HP 3000 Manuals HP 3000 Manuals
Sequential Access and Random Access
Two of the most frequently used methods of transferring data to a file
from your program are sequential access and random access.
When you use sequential access to write data to a file, you write data to
the record currently pointed to by the record pointer. You use the
FWRITE intrinsic to write data sequentially to a disk file or device
file. When you open a file with any form of write access (except Append)
specified in the access type option of HPFOPEN/FOPEN, the file is opened
with the record pointer set to the first record in the file. When you
have accomplished the write operation, the file system automatically sets
the record pointer to point to the beginning of the next record in the
file. Both disk files and device files can be accessed with the FWRITE
intrinsic.
When you use random access to write data to a disk file, you write data
to any record in the file by specifying where you want the file system to
set the record pointer prior to the write operation. You use the
FWRITEDIR intrinsic to write data randomly to a disk file. You must
specify in FWRITEDIR which record that you want to write to. The file
system sets the record pointer to the selected record, then transfers the
data to the record from your program's stack. When you have accomplished
the write operation, the file system automatically sets the record
pointer to point to the beginning of the next record in the file. Only
disk files can be accessed with the FWRITEDIR intrinsic.
The following examples illustrate the use of file system intrinsics to
perform sequential access writes and random access writes to a disk file.
Writing to a disk file using sequential access
Example 8-1 is an HP Pascal/iX code segment that copies logical records
sequentially from an unlabeled tape file (indicated by variable
tape_file_num) and uses FWRITE to write them to a disk file (indicated by
variable disk_file_num). The operation is performed in a loop. The loop
ends when the FREAD intrinsic encounters an EOF marker on the tape
(indicating the end of the file).
Example 8-1. Writing to a Disk File Using Sequential Access
procedure copy_tape_file_to_disk_file;
var
record : packed array [1..80] of char; {declare record }
end_of_file : boolean; {declare exit condition}
record_length : shortint; {size of record read }
length : shortint; {declare parameter }
control_code : 0..65535; {declare parameter }
begin
end_of_file := false; {initialize exit condition }
control_code := 0; {initialize to default }
length : -80; {size of record to be copied }
repeat {loop until exit condition }
record_length := FREAD (tape_file_num, record, length);
if ccode = ccl then {check condition code for error }
handle_file_error (tape_file, 3)
else
if ccode = ccg then {FREAD returns ccg if EOF }
end_of_file := true {exit condition encounter encountered}
else
begin
FWRITE( disk_file_num, {identity returned by HPFOPEN }
record, {read from tape_file_num }
record length, {actual size of record }
control_code {default }
);
if ccode <> cce then {check condition code for error }
handle_file_error (disk_file, 5);
end
until end_of_file;
end {end procedure }
If an error is encountered by either FWRITE or FREAD, the condition
code CCL is returned to the program, thus invoking the procedure
handle_file_error. For more information about FWRITE parameters, refer
to the MPE/iX Intrinsics Reference Manual (32650-90028). For more
information about using the FREAD intrinsic, refer to chapter 9, "Reading
from a File". For more information about opening a file, refer to
chapter 5, "Opening a File". In appendix A, "HP Pascal/iX Program
Examples," example A-1 uses a similar procedure to copy records from a
tape file to a disk file.
Writing to a disk file using random access
Example 8-2 is an HP Pascal/iX code segment that reads records
sequentially from old_disk_file and writes them into new_disk_file.
Assume that both files have been opened already with calls to
HPFOPEN/FOPEN. The end-of-file (EOF) using the FWRITEDIR of old_disk_file
is determined with the FGETINFO intrinsic and assigned to the variable
record_num.
Example 8-2. Writing to a Disk File Using Random Access.
procedure copy_from_old_file_to_new_file;
var
record_num : integer;
buffer : packed array [256] of char;
end_of_file : boolean;
read_length : integer;
length : shortint;
begin
end_of_file := false; {initialize exit condition }
record_num := 0; {initialize record pointer }
length := 128 {also means 256 bytes }
FGETINFO (old disk_file,,,,,,,,,,rec); {locate the EOF in old_disk_file}
if ccode = ccl then
handle_file_error (old_disk_file); {error check on intrinsic call}
repeat {Copy the records in the reverse}
{orders from old disk file }
{to the new disk file }
read_length := FREAD (old_disk_file, buffer, length);
if ccode = ccl then
handle_file_error (old_disk_file)
else
if ccode = ccg then {check for exit condition}
end_of_file := true
else begin
rec := rec - 1 {decrement record pointer}
FWRITEDIR(new_disk_file, buffer, read_length, record_num);
if ccode <> cce then
handle_file_error (new_discfile); {error check }
end
until end_of_file {exit loop if exit condition true}
end; {end procedure }
The operation is performed in a loop. Before each write operation,
record_num is decremented. The loop ends when the FREAD intrinsic
encounters an EOF in old_disk_file (indicating the end of the file). For
more information about FWRITEDIR intrinsic parameters, refer to the
MPE/iX Intrinsics Reference Manual (32650-90028). For more information
about the FREAD intrinsic, refer to chapter 9, "Reading from a File". In
appendix A, "HP Pascal/iX Program Examples", example A-3 uses a similar
routine to copy records using the random access method of data transfer
to write date from one file to another.