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8.12. Using Random-Access I/O
8.12.1. Problem
You have to read a binary record from the
middle of a large file but don't want to read a record at a time to
get there.
8.12.2. Solution
Once you know the record's size, multiply it by the record number to
get the byte address, and then seek to that byte
address and read the record:$ADDRESS = $RECSIZE * $RECNO;
seek(FH, $ADDRESS, 0) or die "seek:$!";
read(FH, $BUFFER, $RECSIZE);
8.12.3. Discussion
The Solution assumes the first record has a RECNO
of 0. If you're counting from one, use:$ADDRESS = $RECSIZE * ($RECNO-1);
This is best applied to binary data. Applying it to text files
assumes you have a constant character width and constant line length.
This rules out most Unicode encodings, any kind of Windows text file,
and any text file where lines can have different lengths.
8.12.4. See Also
The seek function in
perlfunc(1) and in Chapter 29 of
Programming Perl; Recipe 8.13
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| 8.11. Processing Binary Files |  | 8.13. Updating a Random-Access File |
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