 |
» |
|
|
|
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. |
