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Re^2: What the flock!? Concurrency issues in file writing.

by suaveant (Parson)
on Oct 01, 2008 at 16:33 UTC ( #714834=note: print w/replies, xml ) Need Help??


in reply to Re: What the flock!? Concurrency issues in file writing.
in thread What the flock!? Concurrency issues in file writing.

Well, here is an example I ran on the redhat boxes at work and my debian box at home, it shows bad data on both systems if run enough (usually 20 iterations will do it). I also added a check for syswrite... doesn't seem to be writing short data.

#!/usr/bin/perl use Fcntl qw(:DEFAULT :flock); use strict; my $FH; my $fn = 'flockdata'; my $test_data = (join('','a'..'z')."\n") x 10; #print length($test_data),"\n"; sysopen($FH,$fn,O_WRONLY|O_CREAT); for(1..10) { last unless fork(); } for(1..20) { die "FLOCK ERROR\n" unless flock($FH,LOCK_EX); die "SYSWRITE ERROR\n" unless syswrite($FH,$test_data,length($test_d +ata)) == \ length($test_data); flock($FH,LOCK_UN); } close $FH;
and to run it when named testflock.pl
perl -e 'system(q{perl testflock.pl ; ls -al flockdata; rm flockdata}) + for 1..20'

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Re^3: What the flock!? Concurrency issues in file writing.
by bluto (Curate) on Oct 06, 2008 at 16:19 UTC
    As I mentioned before you must put a sysseek in there (e.g. after the flock, but before the syswrite). If you don't you will see corruption since a single process will have a stale version of the current EOF for it's file descriptor.

    I also see corruption with your code on my mac. I don't see it if there is a sysseek($FH, 0, 2). You mentioned that sysseek() "broke everything". When you put it back in this code, and check for error returns, what do you see?

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