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Test if file handle exists

Posted on 2004-08-03
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Last Modified: 2012-05-05
In some code with exhaustive error handling, it calls a particular function to write to a log file, which does a "print LOG $message;".  However, if an error occurs before LOG is opened, then (obviously) I see a "print() on unopened filehandle LOG" error.

If I try wrapping the "print LOG $message" with "if (defined(LOG)) { ... }", I get a "Bareword "LOG" not allowed while "strict subs" in use".

So, my question is: How can I test whether LOG is an open file handle?

Fast answer appreciated.
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Question by:rtsh
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by:davorg
davorg earned 100 total points
ID: 11701783
I'd probably use a lexically scoped filehandle for this.

#!/usr/bin/perl
                                                                               
use strict;
use warnings;
                                                                               
my $log;
                                                                               
log_this("error 1\n");
openlog();
log_this("error 2\n");
                                                                               
{
  my $log;
                                                                               
  sub openlog {
    open $log, '>', 'log.txt' or die $!;
  }
                                                                               
  sub log_this {
    if ($log) {
      print $log @_;
    } else {
      warn @_;
    }
  }
}

Or something like that.

Dave...
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by:rtsh
ID: 11701876
(NB For future people viewing this question: I haven't actually tested this code (yet), but it gives a good enough idea as to how it could be done - ie not using LOG but $log as the file handle)
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Expert Comment

by:ozo
ID: 11711287
You might also try something like
if( -f LOG ){ print LOG @_; }
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Author Comment

by:rtsh
ID: 11712945
ozo... now that looks a lot neater for what I want to do, but I still get a warning:

"-f on unopened filehandle LOG at file.pl line X"

Anyone else got any good ideas?
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Accepted Solution

by:
ozo earned 250 total points
ID: 11713213
eval{no warnings; print LOG @_} or warn @_;
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Author Comment

by:rtsh
ID: 11714335
ozo. Brilliant. Worked perfectly. You can sleep easy tonight that you've helped a busy coder solve an annoying error.

If there's an admin around that can split the points I awarded to davorg between him and ozo, or let me know how I can do it, that'd be excellent.
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