Perl Cookbook

Reading Fixed-Length Records

Problem

You want to read a file whose records have a fixed length.

Solution

Use read and unpack:

# $RECORDSIZE is the length of a record, in bytes.
# $TEMPLATE is the unpack template for the record
# FILE is the file to read from
# @FIELDS is an array, one element per field

until ( eof(FILE) ) {
    read(FILE, $record, $RECORDSIZE) == $RECORDSIZE
        or die "short read\n";
    @FIELDS = unpack($TEMPLATE, $record);
}

Discussion

Because the file in question is not a text file, you can’t use <FH> or IO:: modules’ getline method to read in records. Instead, you must simply read a particular number of bytes into a buffer. This buffer then contains one record’s data, which you decode using unpack with the right format.

For binary data, the catch is often determining the right format. If you’re reading data written by a C program, this can mean peeking at C include files or manpages describing the structure layout, and this requires knowledge of C. It also requires that you become unnaturally chummy with your C compiler, because otherwise it’s hard to predict field padding and alignment (such as the x2 in the format used in Section 8.18). If you’re lucky enough to be on a Berkeley Unix system or a system supporting gcc, then you may be able to use the c2ph tool distributed with Perl to cajole your C compiler into helping you with this.

The tailwtmp program at the end of this chapter uses the format described in utmp (5) under Linux and works on its /var/log/wtmp and /var/run/utmp ...