if(open FILE, ">$filename") {
  flock FILE, 2;
  ... # Write stuff to FILE
  close FILE;
} else {
  print "An I/O error occurred!";
  log_error $!; # Call a sub
}
[download]

And so you have an error, you know that there is no such file or directory. What then? How does this help in debugging.

As perlstyle says:

       o   Always check the return codes of system calls.  Good
           error messages should go to STDERR, include which
           program caused the problem, what the failed system
           call and arguments were, and (VERY IMPORTANT) should
           contain the standard system error message for what
           went wrong.  Here's a simple but sufficient example:

               opendir(D, $dir)     or die "can't opendir $dir: $!";
[download]

By not including debugging information on your arguments you will make such basic things as a renamed directory painful to track down.

And as a side-note, something I learned the other day for error handling in system calls :

system("/usr/bin/scp $file $username\@$host:$remoteFile");
die "System call to scp command failed! : $!\n" if ($? != 0);
[download]

The special variable $? returns a boolean value for success or fail on system calls. So the above snippet performs a reliable "or die" check on your command. A handy one to know, especially as system tends to be a little unreliable in terms of error checking / return values when using a straight die, IMHO.

Update: as per comments below, not a "boolean" in the strict sense of the word. Call it an "azatoth boolean" :)

Azatoth a.k.a Captain Whiplash

Make Your Die Messages Full of Wisdom!
Get YOUR PerlMonks Stagename here!
Want to speak like a Londoner?

Well, my CGI program was installed in a controlled environment, where errors should be very rare, and users were generally _very_ clueless. I can't just have my program die on them! Instead, my program would skip the file operation in question, but log an error, and display an error message with a possible solution, print out the rest of the page layout, and then end at the same spot the program would normally.

Maybe it's a matter of taste, but I just don't like the or die construct, it doesn't give me the ability to neatly and readably handle the exception.

Example:

sub display_values_from_file {
  my $filename = shift;
  if(open FILE, "<$filename") { # check return value of open
    flock FILE, 1;
    while(<FILE>) {
      print &process_line_from_file;
    }
    close FILE;
  } else {
    print "The file could not be opened. Please try again later!";
    log_error "open $filename : $!";
  }
}
# main program
... # do something
print $some_nice_html;
display_values_from_file "/foo/bar.baz";
print $rest_of_html_page;
# end
[download]

This way, when an error occurs, the user will be informed of the error, and all the links the user would need would still be there.

what?  if () {} else {} ?

what about:

open FILE, ">$filename" or &exit_with_error ('An I/O error occured!', 
+$!);
[download]

and in your &exit_with_error sub you do more stuff and exit

just a thought

---

He who asks will be a fool for five minutes, but he who doesn't ask will remain a fool for life.

Chady | http://chady.net/