recorded serially onto 1/4" cartridge tape. The number of bytes per record is determined by the physical record size of the device. The I/O request size must be a multiple of the physical record size of the device. For QIC-11, QIC-24, and QIC-150 tape drives, the block size is 512 bytes.

The records are recorded on tracks in a serpentine motion. As one track is completed, the drive switches to the next and begins writing in the opposite direction, eliminating the wasted motion of rewinding. Each file, including the last, ends with one file mark.

Storage capacity is based on the number of tracks the drive is capable of recording. For example, 4-track drives can only record 20 Mbyte of data on a 450 foot tape; 9-track drives can record up to 45 Mbyte of data on a tape of the same length. QIC-11 is the only tape format available for 4-track tape drives. In contrast, 9-track tape drives can use either QIC-24 or QIC-11. Storage capacity is not appreciably affected by using either format. QIC-24 is preferable to QIC-11 because it records a reference signal to mark the position of the first track on the tape, and each block has a unique block number.

The QIC-150 tape drives require DC-6150 (or equivalent) tape cartridges for writing. However, they can read other tape cartridges in QIC-11, QIC-24, or QIC-120 tape formats.

8mm Cartridge Tape
Data is recorded serially onto 8mm helical scan cartridge tape. Since it is a variable-length tape device, the number of bytes in a physical record may vary. The recording formats available (check specific tape drive) are standard 2Gbyte, 5Gbyte, and compressed format.

4mm DAT Tape
Data is recorded either in Digital Data Storage (DDS) tape format or in Digital Data Storage, Data Compressed (DDS-DC) tape format. Since it is a variable-length tape device, the number of bytes in a physical record may vary. The recording formats available are standard 2Gbyte and compressed format.

Persistent Error Handling
Persistent error handling is a modification of the current error handling behaviors, BSD and SVR4. With persistent error handling enabled, all tape operations after an error or exception will return immediately with an error. Persistent error handling can be most useful with asynchronous tape operations that use the aioread(3AIO) and aiowrite(3AIO) functions.

To enable persistent error handling, the ioctl MTIOCPERSISTENT must be issued. If this ioctl succeeds, then persistent error handling is enabled and changes the current error behavior. This ioctl will fail if the device driver does not support persistent error handling.

With persistent error handling enabled, all tape operations after an exception or error will return with the same error as the first command that failed; the operations will not be executed. An exception is some event that might stop normal tape operations, such as an End Of File (EOF) mark or an End Of Tape (EOT) mark. An example of an error is a media error. The MTIOCLRERR ioctl must be issued to allow normal tape operations to continue and to clear the error.

Disabling persistent error handling returns the error behavior to normal SVR4 error handling, and will not occur until all outstanding operations are completed. Applications should wait for all outstanding operations to complete before disabling persistent error handling. Closing the device will also disable persistent error handling and clear any errors or exceptions.

The Read Operation and Write Operation subsections contain more pertinent information reguarding persistent error handling.

Read Operation
The read(2) function reads the next record on the tape. The record size is passed back as the number of bytes read, provided it is not greater than the number requested. When a tape mark or end of data is read, a zero byte count is returned; all successive reads after the zero read will return an error and errno will be set to EIO. To move to the next file, an MTFSF ioctl can be issued before or after the read causing the error. This error handling behavior is different from the older BSD behavior, where another read will fetch the first record of the next tape file. If the BSD behavior is required, device names containing the letter b (for BSD behavior) in the final component should be used. If persistent error handling was enabled with either the BSD or SVR4 tape device behavior, all operations after this read error will return EIO errors until the MTIOCLRERR ioctl is issued. An MTFSF ioctl can then he issued.

Two successful successive reads that both return zero byte counts indicate EOM on the tape. No further reading should be performed past the EOM.

Fixed-length I/O tape devices require the number of bytes read to be a multiple of the physical record size. For example, 1/4" cartridge tape devices only read multiples of 512 bytes. If the blocking factor is greater than 64,512 bytes (minphys limit), fixed-length I/O tape devices read multiple records.

Most tape devices which support variable-length I/O operations may read a range of 1 to 65,535 bytes. If the record size exceeds 65,535 bytes, the driver reads multiple records to satisfy the request. These multiple records are limited to 65,534 bytes. Newer variable-length tape drivers may relax the above limitation and allow applications to read record sizes larger than 65,534. Refer to the specific tape driver man page for details.

Reading past logical EOT is transparent to the user. A read operation should never hit physical EOT.

Read requests that are lesser than a physical tape record are not allowed. Appropriate error is returned.

Write Operation
The write(2) function writes the next record on the tape. The record has the same length as the given buffer.

Writing is allowed on 1/4" tape at either the beginning of tape or after the last written file on the tape. With the Exabyte 8200, data may be appended only at the beginning of tape, before a filemark, or after the last written file on the tape.

Writing is not so restricted on 1/2", 4mm, and the other 8mm cartridge tape drives. Care should be used when appending files onto 1/2" reel tape devices, since an extra file mark is appended after the last file to mark the EOM. This extra file mark must be overwritten to prevent the creation of a null file. To facilitate write append operations, a space to the EOM ioctl is provided. Care should be taken when overwriting records; the erase head is just forward of the write head and any following records will also be erased.

Fixed-length I/O tape devices require the number of bytes written to be a multiple of the physical record size. For example, 1/4" cartridge tape devices only write multiples of 512 bytes.

Fixed-length I/O tape devices write multiple records if the blocking factor is greater than 64,512 bytes (minphys limit). These multiple writes are limited to 64,512 bytes. For example, if a write request is issued for 65,536 bytes using a 1/4" cartridge tape, two writes are issued; the first for 64,512 bytes and the second for 1024 bytes.

Most tape devices which support variable-length I/O operations may write a range of 1 to 65,535 bytes. If the record size exceeds 65,535 bytes, the driver writes multiple records to satisfy the request. These multiple records are limited to 65,534 bytes. As an example, if a write request for 65,540 bytes is issued, two records are written; one for 65,534 bytes followed by another record for 6 bytes. Newer variable-length tape drivers may relax the above limitation and allow applications to write record sizes larger than 65,534. Refer to the specific tape driver man page for details.

When logical EOT is encountered during a write, that write operation completes and the number of bytes successfully transferred is returned (note that a ’short write’ may have occurred and not all the requested bytes would have been transferred. The actual amount of data written will depend on the type of device being used). The next write will return a zero byte count. A third write will successfully transfer some bytes (as indicated by the returned byte count, which again could be a short write); the fourth will transfer zero bytes, and so on, until the physical EOT is reached and all writes will fail with EIO.

When logical EOT is encountered with persistent error handling enabled, the current write may complete or be a short write. The next write will return a zero byte count. At this point an application should act appropriately for end of tape cleanup or issue yet another write, which will return the error ENOSPC. After clearing the exception with MTIOCLRERR, the next write will succeed (possibly short), followed by another zero byte write count, and then another ENOSPC error.

Allowing writes after LEOT has been encountered enables the flushing of buffers. However, it is strongly recommended to terminate the writing and close the file as soon as possible.

Seeks are ignored in tape I/O.

Close Operation
Magnetic tapes are rewound when closed, except when the "no-rewind" devices have been specified. The names of no-rewind device files use the letter n as the end of the final component. The no-rewind version of /dev/rmt/0l is /dev/rmt/0ln. In case of error for a no-rewind device, the next open rewinds the device.

If the driver was opened for reading and a no-rewind device has been specified, the close advances the tape past the next filemark (unless the current file position is at EOM), leaving the tape correctly positioned to read the first record of the next file. However, if the tape is at the first record of a file it doesn’t advance again to the first record of the next file. These semantics are different from the older BSD behavior. If BSD behavior is required where no implicit space operation is executed on close, the non-rewind device name containing the letter b (for BSD behavior) in the final component should be specified.

If data was written, a file mark is automatically written by the driver upon close. If the rewinding device was specified, the tape will be rewound after the file mark is written. If the user wrote a file mark prior to closing, then no file mark is written upon close. If a file positioning ioctl, like rewind, is issued after writing, a file mark is written before repositioning the tape.

All buffers are flushed on closing a tape device. Hence, it is strongly recommended that the application wait for all buffers to be flushed before closing the device. This can be done by writing a filemark via MTWEOF, even with a zero count.

Note that for 1/2" reel tape devices, two file marks are written to mark the EOM before rewinding or performing a file positioning ioctl. If the user wrote a file mark before closing a 1/2" reel tape device, the driver will always write a file mark before closing to insure that the end of recorded media is marked properly. If the non-rewinding device was specified, two file marks are written and the tape is left positioned between the two so that the second one is overwritten on a subsequent open(2) and write(2).

If no data was written and the driver was opened for WRITE-ONLY access, one or two file marks are written, thus creating a null file.

After closing the device, persistent error handling will be disabled and any error or exception will be cleared.

IOCTLS

Not all devices support all ioctls. The driver returns an ENOTTY error on unsupported ioctls.

The following structure definitions for magnetic tape ioctl commands are from <sys/mtio.h>.

The minor device byte structure is::

15      7      6          5          4         3          2       1   0
________________________________________________________________________
Unit #       BSD      Reserved   Density   Density   No rewind    Unit #
Bits 7-15   behavior              Select    Select    on Close    Bits 0-1
/*
 * Layout of minor device byte:
 */
#define MTUNIT(dev) (((minor(dev) & 0xff80) >> 5) +
(minor(dev) & 0x3))
#define MT_NOREWIND (1 <<2)
#define MT_DENSITY_MASK  (3 <<3)
#define MT_DENSITY1 (0 <<3)   /* Lowest density/format */
#define MT_DENSITY2 (1 <<3)
#define MT_DENSITY3 (2 <<3)
#define MT_DENSITY4 (3 <<3)   /* Highest density/format */
#define MTMINOR(unit)    (((unit & 0x7fc) << 5) + (unit & 0x3))
#define MT_BSD (1 <<6)       /* BSD behavior on close */


/* Structure for MTIOCTOP − magnetic tape operation command */


struct  mtop {
  short   mt_op;       /* operation */
  daddr_t mt_count;    /* number of operations */
};

The following operations of MTIOCTOP ioctl are supported:
MTWEOF

write an end-of-file record

MTFSF

forward space over file mark

MTBSF

backward space over file mark (1/2", 8mm only)

MTFSR

forward space to inter-record gap

MTBSR

backward space to inter-record gap

MTREW

rewind

MTOFFL

rewind and take the drive off-line

MTNOP

no operation, sets status only

MTRETEN

retension the tape (cartridge tape only)

MTERASE

erase the entire tape and rewind

MTEOM

position to EOM

MTNBSF

backward space file to beginning of file

MTSRSZ

set record size

MTGRSZ

get record size

MTLOAD

load the next tape cartridge into the tape drive

/* structure for MTIOCGET − magnetic tape get status command */


struct  mtget {
  short   mt_type;  /* type of magtape device */
/* the following two registers are device dependent */
  short  mt_dsreg;      /* "drive status" register */
  short  mt_erreg;      /* "error" register */
/* optional error info. */
  daddr_t   mt_resid;   /* residual count */
  daddr_t   mt_fileno;  /* file number of current position */
  daddr_t   mt_blkno;   /* block number of current position */
  ushort_t  mt_flags;
  short     mt_bf;      /* optimum blocking factor */
};
/* structure for MTIOCGETDRIVETYPE − get tape config data command */
struct mtdrivetype_request {
  int  size;
  struct  mtdrivetype    *mtdtp;
};
struct mtdrivetype {
  char    name[64];                  /* Name, for debug */
  char    vid[25];                   /* Vendor id and product id */
  char    type;                      /* Drive type for driver */
  int     bsize;                     /* Block size */
  int     options;                   /* Drive options */
  int     max_rretries;              /* Max read retries */
  int     max_wretries;              /* Max write retries */
  uchar_t densities[MT_NDENSITIES];  /* density codes,low->hi */
  uchar_t default_density;           /* Default density chosen */
  uchar_t speeds[MT_NSPEEDS];        /* speed codes, low->hi */
  ushort_t non_motion_timeout;       /* Seconds for non-motion */
  ushort_t io_timeout;               /* Seconds for data to from tape */
  ushort_t rewind_timeout;           /* Seconds to rewind */
  ushort_t space_timeout;            /* Seconds to space anywhere */
  ushort_t load_timeout;             /* Seconds to load tape and ready */
  ushort_t unload_timeout;           /* Seconds to unload */
  ushort_t erase_timeout;            /* Seconds to do long erase */
};

The MTWEOF ioctl is used for writing file marks to tape. Not only does this signify the end of a file, but also usually has the side effect of flushing all buffers in the tape drive to the tape medium. A zero count MTWEOF will just flush all the buffers and will not write any file marks. Because a successful completion of this tape operation will guarantee that all tape data has been written to the tape medium, it is recommended that this tape operation be issued before closing a tape device.

When spacing forward over a record (either data or EOF), the tape head is positioned in the tape gap between the record just skipped and the next record. When spacing forward over file marks (EOF records), the tape head is positioned in the tape gap between the next EOF record and the record that follows it.

When spacing backward over a record (either data or EOF), the tape head is positioned in the tape gap immediately preceding the tape record where the tape head is currently positioned. When spacing backward over file marks (EOF records), the tape head is positioned in the tape gap preceding the EOF. Thus the next read would fetch the EOF.

Record skipping does not go past a file mark; file skipping does not go past the EOM. After an MTFSR <huge number> command, the driver leaves the tape logically positioned before the EOF. A related feature is that EOFs remain pending until the tape is closed. For example, a program which first reads all the records of a file up to and including the EOF and then performs an MTFSF command will leave the tape positioned just after that same EOF, rather than skipping the next file.

The MTNBSF and MTFSF operations are inverses. Thus, an " MTFSF −1" is equivalent to an " MTNBSF 1". An " MTNBSF 0" is the same as " MTFSF 0"; both position the tape device at the beginning of the current file.

MTBSF moves the tape backwards by file marks. The tape position will end on the beginning of the tape side of the desired file mark. An " MTBSF 0" will position the tape at the end of the current file, before the filemark.

MTBSR and MTFSR operations perform much like space file operations, except that they move by records instead of files. Variable-length I/O devices (1/2" reel, for example) space actual records; fixed-length I/O devices space physical records (blocks). 1/4" cartridge tape, for example, spaces 512 byte physical records. The status ioctl residual count contains the number of files or records not skipped.

MTOFFL rewinds and, if appropriate, takes the device off-line by unloading the tape. It is recommended that the device be closed after offlining and then re-opened after a tape has been inserted to facilitate portability to other platforms and other operating systems. Attempting to re-open the device with no tape will result in an error unless the O_NDELAY flag is used. (See open(2).)

The MTRETEN retension ioctl applies only to 1/4" cartridge tape devices. It is used to restore tape tension, improving the tape’s soft error rate after extensive start-stop operations or long-term storage.

MTERASE rewinds the tape, erases it completely, and returns to the beginning of tape. Erasing may take a long time depending on the device and/or tapes. For time details, refer to the the drive specific manual.

MTEOM positions the tape at a location just after the last file written on the tape. For 1/4" cartridge and 8mm tape, this is after the last file mark on the tape. For 1/2" reel tape, this is just after the first file mark but before the second (and last) file mark on the tape. Additional files can then be appended onto the tape from that point.

Note the difference between MTBSF (backspace over file mark) and MTNBSF (backspace file to beginning of file). The former moves the tape backward until it crosses an EOF mark, leaving the tape positioned before the file mark. The latter leaves the tape positioned after the file mark. Hence, "MTNBSF n" is equivalent to "MTBSF (n+1)" followed by "MTFSF 1". The 1/4" cartridge tape devices do not support MTBSF.

MTSRSZ and MTGRSZ are used to set and get fixed record lengths. The MTSRSZ ioctl allows variable length and fixed length tape drives that support multiple record sizes to set the record length. The mt_count field of the mtop struct is used to pass the record size to/from the st driver. A value of 0 indicates variable record size. The MTSRSZ ioctl makes a variable-length tape device behave like a fixed-length tape device. Refer to the specific tape driver man page for details.

MTLOAD loads the next tape cartridge into the tape drive. This is generally only used with stacker and tower type tape drives which handle multiple tapes per tape drive. A tape device without a tape inserted can be opened with the O_NDELAY flag, in order to execute this operation.

The MTIOCGET get status ioctl call returns the drive ID (mt_type), sense key error (mt_erreg), file number (mt_fileno), optimum blocking factor (mt_bf) and record number (mt_blkno) of the last error. The residual count (mt_resid) is set to the number of bytes not transferred or files/records not spaced. The flags word (mt_flags) contains information such as whether the device is SCSI, whether it is a reel device, and whether the device supports absolute file positioning.

The MTIOCGETDRIVETYPE get drivetype ioctl call returns the name of the tape drive as defined in st.conf (name), Vendor ID and model (product), ID (vid), type of tape device (type), block size (bsize), drive options
(options), maximum read retry count (max_rretries), maximum write retry count (max_wretries), densities supported by the drive (densities), and default density of the tape drive (default_density).

Persistent Error Handling IOCTLs and Asynchronous Tape Operations
MTIOCPERSISTENT

enables/disables persistent error handling

MTIOCPERSISTENTSTATUS

queries for persistent error handling

MTIOCLRERR

clears persistent error handling

MTIOCGUARANTEEDORDER

checks whether driver guarantees order of I/O’s

The MTIOCPERSISTENT ioctl enables or disables persistent error handling. It takes as an argument a pointer to an integer that turns it either on or off. If the ioctl succeeds, the desired operation was successful. It will wait for all outstanding I/O’s to complete before changing the persistent error handling status. For example,

int on = 1;
ioctl(fd, MTIOCPERSISTENT, &on);
int off = 0;
ioctl(fd, MTIOCPERSISTENT, &off);

The MTIOCPERSISTENTSTATUS ioctl enables or disables persistent error handling. It takes as an argument a pointer to an integer inserted by the driver. The integer can be either 1 if persistent error handling is ’on’, or 0 if persistent error handling is ’off’. It will not wait for outstanding I/O’s. For example,

int query;
ioctl(fd, MTIOCPERSISTENTSTATUS, &query);

The MTIOCLRERR ioctl clears persistent error handling and allows tape operations to continual normally. This ioctl requires no argument and will always succeed, even if persistent error handling has not been enabled. It will wait for any outstanding I/O’s before it clears the error.

The MTIOCGUARANTEEDORDER ioctl is used to determine whether the driver guarantees the order of I/O’s. It takes no argument. If the ioctl succeeds, the driver will support guaranteed order. If the driver does not support guaranteed order, then it should not be used for asynchronous I/O with libaio. It will wait for any outstanding I/O’s before it returns. For example,

ioctl(fd, MTIOCGUARANTEEDORDER)

See the Persistent Error Handling subsection above for more information on persistent error handling.

Asynchronous and State Change IOCTLS
MTIOCSTATE

This ioctl blocks until the state of the drive, inserted or ejected, is changed. The argument is a pointer to a mtio_state, enum, whose possible enumerations are listed below. The initial value should be either the last reported state of the drive, or MTIO_NONE. Upon return, the enum pointed to by the argument is updated with the current state of the drive.

enum mtio_state {
MTIO_NONE      /* Return tape’s current state */
MTIO_EJECTED   /* Tape state is "ejected" */
MTIO_INSERTED  /* Tape state is "inserted" */
;

When using asynchronous operations, most ioctls will wait for all outstanding commands to complete before they are executed.

IOCTLS for Multi-initiator Configurations
MTIOCRESERVE

reserve the tape drive

MTIOCRELEASE

revert back to the default behavior of reserve on open/release on close

MTIOCFORCERESERVE

reserve the tape unit by breaking reservation held by another host

The MTIOCRESERVE ioctl reserves the tape drive such that it does not release the tape drive at close. This changes the default behavior of releasing the device upon close. Reserving the tape drive that is already reserved has no effect. For example,

ioctl(fd, MTIOCRESERVE);

The MTIOCRELEASE ioctl reverts back to the default behavior of reserve on open/release on close operation, and a release will occur during the next close. Releasing the tape drive that is already released has no effect. For example,

ioctl(fd, MTIOCRELEASE);

The MTIOCFORCERESERVE ioctl breaks a reservation held by another host, interrupting any I/O in progress by that other host, and then reserves the tape unit. This ioctl can be executed only with super-user privileges. It is recommended to open the tape device in O_NDELAY mode when this ioctl needs to be executed, otherwise the open will fail if another host indeed has it reserved. For example,

ioctl(fd, MTIOCFORCERESERVE);

IOCTLS for Handling Tape Configuration Options
MTIOCSHORTFMK

enables/disable support for writing short filemarks. This is specific to Exabyte drives.

MTIOCREADIGNOREILI

enables/disable supress incorrect length indicator support during reads

MTIOCREADIGNOREEOFS

enables/disable support for reading past two EOF marks which otherwise indicate End-Of-recording-Media (EOM) in the case of 1/2" reel tape drives

The MTIOCSHORTFMK ioctl enables or disables support for short filemarks. This ioctl is only applicable to Exabyte drives which support short filemarks. As an argument, it takes a pointer to an integer. If 0 (zero) is the specified integer, then long filemarks will be written. If 1 is the specified integer, then short filemarks will be written. The specified tape bahavior will be in effect until the device is closed.

For example:

int on = 1;
int off = 0;
/* enable short filemarks */
ioctl(fd, MTIOSHORTFMK, &on);
/* disable short filemarks */
ioctl(fd, MTIOCSHORTFMK, &off);

Tape drives which do not support short filemarks will return an errno of ENOTTY.

The MTIOCREADIGNOREILI ioctl enables or disables the suppress incorrect length indicator (SILI) support during reads. As an argument, it takes a pointer to an integer. If 0 (zero) is the specified integer, SILI will not be used during reads and incorrect length indicator will not be supressed. If 1 is the specified integer, SILI will be used during reads and incorrect length indicator will be supressed.
The specified tape bahavior will be in effect until the device is closed.

For example:

int on = 1;
int off = 0;
ioctl(fd, MTIOREADIGNOREILI, &on);
ioctl(fd, MTIOREADIGNOREILI, &off);

The MTIOCREADIGNOREEOFS ioctl enables or disables support for reading past double EOF marks which otherwise indicate End-Of-recorded-media (EOM) in the case of 1/2" reel tape drives.
As an argument, it takes a pointer to an integer. If 0 (zero) is the specified integer, then double EOF marks indicate End-Of-recodred-media (EOD). If 1 is the specified integer, the double EOF marks no longer indicate EOM, thus allowing applications to read past two EOF marks. In this case it is the responsibility of the application to detect end-of-recorded-media (EOM). The specified tape bahavior will be in effect until the device is closed.

For example:

int on = 1;
int off = 0;
ioctl(fd, MTIOREADIGNOREEOFS, &on);
ioctl(fd, MTIOREADIGNOREEOFS, &off);

Tape drives other than 1/2" reel tapes will return an errno of ENOTTY.

EXAMPLES

Example 1: Tape Positioning and Tape Drives

Suppose you have written three files to the non-rewinding 1/2" tape device, /dev/rmt/0ln, and that you want to go back and dd(1M) the second file off the tape. The commands to do this are:

mt -F /dev/rmt/0lbn bsf 3
mt -F /dev/rmt/0lbn fsf 1
dd if=/dev/rmt/0ln

To accomplish the same tape positioning in a C program, followed by a get status ioctl:

struct mtop mt_command;
struct mtget mt_status;
mt_command.mt_op = MTBSF;
mt_command.mt_count = 3;
ioctl(fd, MTIOCTOP, &mt_command);
mt_command.mt_op = MTFSF;
mt_command.mt_count = 1;
ioctl(fd, MTIOCTOP, &mt_command);
ioctl(fd, MTIOCGET, (char *)&mt_status);

or

mt_command.mt_op = MTNBSF;
mt_command.mt_count = 2;
ioctl(fd, MTIOCTOP, &mt_command);
ioctl(fd, MTIOCGET, (char *)&mt_status);

To get information about the tape drive:

struct mtdrivetype mtdt;
struct mtdrivetype_request mtreq;
mtreq.size = sizeof(struct mtdrivetype);
mtreq.mtdtp = &mtdt;
ioctl(fd, MTIOCGETDRIVETYPE, &mtreq);

FILES

/dev/rmt/<unit number><density>[<BSD behavior>][<no rewind>]

Where density can be l, m, h, u/c (low, medium, high, ultra/compressed, respectively), the BSD behavior option is b, and the no rewind option is n.

For example, /dev/rmt/0hbn specifies unit 0, high density, BSD behavior and no rewind.

SEE ALSO

mt(1), tar(1), dd(1M), open(2), read(2), write(2), aioread(3AIO), aiowrite(3AIO), ar(3HEAD), st(7D)

1/4 Inch Tape Drive Tutorial