in reply to Atomic update counter in a file

This one still has a race condition between the test for existance and the creation of the new file.

Which is why typically you never perform that test -- its redundant and introduces a race condition

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Re^2: Atomic update counter in a file
by Anonymous Monk on Nov 18, 2015 at 10:37 UTC
    Then what do you suggest to atomically differentiate between creation of the file and update of the file?
[reply]
      Answering my own question... This seems to do the trick: 
      use Path::Tiny;
#... some perl stuff
my $FH = path(".counter")->filehandle({locked=>1, exclusive=>1}, "+>>", ":raw") or die("open .counter: $!\n");
seek($FH,0,0);
my $nr; { local $/; $nr = <$FH> };
$nr ||= "000\n";
die("garbage in .counter\n")  unless $nr =~ /^\d+\n$/;
chomp($nr);
++$nr;
truncate($FH,0);
seek($FH,0,0);
print $FH $nr, "\n";
close($FH) or die("close .counter: $!\n");
# and continuing using $nr
      The mode "+>>" creates the file when it does not exist yet, and allows for read and write. The "locked" and "exclusive" mode make sure we're the only one using it.
[reply]

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