Two of the most frequently used methods of
transferring data from a file to your program are sequential access and random
access.
When you use sequential access to read data from a file, you read data from the
record currently pointed to by the record pointer. You use the FREAD intrinsic
to read data sequentially from a disk file or device file. When you open a
file with any form of Read access 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 read 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 FREAD intrinsic. When you use random access to read data from a disk file,
you read data from any record in the file by specifying where you want the file
system to set the record pointer prior to the read operation. You use the
FREADDIR intrinsic to randomly access records in a disk file. You must specify
in FREADDIR which record that you want to read from. The file system sets the
record pointer to the selected record, then transfers the data from the record
to your program's stack. When you have accomplished the read 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 FREADDIR
intrinsic
The following examples illustrate the use of file system intrinsics to perform
sequential access reads and random access reads from a disk file.
Reading from a disk file using sequential access |
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Example 9-1 is an HP Pascal/iX
code segment that uses the FREAD intrinsic to read records sequentially from a
disk file. Example 9-1 contains a loop construct, where records are read
sequentially from disk_file and written to the file new_file (both files opened
elsewhere by HPFOPEN/FOPEN calls). The files are both standard ASCII files
with fixed-record format, each record 256 bytes in length. When a logical
end-of-file (EOF) is reached, a condition code of CCG is returned by FREAD.
The loop ends when FREAD encounters the EOF and returns the CCG condition to
the program.
Example 9-1. Reading from a Disk File Using Sequential Access
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.
.
var
expected_length: shortint; {required by FREAD }
record : packed array [1..256] of char; {declare record type }
control_code : 0..65535; {required by FWRITE }
record_length : shortint; {expected record length}
end_of_file : boolean; {declare exit condition}
.
.
.
begin
end_of_file := false; {initialize exit condition }
expected_length := -256; {record size of file }
control_code := 0 {set to default }
repeat {begin loop }
record_length := FREAD ( disk_file, {file number from HPFOPEN }
record, {data transferred to here }
expected_length {record size of file }
);
if ccode = ccl then handle_file_error (disk_file) {error check }
else if ccode = ccg then end_of_file ;= true {exit condition check}
else begin
FWRITE( new_file, {file number from HPFOPEN }
record, {data transferred here }
record_length, {value returned by FREAD }
control_code {required; set to default }
);
end
until end_of_file; {loop ends when exit condition encounted }
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.
.
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If an error is encountered by either FREAD or FWRITE, the condition code CCL is
returned to the program, thus invoking the procedure handle_file_error. For
more information about FREAD parameters, refer to the MPE/iX Intrinsics
Reference Manual (32650-90028). For more information about using the FWRITE
intrinsic, refer to chapter 8, "Writing to a File". For more information about
opening a file, refer to chapter 5, "Opening a File".
Reading from a disk file using random access |
|
Example 9-2 is an HP Pascal/iX code
segment that, within a loop construct, calls the FREADDIR intrinsic to read a
record whose record number has been selected by the procedure select_record and
returned in the variable record_number. The example then prints the selected
record to the standard list device $STDLIST using the PRINT intrinsic.
Example 9-2. Reading from a Disk File Using Random Access
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var
record : packed array [1..30] of char; {declare record type }
record_length : shortint; {expected record length }
read_length : shortint; {actual bytes read by FREAD}
record_number : integer; {which record to read }
control_code : shortint; {required by PRINT }
end_of_file : boolean; {declare exit condition }
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.
.
control_code := 0; {default condition }
record_length := -30; {file record size 30 bytes }
record_number := 0; {initialize variable }
end_of_file := false; {initialize exit condition }
repeat {begin loop }
select_record (record_number);
read_length := FREADDIR (data_file, {HPFOPEN file number }
record, {record read from data_file }
record_length,{expected length of record }
record_number {returned from select_record }
);
if ccode = ccl then handle_file_error (data_file) {error check }
else if ccode = ccg then end_of_file := true {check for exit condition}
else begin
PRINT (record, {returned by FREADDIR }
read_length, {returned by FREADDIR }
control_code {set to default condition}
);
if ccode <> cce then handle_file_error (data_file)
end
until end_of_file; {exit if exit condition true }
.
.
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Assume that a disk file identified by data_file has been opened elsewhere by an
HPFOPEN/FOPEN call. Also, assume that procedure select_record prompts the user
for a valid record number of a record in data_file. The loop is repeated until
the FREADDIR intrinsic encounters an end-of-file condition, or an error
condition is returned by an intrinsic.
If an error is encountered by either FREADDIR or PRINT, procedure
handle_file_error is invoked. For more information about FREADDIR parameters,
refer to the MPE/iX Intrinsics Reference Manual (32650-90028). For more
information about using the PRINT intrinsic, refer to chapter 8, "Writing to a
File". For more information about opening a file, refer to chapter 5, "Opening
a File".
Increasing I/O performance using FREADSEEK |
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If you know in advance that a certain
record is to be read from a file with the FREADDIR intrinsic, you can speed up
the I/O process by issuing an FREADSEEK intrinsic call.
The FREADSEEK intrinsic moves the record from disk to virtual memory. Then,
when the FREADDIR intrinsic call is issued, the record is transferred from
virtual memory to the buffer in the stack specified by FREADDIR without having
to perform I/O. The use of FREADSEEK enhances the I/O process, because the
FREADDIR call does not make the file system perform a physical I/O.
