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

Is it possible to tie a callback/subroutine that will be automatically executed when another function/subroutine is called?

For example: If I call the built-in open() function, it does it's thing.

I would like to watch for calls to open() and execute my own subroutine at the same time (before or after, but automatically). Is this possible?

  • Comment on Tie subroutine to a built-in function call?

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Re: Tie subroutine to a built-in function call?
by BrowserUk (Patriarch) on Aug 23, 2016 at 17:15 UTC

    See Overriding core functions.

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[reply]
Re: Tie subroutine to a built-in function call?
by stevieb (Canon) on Aug 23, 2016 at 17:25 UTC

    Here's an example. You assign a reference to your own subroutine to the CORE::GLOBAL namespace for the function in question within a BEGIN block. In this case, the custom open() will only accept the 3-arg form of open.

    use warnings;
use strict;

BEGIN {
    *CORE::GLOBAL::open = sub (*;$@) {
        print "in my open...\n";

        # the following line calls the built-in open() function
        # directly with the params your sub received

        CORE::open($_[0], $_[1], $_[2]);
    }
};

open my $fh, '<', 'blah' or die $!;

print <$fh>;

    [download]

    Output:

    in my open...
file contents

    [download]
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[d/l]
[select]
      This is exactly what I was looking for!
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