Beefy Boxes and Bandwidth Generously Provided by pair Networks
"be consistent"

How can I manipulate fixed-record-length files?

by faq_monk (Initiate)
on Oct 08, 1999 at 00:23 UTC ( #648=perlfaq nodetype: print w/ replies, xml ) Need Help??

Current Perl documentation can be found at

Here is our local, out-dated (pre-5.6) version:

The most efficient way is using pack() and unpack(). This is faster than using substr() when take many, many strings. It is slower for just a few.

Here is a sample chunk of code to break up and put back together again some fixed-format input lines, in this case from the output of a normal, Berkeley-style ps:

    # sample input line:
    #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
    $PS_T = 'A6 A4 A7 A5 A*';
    open(PS, "ps|");
    print scalar <PS>; 
    while (<PS>) {
        ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
        for $var (qw!pid tt stat time command!) {
            print "$var: <$$var>\n";
        print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),

We've used $$var in a way that forbidden by use strict 'refs'. That is, we've promoted a string to a scalar variable reference using symbolic references. This is ok in small programs, but doesn't scale well. It also only works on global variables, not lexicals.

Log In?

What's my password?
Create A New User
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others musing on the Monastery: (5)
As of 2015-10-09 04:15 GMT
Find Nodes?
    Voting Booth?

    Does Humor Belong in Programming?

    Results (233 votes), past polls