.\" Copyright 1995 Robert K. Nichols (Robert.K.Nichols@att.com)

.\" Copyright 1999-2005 Kai Mäkisara (Kai.Makisara@kolumbus.fi)

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

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.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.TH ST 4  2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

st \- SCSI tape device

.SH SYNOPSIS

.nf

.B #include <sys/mtio.h>

.PP

.BI "int ioctl(int " fd ", int " request " [, (void *)" arg3 "]);"

.BI "int ioctl(int " fd ", MTIOCTOP, (struct mtop *)" mt_cmd );

.BI "int ioctl(int " fd ", MTIOCGET, (struct mtget *)" mt_status );

.BI "int ioctl(int " fd ", MTIOCPOS, (struct mtpos *)" mt_pos );

.fi

.SH DESCRIPTION

The

.B st

driver provides the interface to a variety of SCSI tape devices.

Currently, the driver takes control of all detected devices of type

\(lqsequential-access\(rq.

The

.B st

driver uses major device number 9.

.PP

Each device uses eight minor device numbers.

The lowermost five bits

in the minor numbers are assigned sequentially in the order of

detection.

In the 2.6 kernel, the bits above the eight lowermost bits are

concatenated to the five lowermost bits to form the tape number.

The minor numbers can be grouped into

two sets of four numbers: the principal (auto-rewind) minor device numbers,

.IR n ,

and the \(lqno-rewind\(rq device numbers,

.RI ( n " + 128)."

Devices opened using the principal device number will be sent a

.BR REWIND

command when they are closed.

Devices opened using the \(lqno-rewind\(rq device number will not.

(Note that using an auto-rewind device for positioning the tape with,

for instance, mt does not lead to the desired result: the tape is

rewound after the mt command and the next command starts from the

beginning of the tape).

.PP

Within each group, four minor numbers are available to define

devices with different characteristics (block size, compression,

density, etc.)

When the system starts up, only the first device is available.

The other three are activated when the default

characteristics are defined (see below).

(By changing compile-time

constants, it is possible to change the balance between the maximum

number of tape drives and the number of minor numbers for each

drive.

The default allocation allows control of 32 tape drives.

For instance, it is possible to control up to 64 tape drives

with two minor numbers for different options.)

.PP

Devices are typically created by:

.PP

.in +4n

.EX

mknod \-m 666 /dev/st0 c 9 0

mknod \-m 666 /dev/st0l c 9 32

mknod \-m 666 /dev/st0m c 9 64

mknod \-m 666 /dev/st0a c 9 96

mknod \-m 666 /dev/nst0 c 9 128

mknod \-m 666 /dev/nst0l c 9 160

mknod \-m 666 /dev/nst0m c 9 192

mknod \-m 666 /dev/nst0a c 9 224

.EE

.in

.PP

There is no corresponding block device.

.PP

The driver uses an internal buffer that has to be large enough to hold

at least one tape block.

In kernels before 2.1.121, the buffer is

allocated as one contiguous block.

This limits the block size to the

largest contiguous block of memory the kernel allocator can provide.

The limit is currently 128\ kB for 32-bit architectures and

256\ kB for 64-bit architectures.

In newer kernels the driver

allocates the buffer in several parts if necessary.

By default, the

maximum number of parts is 16.

This means that the maximum block size

is very large (2\ MB if allocation of 16 blocks of 128\ kB succeeds).

.PP

The driver's internal buffer size is determined by a compile-time

constant which can be overridden with a kernel startup option.

In addition to this, the driver tries to allocate a larger temporary

buffer at run time if necessary.

However, run-time allocation of large

contiguous blocks of memory may fail and it is advisable not to rely

too much on dynamic buffer allocation with kernels older than 2.1.121

(this applies also to demand-loading the driver with kerneld or kmod).

.PP

The driver does not specifically support any tape drive brand or

model.

After system start-up the tape device options are defined by

the drive firmware.

For example, if the drive firmware selects fixed-block mode,

the tape device uses fixed-block mode.

The options can

be changed with explicit

.BR ioctl (2)

calls and remain in effect when the device is closed and reopened.

Setting the options affects both the auto-rewind and the nonrewind

device.

.PP

Different options can be specified for the different devices within

the subgroup of four.

The options take effect when the device is

opened.

For example, the system administrator can define

one device that writes in fixed-block mode with a certain block size,

and one which writes in variable-block mode (if the drive supports

both modes).

.PP

The driver supports

.B tape partitions

if they are supported by the drive.

(Note that the tape partitions

have nothing to do with disk partitions.

A partitioned tape can be

seen as several logical tapes within one medium.)

Partition support has to be enabled with an

.BR ioctl (2).

The tape

location is preserved within each partition across partition changes.

The partition used for subsequent tape operations is

selected with an

.BR ioctl (2).

The partition switch is executed together with

the next tape operation in order to avoid unnecessary tape

movement.

The maximum number of partitions on a tape is defined by a

compile-time constant (originally four).

The driver contains an

.BR ioctl (2)

that can format a tape with either one or two partitions.

.PP

Device

.I /dev/tape

is usually created as a hard or soft link to the default tape device

on the system.

.PP

Starting from kernel 2.6.2, the driver exports in the sysfs directory

.I /sys/class/scsi_tape

the attached devices and some parameters assigned to the devices.

.SS Data transfer

The driver supports operation in both fixed-block mode and

variable-block mode (if supported by the drive).

In fixed-block mode the drive

writes blocks of the specified size and the block size is not

dependent on the byte counts of the write system calls.

In variable-block mode one tape block is written for each write call

and the byte

count determines the size of the corresponding tape block.

Note that

the blocks on the tape don't contain any information about the

writing mode: when reading, the only important thing is to use

commands that accept the block sizes on the tape.

.PP

In variable-block mode the read byte count does not have to match

the tape block size exactly.

If the byte count is larger than the

next block on tape, the driver returns the data and the function

returns the actual block size.

If the block size is larger than the

byte count, an error is returned.

.PP

In fixed-block mode the read byte counts can be arbitrary if

buffering is enabled, or a multiple of the tape block size if

buffering is disabled.

Kernels before 2.1.121 allow writes with

arbitrary byte count if buffering is enabled.

In all other cases

(kernel before 2.1.121 with buffering disabled or newer kernel) the

write byte count must be a multiple of the tape block size.

.PP

In the 2.6 kernel, the driver tries to use direct transfers between the user

buffer and the device.

If this is not possible, the driver's internal buffer

is used.

The reasons for not using direct transfers include improper alignment

of the user buffer (default is 512 bytes but this can be changed by the HBA

driver), one or more pages of the user buffer not reachable by the

SCSI adapter, and so on.

.PP

A filemark is automatically written to tape if the last tape operation

before close was a write.

.PP

When a filemark is encountered while reading, the following

happens.

If there are data remaining in the buffer when the filemark

is found, the buffered data is returned.

The next read returns zero

bytes.

The following read returns data from the next file.

The end of

recorded data is signaled by returning zero bytes for two consecutive

read calls.

The third read returns an error.

.SS Ioctls

The driver supports three

.BR ioctl (2)

requests.

Requests not recognized by the

.B st

driver are passed to the

.B SCSI

driver.

The definitions below are from

.IR /usr/include/linux/mtio.h :

.SS MTIOCTOP \(em perform a tape operation

.PP

This request takes an argument of type

.IR "(struct mtop\ *)" .

Not all drives support all operations.

The driver returns an

.B EIO

error if the drive rejects an operation.

.PP

.in +4n

.EX

/* Structure for MTIOCTOP \- mag tape op command: */

struct mtop {

    short   mt_op;       /* operations defined below */

    int     mt_count;    /* how many of them */

};

.EE

.in

.PP

Magnetic Tape operations for normal tape use:

.TP 14

.B MTBSF

Backward space over

.I mt_count

filemarks.

.TP

.B MTBSFM

Backward space over

.I mt_count

filemarks.

Reposition the tape to the EOT side of the last filemark.

.TP

.B MTBSR

Backward space over

.I mt_count

records (tape blocks).

.TP

.B MTBSS

Backward space over

.I mt_count

setmarks.

.TP

.B MTCOMPRESSION

Enable compression of tape data within the drive if

.I mt_count

is nonzero and disable compression if

.I mt_count

is zero.

This command uses the MODE page 15 supported by most DATs.

.TP

.B MTEOM

Go to the end of the recorded media (for appending files).

.TP

.B MTERASE

Erase tape.

With 2.6 kernel, short erase (mark tape empty) is performed if the

argument is zero.

Otherwise, long erase (erase all) is done.

.TP

.B MTFSF

Forward space over

.I mt_count

filemarks.

.TP

.B MTFSFM

Forward space over

.I mt_count

filemarks.

Reposition the tape to the BOT side of the last filemark.

.TP

.B MTFSR

Forward space over

.I mt_count

records (tape blocks).

.TP

.B MTFSS

Forward space over

.I mt_count

setmarks.

.TP

.B MTLOAD

Execute the SCSI load command.

A special case is available for some HP

autoloaders.

If

.I mt_count

is the constant

.B MT_ST_HPLOADER_OFFSET

plus a number, the number is

sent to the drive to control the autoloader.

.TP

.B MTLOCK

Lock the tape drive door.

.TP

.B MTMKPART

Format the tape into one or two partitions.

If

.I mt_count

is positive, it gives the size of partition 1 and partition

0 contains the rest of the tape.

If

.I mt_count

is zero, the tape is formatted into one partition.

From kernel version 4.6,

.\" commit 8038e6456a3e6f5c4759e0d73c4f9165b90c93e7

a negative

.I mt_count

specifies the size of partition 0 and

the rest of the tape contains partition 1.

The physical ordering of partitions depends on the drive.

This command is not allowed for a drive unless the partition support

is enabled for the drive (see

.BR MT_ST_CAN_PARTITIONS

below).

.TP

.B MTNOP

No op\(emflushes the driver's buffer as a side effect.

Should be used before reading status with

.BR MTIOCGET .

.TP

.B MTOFFL

Rewind and put the drive off line.

.TP

.B MTRESET

Reset drive.

.TP

.B MTRETEN

Re-tension tape.

.TP

.B MTREW

Rewind.

.TP

.B MTSEEK

Seek to the tape block number specified in

.IR mt_count .

This operation requires either a SCSI-2 drive that supports the

.B LOCATE

command (device-specific address)

or a Tandberg-compatible SCSI-1 drive (Tandberg, Archive

Viper, Wangtek, ...).

The block number should be one that was previously returned by

.BR MTIOCPOS

if device-specific addresses are used.

.TP

.B MTSETBLK

Set the drive's block length to the value specified in

.IR mt_count .

A block length of zero sets the drive to variable block size mode.

.TP

.B MTSETDENSITY

Set the tape density to the code in

.IR mt_count .

The density codes supported by a drive can be found from the drive

documentation.

.TP

.B MTSETPART

The active partition is switched to

.IR mt_count .

The partitions are numbered from zero.

This command is not allowed for

a drive unless the partition support is enabled for the drive (see

.B MT_ST_CAN_PARTITIONS

below).

.TP

.B MTUNLOAD

Execute the SCSI unload command (does not eject the tape).

.TP

.B MTUNLOCK

Unlock the tape drive door.

.TP

.B MTWEOF

Write

.I mt_count

filemarks.

.TP

.B MTWSM

Write

.I mt_count

setmarks.

.PP

Magnetic Tape operations for setting of device options (by the superuser):

.TP 8

.B MTSETDRVBUFFER

Set various drive and driver options according to bits encoded in

.IR mt_count .

These consist of the drive's buffering mode, a set of Boolean driver

options, the buffer write threshold, defaults for the block size and

density, and timeouts (only in kernels 2.1 and later).

A single operation can affect only one item in the list above (the

Booleans counted as one item.)

.IP

A value having zeros in the high-order 4 bits will be used to set the

drive's buffering mode.

The buffering modes are:

.RS 12

.IP 0 4

The drive will not report

.BR GOOD

status on write commands until the data

blocks are actually written to the medium.

.IP 1

The drive may report

.BR GOOD

status on write commands as soon as all the

data has been transferred to the drive's internal buffer.

.IP 2

The drive may report

.BR GOOD

status on write commands as soon as (a) all

the data has been transferred to the drive's internal buffer, and

(b) all buffered data from different initiators has been successfully

written to the medium.

.RE

.IP

To control the write threshold the value in

.I mt_count

must include the constant

.BR MT_ST_WRITE_THRESHOLD

bitwise ORed with a block count in the low 28 bits.

The block count refers to 1024-byte blocks, not the physical block

size on the tape.

The threshold cannot exceed the driver's internal buffer size (see

DESCRIPTION, above).

.IP

To set and clear the Boolean options

the value in

.I mt_count

must include one of the constants

.BR MT_ST_BOOLEANS ,

.BR MT_ST_SETBOOLEANS ,

.BR MT_ST_CLEARBOOLEANS ,

or

.BR MT_ST_DEFBOOLEANS

bitwise ORed with

whatever combination of the following options is desired.

Using

.BR MT_ST_BOOLEANS

the options can be set to the values

defined in the corresponding bits.

With

.BR MT_ST_SETBOOLEANS

the options can be selectively set and with

.BR MT_ST_DEFBOOLEANS

selectively cleared.

.IP ""

The default options for a tape device are set with

.BR MT_ST_DEFBOOLEANS .

A nonactive tape device (e.g., device with

minor 32 or 160) is activated when the default options for it are

defined the first time.

An activated device inherits from the device

activated at start-up the options not set explicitly.

.IP ""

The Boolean options are:

.RS

.TP

.BR MT_ST_BUFFER_WRITES " (Default: true)"

Buffer all write operations in fixed-block mode.

If this option is false and the drive uses a fixed block size, then

all write operations must be for a multiple of the block size.

This option must be set false to write reliable multivolume archives.

.TP

.BR MT_ST_ASYNC_WRITES " (Default: true)"

When this option is true, write operations return immediately without

waiting for the data to be transferred to the drive if the data fits

into the driver's buffer.

The write threshold determines how full the buffer must be before a

new SCSI write command is issued.

Any errors reported by the drive will be held until the next

operation.

This option must be set false to write reliable multivolume archives.

.TP

.BR MT_ST_READ_AHEAD " (Default: true)"

This option causes the driver to provide read buffering and

read-ahead in fixed-block mode.

If this option is false and the drive uses a fixed block size, then

all read operations must be for a multiple of the block size.

.TP

.BR MT_ST_TWO_FM " (Default: false)"

This option modifies the driver behavior when a file is closed.

The normal action is to write a single filemark.

If the option is true, the driver will write two filemarks and

backspace over the second one.

.IP

Note:

This option should not be set true for QIC tape drives since they are

unable to overwrite a filemark.

These drives detect the end of recorded data by testing for blank tape

rather than two consecutive filemarks.

Most other current drives also

detect the end of recorded data and using two filemarks is usually

necessary only when interchanging tapes with some other systems.

.TP

.BR MT_ST_DEBUGGING " (Default: false)"

This option turns on various debugging messages from the driver

(effective only if the driver was compiled with

.B DEBUG

defined nonzero).

.TP

.BR MT_ST_FAST_EOM " (Default: false)"

This option causes the

.B MTEOM

operation to be sent directly to the

drive, potentially speeding up the operation but causing the driver to

lose track of the current file number normally returned by the

.B MTIOCGET

request.

If

.B MT_ST_FAST_EOM

is false, the driver will respond to an

.B MTEOM

request by forward spacing over files.

.TP

.BR MT_ST_AUTO_LOCK " (Default: false)"

When this option is true, the drive door is locked when the device is

opened and unlocked when it is closed.

.TP

.BR MT_ST_DEF_WRITES " (Default: false)"

The tape options (block size, mode, compression, etc.) may change

when changing from one device linked to a drive to another device

linked to the same drive depending on how the devices are

defined.

This option defines when the changes are enforced by the

driver using SCSI-commands and when the drives auto-detection

capabilities are relied upon.

If this option is false, the driver

sends the SCSI-commands immediately when the device is changed.

If the

option is true, the SCSI-commands are not sent until a write is

requested.

In this case, the drive firmware is allowed to detect the

tape structure when reading and the SCSI-commands are used only to

make sure that a tape is written according to the correct specification.

.TP

.BR MT_ST_CAN_BSR " (Default: false)"

When read-ahead is used, the tape must sometimes be spaced backward to the

correct position when the device is closed and the SCSI command to

space backward over records is used for this purpose.

Some older

drives can't process this command reliably and this option can be used

to instruct the driver not to use the command.

The end result is that,

with read-ahead and fixed-block mode, the tape may not be correctly

positioned within a file when the device is closed.

With 2.6 kernel, the

default is true for drives supporting SCSI-3.

.TP

.BR MT_ST_NO_BLKLIMS " (Default: false)"

Some drives don't accept the

.B "READ BLOCK LIMITS"

SCSI command.

If this is used, the driver does not use the command.

The drawback is

that the driver can't check before sending commands if the selected

block size is acceptable to the drive.

.TP

.BR MT_ST_CAN_PARTITIONS " (Default: false)"

This option enables support for several partitions within a

tape.

The option applies to all devices linked to a drive.

.TP

.BR MT_ST_SCSI2LOGICAL " (Default: false)"

This option instructs the driver to use the logical block addresses

defined in the SCSI-2 standard when performing the seek and tell

operations (both with

.B MTSEEK

and

.B MTIOCPOS

commands and when changing tape

partition).

Otherwise, the device-specific addresses are used.

It is highly advisable to set this option if the drive supports the

logical addresses because they count also filemarks.

There are some

drives that support only the logical block addresses.

.TP

.BR MT_ST_SYSV " (Default: false)"

When this option is enabled, the tape devices use the SystemV

semantics.

Otherwise, the BSD semantics are used.

The most important

difference between the semantics is what happens when a device used

for reading is closed: in System V semantics the tape is spaced forward

past the next filemark if this has not happened while using the

device.

In BSD semantics the tape position is not changed.

.TP

.BR MT_NO_WAIT " (Default: false)"

Enables immediate mode (i.e., don't wait for the command to finish) for some

commands (e.g., rewind).

.PP

An example:

.PP

.in +4n

.EX

struct mtop mt_cmd;

mt_cmd.mt_op = MTSETDRVBUFFER;

mt_cmd.mt_count = MT_ST_BOOLEANS |

        MT_ST_BUFFER_WRITES | MT_ST_ASYNC_WRITES;

ioctl(fd, MTIOCTOP, mt_cmd);

.EE

.in

.RE

.IP ""

The default block size for a device can be set with

.B MT_ST_DEF_BLKSIZE

and the default density code can be set with

.BR MT_ST_DEFDENSITY .

The values for the parameters are or'ed

with the operation code.

.IP ""

With kernels 2.1.x and later, the timeout values can be set with the

subcommand

.B MT_ST_SET_TIMEOUT

ORed with the timeout in seconds.

The long timeout (used for rewinds and other commands

that may take a long time) can be set with

.BR MT_ST_SET_LONG_TIMEOUT .

The kernel defaults are very long to

make sure that a successful command is not timed out with any

drive.

Because of this, the driver may seem stuck even if it is only

waiting for the timeout.

These commands can be used to set more

practical values for a specific drive.

The timeouts set for one device

apply for all devices linked to the same drive.

.IP ""

Starting from kernels 2.4.19 and 2.5.43, the driver supports a status

bit which indicates whether the drive requests cleaning.

The method used by the

drive to return cleaning information is set using the

.B MT_ST_SEL_CLN

subcommand.

If the value is zero, the cleaning

bit is always zero.

If the value is one, the TapeAlert data defined

in the SCSI-3 standard is used (not yet implemented).

Values 2\(en17 are

reserved.

If the lowest eight bits are >= 18, bits from the extended

sense data are used.

The bits 9\(en16 specify a mask to select the bits

to look at and the bits 17\(en23 specify the bit pattern to look for.

If the bit pattern is zero, one or more bits under the mask indicate

the cleaning request.

If the pattern is nonzero, the pattern must match

the masked sense data byte.

.SS MTIOCGET \(em get status

.PP

This request takes an argument of type

.IR "(struct mtget\ *)" .

.PP

.in +4n

.EX

/* structure for MTIOCGET \- mag tape get status command */

struct mtget {

    long     mt_type;

    long     mt_resid;

    /* the following registers are device dependent */

    long     mt_dsreg;

    long     mt_gstat;

    long     mt_erreg;

    /* The next two fields are not always used */

    daddr_t  mt_fileno;

    daddr_t  mt_blkno;

};

.EE

.in

.IP \fImt_type\fP 11

The header file defines many values for

.IR mt_type ,

but the current driver reports only the generic types

.B MT_ISSCSI1

(Generic SCSI-1 tape)

and

.B MT_ISSCSI2

(Generic SCSI-2 tape).

.IP \fImt_resid\fP

contains the current tape partition number.

.IP \fImt_dsreg\fP

reports the drive's current settings for block size (in the low 24

bits) and density (in the high 8 bits).

These fields are defined by

.BR MT_ST_BLKSIZE_SHIFT ,

.BR MT_ST_BLKSIZE_MASK ,

.BR MT_ST_DENSITY_SHIFT ,

and

.BR MT_ST_DENSITY_MASK .

.IP \fImt_gstat\fP

reports generic (device independent) status information.

The header file defines macros for testing these status bits:

.RS

.HP 4

\fBGMT_EOF\fP(\fIx\fP):

The tape is positioned just after a filemark

(always false after an

.B MTSEEK

operation).

.HP

\fBGMT_BOT\fP(\fIx\fP):

The tape is positioned at the beginning of the first file (always false

after an

.B MTSEEK

operation).

.HP

\fBGMT_EOT\fP(\fIx\fP):

A tape operation has reached the physical End Of Tape.

.HP

\fBGMT_SM\fP(\fIx\fP):

The tape is currently positioned at a setmark

(always false after an

.B MTSEEK

operation).

.HP

\fBGMT_EOD\fP(\fIx\fP):

The tape is positioned at the end of recorded data.

.HP

\fBGMT_WR_PROT\fP(\fIx\fP):

The drive is write-protected.

For some drives this can also mean that the drive does not support

writing on the current medium type.

.HP

\fBGMT_ONLINE\fP(\fIx\fP):

The last

.BR open (2)

found the drive with a tape in place and ready for operation.

.HP

\fBGMT_D_6250\fP(\fIx\fP), \fBGMT_D_1600\fP(\fIx\fP), \fBGMT_D_800\fP(\fIx\fP):

This \(lqgeneric\(rq status information reports the current

density setting for 9-track \(12" tape drives only.

.HP

\fBGMT_DR_OPEN\fP(\fIx\fP):

The drive does not have a tape in place.

.HP

\fBGMT_IM_REP_EN\fP(\fIx\fP):

Immediate report mode.

This bit is set if there are no guarantees that

the data has been physically written to the tape when the write call

returns.

It is set zero only when the driver does not buffer data and

the drive is set not to buffer data.

.HP

\fBGMT_CLN\fP(\fIx\fP):

The drive has requested cleaning.

Implemented in kernels since 2.4.19 and 2.5.43.

.RE

.IP \fImt_erreg\fP

The only field defined in

.I mt_erreg

is the recovered error count in the low 16 bits (as defined by

.BR MT_ST_SOFTERR_SHIFT

and

.BR MT_ST_SOFTERR_MASK .

Due to inconsistencies in the way drives report recovered errors, this

count is often not maintained (most drives do not by default report

soft errors but this can be changed with a SCSI MODE SELECT command).

.IP \fImt_fileno\fP

reports the current file number (zero-based).

This value is set to \-1 when the file number is unknown (e.g., after

.BR MTBSS

or

.BR MTSEEK ).

.IP \fImt_blkno\fP

reports the block number (zero-based) within the current file.

This value is set to \-1 when the block number is unknown (e.g., after

.BR MTBSF ,

.BR MTBSS ,

or

.BR MTSEEK ).

.SS MTIOCPOS \(em get tape position

.PP

This request takes an argument of type

.I "(struct mtpos\ *)"

and reports the drive's notion of the current tape block number,

which is not the same as

.I mt_blkno

returned by

.BR MTIOCGET .

This drive must be a SCSI-2 drive that supports the

.B "READ POSITION"

command (device-specific address)

or a Tandberg-compatible SCSI-1 drive (Tandberg, Archive

Viper, Wangtek, ... ).

.PP

.in +4n

.EX

/* structure for MTIOCPOS \- mag tape get position command */

struct mtpos {

    long mt_blkno;    /* current block number */

};

.EE

.in

.SH RETURN VALUE

.TP 14

.TP

.B EACCES

An attempt was made to write or erase a write-protected tape.

(This error is not detected during

.BR open (2).)

.TP

.B EBUSY

The device is already in use or the driver was unable to allocate a

buffer.

.TP

.B EFAULT

The command parameters point to memory not belonging to the calling

process.

.TP

.B EINVAL

An

.BR ioctl (2)

had an invalid argument, or a requested block size was invalid.

.TP

.B EIO

The requested operation could not be completed.

.TP

.B ENOMEM

The byte count in

.BR read (2)

is smaller than the next physical block on the tape.

(Before 2.2.18 and 2.4.0-test6 the extra bytes have been

silently ignored.)

.TP

.B ENOSPC

A write operation could not be completed because the tape reached

end-of-medium.

.TP

.B ENOSYS

Unknown

.BR ioctl (2).

.TP

.B ENXIO

During opening, the tape device does not exist.

.TP

.B EOVERFLOW

An attempt was made to read or write a variable-length block that is

larger than the driver's internal buffer.

.TP

.B EROFS

Open is attempted with