HP 3000 Manuals HP 3000 Manuals
Writing to a Magnetic Tape File
The following discussion pertains to writing data to two different types
of magnetic tape files.
* unlabeled magnetic tape files
* labeled magnetic tape files
Unless you specifically create and open a labeled magnetic tape file, the
file system opens an unlabeled magnetic tape file when you specify a tape
drive using either the device name option or device class option of
HPFOPEN/FOPEN. For more information about opening both unlabeled and
labeled magnetic tape files, refer to chapter 5, "Opening a File".
When you are writing records to an unlabeled magnetic tape file, you must
take into consideration characteristics of magnetic tape that do not
apply to files on other devices. For example, if a user program attempts
to write over or beyond the physical EOT marker, the FWRITE intrinsic
returns an error condition code (CCL). The actual data is written to the
tape, and a call to FCHECK reveals a file error indicating END-OF-TAPE.
All writes to the tape after the EOT tape marker has been crossed
transfer the data successfully, but return a CCL condition code until the
tape crosses the EOT marker again in the reverse direction (rewind or
backspace). For more information about magnetic tape considerations,
refer to chapter 7, "Record Selection and Data Transfer".
Writing records to a labeled tape file differs slightly from writing to
an unlabeled tape file. If the magnetic tape is unlabeled and a user
program attempts to write over or beyond the physical EOT marker, the
FWRITE intrinsic returns an error condition code (CCL). The actual data
has been written to the tape, and a call to FCHECK reveals a file error
indicating END-OF-TAPE. All writes to the tape after the EOT tape marker
has been crossed transfer the data successfully, but return a CCL
condition code until the tape crosses the EOT marker again in the reverse
direction (rewind or backward).
If the magnetic tape is labeled, a CCL condition code is not returned
when the tape passes the EOT marker. Attempts to write to the tape after
the EOT marker is encountered cause end-of-volume (EOV) labels to be
written. A message then is printed on the operator's console requesting
another reel of tape to be mounted.
The following headings provide examples of file system intrinsic calls
that illustrate:
* writing to an unlabeled magnetic tape file
* writing to a labeled magnetic tape file
* writing a user-defined file label on a labeled tape file
Writing to an unlabeled magnetic tape file
Example 8-6 is an HP Pascal/iX code segment that writes user-supplied
data to the unlabeled magnetic tape file opened in example 5-5. For
information about the HPFOPEN call that returns the file number in the
variable unlabeled_tape_file, refer to example 5-5.
Example 8-6. Writing to an Unlabeled Magnetic Tape File
:
var
control_code : 0..65535; {Declare FWRITE parm. }
record_length: shortint; {Declare FWRITE parm }
file_record : record_type; {Record to be written to file; }
{record_type is 256-byte }
{fixed-length record. }
:
record_length:= -256; {Number of bytes in record. }
control_code := 0; {Default specified }
FWRITE ( unlabeled_tape_file, {HPFOPEN returned file number. }
file_record {Record to be passed }
record_length {Size of file_record. }
control_code {Required, but ignored. }
);
if ccode = CCL {check FWRITE condition code }
then handle_file_error (labeled_tape_file);
:
If the FWRITE intrinsic encounters an error condition (CCL), an
error-handling procedure, handle_file_error, is invoked. FWRITE returns
a CCG condition code if the EOF is reached. For details concerning
FWRITE intrinsic parameters, refer to the MPE/iX Intrinsics Reference
Manual (32650-90028).
Writing to a labeled magnetic tape file
Example 8-7 is an HP Pascal/iX code segment that writes user-supplied
data to the labeled magnetic tape file opened in Example 5-6. For
information about the HPFOPEN call that returns the file number in the
variable labeled_tape_file, refer to example 5-6.
Example 8-7. Writing to a Labeled Magnetic Tape File
:
var
control_code : 0..65535; {Declare FWRITE parm. }
record_length: shortint; {Declare FWRITE parm }
file_record : record_type; {Record to be written to file; }
{record_type is 256-byte }
{fixed-length record. }
:
record_length:= -256; {Number of bytes in record. }
control_code := 0; {Default specified }
FWRITE ( labeled_tape_file, {HPFOPEN returned file number. }
file_record {Record to be passed }
record_length {Size of file_record. }
control_code {Required, but ignored. }
);
if ccode = CCL {check FWRITE condition code }
then handle_file_error (labeled_tape_file);
:
If the FWRITE intrinsic encounters an error condition (CCL), an error
handling procedure handle_file_error is invoked. FWRITE returns a CCG
condition code if the EOF is reached. For more information about FWRITE
intrinsic parameters, refer to the MPE/iX Intrinsics Reference Manual
(32650-90028). For more information about opening files, refer to
chapter 5, "Opening a File".