gri6507 has asked for the wisdom of the Perl Monks concerning the following question:

Fellow monks,

I have a need to fork() and then redefine a function within the child, but not the parent. Here's my code.

use warnings;
use strict;

sub foo {
    my $bar = shift;
    print "original foo($bar)\n";
}

if (my $pid = fork()) {
    close PARENT;

    foo('parent');

    close CHILD;
    waitpid($pid, 0);
} else {
    die "cannot fork: $!" unless defined $pid;
    close CHILD;

    # overload the function
    no warnings 'redefine';
    sub foo {
        my $bar = shift;
        print "modified foo($bar)\n";
    }

    foo('child');

    close PARENT;
    exit(0);
}

foo('the end');

[download]
This prints:
modified foo(parent)
modified foo(child)
modified foo(the end)

However, I was hoping that the redefinition of the foo() function would only affect the scope of the child process. In other words, I wanted to get
original foo(parent)
modified foo(child)
original foo(the end)

Is something like this possible?

Replies are listed 'Best First'.
Re: Redefining function after fork
by Athanasius (Archbishop) on Jan 18, 2014 at 02:43 UTC
[reply]
[d/l]
[select]
      Thank you. This is what I wanted. However, I also need to do such "remapping" of functions within a child for a set of functions, where each remapping is exactly the same - ideally something like:
      foreach my $origName (qw(foo bar baz)) {
    my $newName = "modified_$origName";
    sub $newName {
        print "modified $newName(" . join(',', @_) . ")\n";
    }
    *$origName = *$newName;
}

      [download]
      However, this is not lexically correct. How does one create a subroutine whos name is a variable?
[reply]
[d/l]

        I'm not sure just what you're asking for here, but maybe one of these will serve: 

        >perl -wMstrict -le
"foreach my $origName (qw(foo bar baz)) {
   my $newName = qq{gen_$origName};
   my $newCode = sub {
     print qq{generated $origName(@_)};
     };
   no strict 'refs';
   *$newName = $newCode;
   }
 ;;
 gen_foo(1, 2, 3);
 gen_bar(4, 5, 6);
 gen_baz(7, 8, 9);
 ;;
 my $name = 'also_works';
 eval qq{sub $name { print qq{this also works: \@_}; } };
 also_works(9, 8, 7);
"
generated foo(1 2 3)
generated bar(4 5 6)
generated baz(7 8 9)
this also works: 9 8 7

        [download]

        Update: This can be made a bit more concise (note the slightly tricky disambiguating semicolon in the
            *{; ... } = sub { ... }
        statement): 

        >perl -wMstrict -le
"foreach my $origName (qw(foo bar)) {
   no strict 'refs';
   *{; qq{gen_$origName} } = sub { print qq{generated $origName(@_)} }
+;
   }
 ;;
 gen_foo(1, 2, 3);
 gen_bar(4, 5, 6);
"
generated foo(1 2 3)
generated bar(4 5 6)

        [download]
[reply]
[d/l]
[select]

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