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file lock and update

by CColin (Scribe)
on Dec 26, 2007 at 11:45 UTC ( #659032=perlquestion: print w/ replies, xml ) Need Help??
CColin has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have a program which modifies a file in place using a temporary file
open(OLD, "< $old") or die "can't open $old: $!"; open(NEW, "> $new") or die "can't open $new: $!"; my @array = <OLD>; my $required_element_details = shift(@array); foreach my $remaining_details (@array) { print NEW "$remaining_details"; } close(OLD) close(NEW) rename($new, $old)
Problem is this file can be accessed at the same time by the same program so the reading and writing process needs to be locked. Have tried:
open(OLD, "< $old") or die "can't open $old: $!"; open(NEW, "> $new") or die "can't open $new: $!"; flock (OLD, 2); flock (NEW, 2); [continue as before including renaming the file before the filehandles + are closed]
But this doesn't work when both programs are trying at the same time, instead both programs read the same initial element (and write the same results). Any suggestions? Colin

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Re: file lock and update
by McDarren (Abbot) on Dec 26, 2007 at 14:56 UTC
    Hi Colin,

    You probably want to use sysopen rather than open. The canonical way to do this (on a single output file) is something along the following lines:

    use Fcntl qw(:DEFAULT :flock); sysopen(OUT, $outfile, O_WRONLY | O_CREAT) or die "Cannot open $outfile for writing:$!\n"; flock(OUT, LOCK_EX) or die "Cannot get a lock on $outfile:$!\n"; truncate(OUT, 0) or die "Cannot truncate $outfile:$!\n";
    There are a couple of very good discussions regarding file locking in the Camel (3rd Ed.). If you happen to have a copy, look in Chapters 16.2.1 and 23.2.2.

    Otherwise, you can refer to the docs for sysopen and flock.
    perldoc -q lock also gives some useful examples.

    Hope this helps,
    Darren :)

Re: file lock and update
by mickeyn (Priest) on Dec 26, 2007 at 13:11 UTC
Modifying a file in place (was Re: file lock and update)
by roboticus (Chancellor) on Dec 26, 2007 at 15:21 UTC

    A minor nit in terminology: Your program doesn't modify the file in place. "Modify in place" means to open the file in read/write mode and alter the file without creating a new copy. (Not terribly safe for many applications, though!) Here's an example of modifying a file in place:

    $ cat #!/usr/bin/perl -w use warnings; use strict; # Open in Read/Write mode and modify it in place open FH, "+<", "a_file.txt" or die "Can't open file!"; print FH "Foo bar baz!"; close FH; $ cat a_file.txt Now is the time for all good men to come to the aid of their party. $ perl $ cat a_file.txt Foo bar baz!ime for all good men to come to the aid of their party. $

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