30
Jun/092

use autodie;

When writing perl code you’ll often find yourself using built-in functions that do not throw a fatal error when there is a problem. Instead you are expected to check the return value of the function, and if it is false, call die() yourself. Consider the following code:

open( my $fh, '>', 'testfile.txt' )
    or die('Unable to open testfile.txt for write');
print $fh "Hello world!\n";
close( $fh )
    or die('Unable to close testfile.txt');

This is a very cumbersome way to write code as you could easily forget to add these extra checks. If you’re lazy, which you should be, you’ll want fatal errors without having to write a bunch of extra code. This is where autodie steps in. The above code can then be rewritten as:

use autodie;

open( my $fh, '>', 'testfile.txt' );
print $fh "Hello world!\n";
close( $fh );

Much better. Generally you should want anything that has an unexpected problem to throw a fatal error. If you want your code to catch the error, handle it, and possibly continue running, wrap the offending code in an eval:

use autodie;

eval{
    open( my $fh, '>', 'testfile.txt' );
    print $fh "Hello world!\n";
    close( $fh );
};
if ($@) {
    unlink('testfile.txt') if -e 'testfile.txt';
    die($@);
}

On a side note: block evals, such as eval{ print ‘hi!’; } are very efficient and you can use them liberally. String evals, such as eval(”print ‘hi!’”), are not nearly as efficient and should be avoided.

Tagged as:
2 Comments
Comments (2) Trackbacks (0)
  1. Chas. Owens
    8:39 am on June 30th, 2009

    Your eval version can be a little cleaner:

    my $file = “testfile.txt”;
    eval {
    open my $fh, “>”, $file;
    print {$fh} “Hello world!\n”;
    close $fh;
    } or {
    warn $@;
    unlink $file if -e $file;
    exit 255;
    }

    Since close will return true if it succeeds, we can use that return to build a try/catch like system that is not dependent on $@ (which could be changed by an errant DESTROY method that failed to localize it). Also, warning about the error means that if the unlink fails it won’t mask the error from the eval.

  2. racer
    3:43 am on July 20th, 2009

    Chas.Owens, you forgot

     do

    after that

     or

    :)

    unlink(’testfile.txt’) if -e ‘testfile.txt’; is a race condition, testfile.txt could exist when -e ‘testfile.txt’; is called, but be deleted before unlink is called.

    Not to worry, since autodie and $fh are lexically scoped, you could simply write

        use autodie;
        eval {
            open( my $fh, '<', 'testfile.txt' );
            print $fh "Hello world!\n";
            1;
        } or do {
            no autodie 'unlink';
            unlink 'testfile.txt';
            die;
        };

    or

        my $file = q'testfile.txt';
        eval {
            use autodie;
            open( my $fh, '<', $file );
            print $fh "Hello world!\n";
            1;
        } or do {
            unlink $file;
            die
        };

Leave a comment

No trackbacks yet.

Recent Posts

Iron Man

Links

RSS Planet Perl Iron Man Posts

RSS bluefeet @ github

Perl Ads

RSS Perl Jobs

Tags

Calendar

June 2009
M T W T F S S
« May   Jul »
1234567
891011121314
15161718192021
22232425262728
2930