flock with anonymous filehandle

Xxaxx has asked for the wisdom of the Perl Monks concerning the following question:

I'm learning (or trying to learn) how to work with filehandles passed along through returns and params.

I believe I have the fileopen and return working. At least is seems to be working.

The part I'm messing up on is the file locking part.

From flock manpage:

sub lock {
    flock(MBOX,LOCK_EX);
      # and, in case someone appended
      # while we were waiting...
      seek(MBOX, 0, 2);
}

sub unlock {
    flock(MBOX,LOCK_UN);
}

open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
    or die "Can't open mailbox: $!";

lock();
print MBOX $msg,"\n\n";
unlock();
[download]

Below was my attempt at making this work. If a kind hearted monk could help unscramble this I'd appreciate it.

sub fileopen {
    my($filename) = @_;
    local *MW;

    my($openokay) = open(MW,"+<$filename"); 

    if (!$openokay) {
        return 0;
    }

    lock(*MW);  

    seek(MW,0,0);
    return *MW;
}

sub lock {
    my($nick) = @_;
    flock({$nick},LOCK_EX);
      # and, in case someone appended
      # while we were waiting...
      seek({$nick}, 0, 2);
}

sub unlock {
    my($nick) = @_;
    flock({$nick},LOCK_UN);
}
[download]

Thanks in advance. Claude

Comment on flock with anonymous filehandle Select or Download Code

Replies are listed 'Best First'.
Re: flock with anonymous filehandle by wog (Curate) on Jun 09, 2001 at 05:10 UTC
Your problem is the extra `{}`s around the filehandle. Although print lets you do that, since it uses a syntax not involving a comma, flock and seek do not. So change like: `flock({$nick},LOCK_EX);` [download] to `flock($nick,LOCK_EX);` [download] The syntax you had there created a hashref with nothing in it. You really should be checking the return value of flock, and open. (was updated because I really should've checked what `{ 'whatever' }` produced.)	[reply] [d/l] [select]
Re: flock with anonymous filehandle by HamNRye (Monk) on Jun 09, 2001 at 23:55 UTC
Just a few more things.... `my($openokay) = open(MW,"+<$filename"); if (!$openokay) { return 0; }` [download] this could be done much more efficiently: `open(MW,"+<$filename") \|\| return 0; # Or even... sub fileopen { my ($filename) = shift; if ( open(MW,"+<$filename") ) { lock(MW); seek(MW,0,0); return MW; } else { return 0; }` [download] Also, try using the var = shift syntax for getting arguments in a subroutine. I've run into too many problems trying to use the @_. (Think about it, shift returns the first value of an array.) And this is the usual reminder, use Strict, and -w... And GOD said unto them, "Dids't thou use strict??"	[reply] [d/l] [select]
Re: Re: flock with anonymous filehandle by wog (Curate) on Jun 10, 2001 at 00:06 UTC
Also, try using the var = shift syntax for getting arguments in a subroutine. I've run into too many problems trying to use the @_. The syntax used is perfectly fine. The thing that one must remember when using `@_` is that: `my $arg = @_;` [download] will assign the number of arguments to `$arg`, but: `my($arg) = @_;` [download] will assign the first argument to `$arg`. (The `()`s give list context to the right-hand side of the `my` assignment.) update: Of course, one must also remember to assign all the arguments at once when using `@_`.	[reply] [d/l] [select]
Re: Re: Re: flock with anonymous filehandle by Xxaxx (Monk) on Jun 10, 2001 at 06:32 UTC
Thanks for the comments. I was wondering though, if the admonition against using `my($arg) = @_;` [download] is because some folks forget and leave out the all importand ()? Or is there some other reason that the shift is seemingly preferred? Thanks Claude	[reply] [d/l]
Re: Re: Re: Re: flock with anonymous filehandle by Anonymous Monk on Jun 11, 2001 at 13:24 UTC


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