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();

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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);
}

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Thanks in advance. Claude

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);

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    to 
    flock($nick,LOCK_EX);

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    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.)

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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;
    }

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    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;
}

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    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??"

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      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 = @_;

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      will assign the number of arguments to $arg, but: 

      my($arg) = @_;

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      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 @_.

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        Thanks for the comments.
        I was wondering though, if the admonition against using 
        my($arg) = @_;

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        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

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