Perl and File Locking

Perl and File Locking

[wynfield]

Re: Perl 5, version 14, subversion 2 (v5.14.2) 
    Cygwin: CYGWIN_NT-5.1  1.7.25(0.270/5/3

In Perl (I am attempting to open a file and put an exclusive lock on it to update
it like this:

###########################################

use Fcntl qw( O_RDWR  O_CREAT  O_NONBLOCK  O_EXLOCK );


     sysopen($FH, $fname, O_RDWR | O_CREAT | O_NONBLOCK |  O_EXLOCK, 0664)


############################################

Result on a test run give me the following error message:

"Your vendor has not defined Fcntl macro O_EXLOCK,"

Is it possible to develope perl code on cygwin that can use file locking to update data in a file from within perl?

Thank you.

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Re: Perl and File Locking

[Warren Young]

On 9/25/2013 02:24, wynfield@gmail.com wrote:
>
> "Your vendor has not defined Fcntl macro O_EXLOCK,"

O_EXLOCK is a BSD feature, and Cygwin tries to emulate Linux, not BSD.

Minimal testing tells me you can use Cygwin's nonstandard 
F_LCK_MANDATORY feature from Perl.  This script, foo.pl, doesn't die:

     #!/usr/bin/perl
     sysopen my $fh, "foo.pl", 0600, O_RDONLY or die "sysopen: $!\n";
     fcntl $fh, 0x99, 1 or die "fcntl: $!\n";

Whether it actually does what you expect is a different question.

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Re: Perl and File Locking

[Corinna Vinschen]

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On Sep 25 08:25, Warren Young wrote:
> On 9/25/2013 02:24, wynfield@gmail.com wrote:
> >
> >"Your vendor has not defined Fcntl macro O_EXLOCK,"
> 
> O_EXLOCK is a BSD feature, and Cygwin tries to emulate Linux, not BSD.
> 
> Minimal testing tells me you can use Cygwin's nonstandard
> F_LCK_MANDATORY feature from Perl.  This script, foo.pl, doesn't
> die:
> 
>     #!/usr/bin/perl
>     sysopen my $fh, "foo.pl", 0600, O_RDONLY or die "sysopen: $!\n";
>     fcntl $fh, 0x99, 1 or die "fcntl: $!\n";
> 
> Whether it actually does what you expect is a different question.

Uh, no, don't do that.  O_EXLOCK in the BSD world does *not* mean that
you want mandatory locking.  It only means that the application wants an
exclusive lock on the entire file right at open time, thus allowing an
atomic open/lock under BSD flock semantics.

What you want to do is to call

  sysopen my $fh, "foo.pl", 0600, O_RDONLY ...
  [insert perl equivalent of the flock system call here]

That's not atomic, but this isn't provided (yet) utside the BSD world.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
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Red Hat

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