HP 3000 Manuals HP 3000 Manuals
FREADBACKWARD
NM and CM callable.
Reads a logical record backward from the current record pointer. Data is
presented as if read forward. Used for tape files only. Can recover
tape errors when handling I/O management and data recovery routines.
Syntax
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| |
| I16 I16V UDS I16V |
| transfercount:=FREADBACKWARD(filenum,buffer,length);|
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Functional Return
transfercount 16-bit signed integer
Returns the length of the data transferred to
buffer:
* If a negative value is passed in the length
parameter, the transfercount is a positive
value indicating the number of bytes
transferred.
* If a positive value is passed in the length
parameter, the transfercount is a positive
value indicating the number of half words
transferred.
NOTE If the file is opened with the nowait I/O option enabled,
transfercount is zero. The IOWAIT intrinsic can be used to
determine the length of the data transferred.
Parameters
filenum 16-bit signed integer by value (required)
Passes the file number of the file to be read.
buffer user-defined structure (required)
Returns the specified record. This structure must
be large enough to hold all of the information to
be returned.
length 16-bit signed integer by value (required)
Returns the length of the data to be transferred to
buffer:
Value Meaning
<0 Length in bytes
=0 No transfer occurs
>0 Length in half words
If length is larger than the size of the logical
record, and the multirecord aoption in
FOPEN/HPFOPEN was not specified, transfer is
limited to the length of the logical record. If
the multirecord aoption in FOPEN/HPFOPEN is
specified, transfer continues until either length
is satisfied or end-of-data is encountered, and
each transfer begins at the start of the next
physical record (block). Any data remaining in the
last physical record read is inaccessible.
Operation Notes
Three restrictions apply when using FREADBACKWARD:
* Used only with a magnetic tape drive with read-reverse capability.
* The magnetic tape drive must be located on an HP-IB system.
* The magnetic tape NOBUF must be accessed.
NOTE Magnetic tape devices with read-reverse capability are not
supported. A call to FREADBACKWARD results in an error (CCL (1)).
Not used for KSAM files.
Condition Codes
CCE (2) Request granted.
CCG (0) Request denied. The logical beginning-of-data was
encountered. When reading a labeled magnetic tape
file that spans more than one volume, CCG (0) is
not returned when beginning-of-tape (BOT) is
encountered. CCG (0) is returned at actual
beginning of file, with a transmission log of 0 if
an attempt is made to read past beginning-of-file.
CCL (1) Request denied. The information was not read
because a tape error occurred, or a tape error was
recovered and the FSETMODE option was enabled.
Check the condition code in both normal I/O and
nowait I/O.
Related Information
Intrinsics None
Commands None
Manuals Accessing Files Programmer's Guide (32650-90017)