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You can use the FUPDATE intrinsic to update a logical record of a
disk file. FUPDATE affects the last logical record (or block for
NOBUF files) accessed by any intrinsic call for the file named,
and writes information from a buffer in the stack into this record.
Following the update operation, the record pointer is set to indicate
the next record position. The record number need not be supplied in
the FUPDATE intrinsic call; FUPDATE automatically updates the
last record referenced in any intrinsic call. Note that the file
system assumes that the record to be updated has just been accessed in
some way. The disk file containing the record to be updated must have been opened with
the access type option parameter of HPFOPEN/FOPEN set to update access. In
addition, the file must not contain a variable-length record format.
Example 10-1 is an HP Pascal/iX code segment that illustrates how to use the
FUPDATE intrinsic to update records in a disk file being shared by multiple
concurrent accessors. The program segment also uses file system locking
intrinsics (FLOCK and FUNLOCK) to guarantee exclusive access to the file while
the update occurs. The pertinent code from Example 10-1 is shown below:
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read_length := FREAD (disk_file_num, inbuf, 128);
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FUPDATE (disk_file_num, inbuf, 128);
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The statements above are in a loop that follows this algorithm: Read the record from the file identified by disk_file_num using the FREAD intrinsic. Write the record to STDLIST to be reviewed by user. Read new data input to STDIN by user and modify record in program. Using FUPDATE intrinsic, write the updated record to the location in disk_file_num indicated by last intrinsic call (in this case, the FREAD call shown above).
Example 10-1. Updating a Disk File
procedure update_disk_file;
{**************************************************************}
{ procedure update_disk_file updates records in the disk file }
{ with the replacement records read from $STDIN }
{**************************************************************}
var
dummy : integer;
inbuf : array [1..80] of char;
end_of_file : boolean;
read_length : integer;
begin
{Lock the file and suspend }
end_of_file := false;
FLOCK (disk_file_num, 1);
if ccode = ccl then
handle_file_error (disk_file_num, 0);
{Begin loop }
repeat
{ Read record from disk file, then write record to $STDLIST; }
{ read updated record number from $STDIN and update }
{ the disk file with the input record and unlock disk file. }
read_length := FREAD (disk_file_num, inbuf, 128);
{read in record}
if ccode = ccg then
end_of_file := true {exit condition }
else
begin
FWRITE (std_list, inbuf, -20, octal('320'));
{user reviews record}
if ccode <> cce then
handle_file_error (std_list, 5);
dummy := FREAD (std_in, inbuf[20], 5);
{input updated field}
if ccode = ccl then
handle_file_error (std_list, 6);
else
if ccode = ccg then
end_of_file := true; {exit condition}
FUPDATE (disk_file_num, inbuf, 128); {update record }
if ccode <> cce then
handle_file_error (disk_file_num, 7);
end
until end_of_file; {test for EOF }
FUNLOCK (disk_file_num); {final unlock of disk file}
if ccode <> cce then
handle_file_error (disk_file_num, 2);
end: {end update_file }
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In appendix A, "HP Pascal Program Examples," example A-5 is an HP Pascal/iX
program that uses the procedure in example 10-1 to update records in a disk
file. For more information about FUPDATE parameters, refer to the MPE/iX
Intrinsics Reference Manual (32650-90028). | | | |  | NOTE: A magnetic tape device is not designed to enable the update/replacement of a single record in an existing file. Problems occurs in maintaining the integrity of tape records if you attempt to perform a record update operation directly to a magnetic tape file. You should update individual records of a magnetic tape file only when you are copying the entire contents of that file to another file. | | | | |
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