.po 1i
.TH CT 4 Zilog
.V 08/15/84 17:30:27
.\" @[$]ct.4	1.11 08/15/84 17:30:27 - Zilog Inc.
.ad
.fi
.SH NAME
ct \- cartridge tape interface
.SH DESCRIPTION
The files
.BR ct0 ,
.BR ct1 ,\ ...
refer to the cartridge tape units.
The files
.BR ct0 ,
.BR ct1 ,\ ...
are normal-rewind on close, and the files
.BR nct0 ,
.BR nct1 ,\ ...
are no-rewind on close.  When opened for reading or writing, the
tape is assumed to be in the desired position.
If the tape was open for writing, an end-of-file is written,
and the next four feet of tape are erased (indicating end-of-tape).
If the tape is not to be rewound, the tape is backspaced to after
the filemark.
.PP
Exiting a program which has opened a tape device automatically closes
the device, even if there is no explicit close of the device.
If the tape was opened for writing, an end-of-file is written
at the current location, and the next four feet of tape are erased.
Therefore, one should not del out of programs 
which have opened a tape device
for writing unless
truncation is desired.
It is generally desirable
to be at the end of the tape upon close of the tape device
if it was opened for writing.
.PP
The
.B ct
files allow reads and writes of arbitrary length.
Seeks are also supported. These files are intended to allow tape files
to be accessed similarly to other files to a certain extent.
.PP
The files
.BR rct0 ,
.BR rct1 ,\ ...
refer to the raw interface.
These are useful to read and write long records 
on the ZEUS system.
.BR Tar (1),
for example, uses the raw interface.
The rest of this discussion continues to refer to the raw device names.
Except that seeks are not supported on the raw device, the
nonraw device can be accessed similarly by dropping the r from the
device name.
.PP
The cartridge tape unit has the capability of accessing each of the
four tracks on a tape individually. The files
.BR rct0a ,\ ...,\  rct0d ,\  rct1a ,\ ...
refer to the individual tracks on a tape.
.PP
The lowest three bits of the minor device number
select the tape track to be used.  The next two
bits specify the drive.  
If Bit 7 is on, the tape is not rewound
on close.
The minor device number has no necessary connection
with the file name.
.PP
The name of the raw tape device is
.RI rct n ,
where
.I n
is the drive number.
A leading
.I n
(for example, nrct0) specifies the no-rewind device, and
a trailing letter in the range a-d indicates single-track operation.
.PP
Each
.B read
or
.B write
call to a raw tape device 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 not greater
than the buffer size.
If the record is long, the extra data is skipped over
without notification.
The buffer must begin on a word boundary.
Writes can be of any length.  The buffer length for
reads must be even; if the record is shorter, the
actual length (even or odd) will be returned.
Seeks are ignored on the raw device.
A zero count is returned when a tape mark is read.
When the no-rewind device is closed and reopened,
the tape head is positioned at the beginning
of the next tape file.
.PP
There are several 
.BR ioctl (2)
system calls that apply to the cartridge tape drive.
The calls reference an open /dev/*ct* special file.
The syntax of the calls is: 
.sp
.nf
                \fBioctl(\fIfd, REQUEST, count\fR);
.fi
.sp
.I fd
is a valid file descriptor of a /dev/*ct* file.
.br
\fIREQUEST\fR is one of the following requests.
.br
\fICount\fR applies to certain of the requests below.
.sp
.B CTIOBF 
\- Skips forward the number of tape blocks specified 
in the count field.
This command will not skip over file marks or past the end-of-tape mark.
The tape blocks can be any size.
.sp
.B CTIOBR 
\- Skips backward the specified number of tape blocks.
This command will not skip back over file marks or before the 
beginning-of-tape mark.
The tape blocks can be of any size.
.sp
.B CTIOFF 
\- Skips forward the number of files (determined 
by file marks) specified in the count field.
The tape is positioned at the beginning of the next file.
The tape will not skip past the end-of-tape mark.
.sp
.B CTIOFR
\- Skips backward the number of files (determined
by file marks) specified in the count field.
The tape is positioned at the beginning of the appropriate file.
The tape will not skip past the beginning-of-tape mark.
.sp
.B CTIOFM
\- Writes a file mark at the current position.
This command does not take a count.
.sp
.B CTIORWT
\- Rewind the tape.  Leave the tape positioned at the 
load point.
This command does not take a count.
.sp
.B CTIOGAP
\- Write the specified number of gaps.
.sp
.B CTIOSB
\- Seek to a record.
The count field contains the record number relative to the beginning of
the file of where to position the tape.
.PP
.B ioctl
returns the number of blocks or files skipped,
or -1 if an end of data or end of media error has occurred.
.SH FILES
/dev/rct?,
.br
/dev/nrct?,
.br
/dev/ct?,
.br
/dev/nct?
.br
/dev/rct?[a-d],
.br
/dev/nrct?[a-d],
.br
/dev/ct?[a-d],
.br
/dev/nct?[a-d]
.SH SEE ALSO
dd(1),
tar(1),
ioctl(2).