Bug #120562 for Getopt-Long: please circumvent a core Perl bug in Getopt::Long

Thank you for your kind answers. I had failed to realize Getopt::Long meant to tweak @ARGV with side effects, and this had a nasty interaction with my habit of using C-like entry points in Perl scripts (in order to incite future maintainers to not hide active code at top level or such nasty ideas). Recent Enlightenment made me realize the other practical thorn in the foot I had with respect to this core Perl bug probably boiled down to the same cause: $ cat /tmp/cr.pl use Carp qw( carp ); use Getopt::Long qw( GetOptions ); sub main {   GetOptions();   carp("Hello"); } exit main(@ARGV); $ perl /tmp/cr.pl jkjkl jkljlk panic: attempt to copy freed scalar 12521c0 to 12f1b08 at /usr/share/perl/5.20/Carp.pm line 228. I understand the core Perl bug is nasty and tricky and takes a lot of efforts to fix. Let's hope it does not get buried and forgotten. In the mean time, I will remember to tell people to not use "global" ("top level"?) variables (such as the @L in my first example) and avoid using GetOptions(), but rather use GetOptionsFromArray() (and passing it a reference to a copy of @ARGV or whatever). On Thursday, March 9, 2017 8:00 PM, Father Chrysostomos via RT <bug-Getopt-Long@rt.cpan.org> wrote: <URL: https://rt.cpan.org/Ticket/Display.html?id=120562 > On Thu Mar 09 02:21:31 2017, sblondeel@yahoo.com wrote: Show quoted text
The reason you are getting a freed scalar (as I am sure you are aware) is that the elements of @_ are aliased to the elements of @ARGV without holding a reference count. If you take a reference to \@_ from inside the main sub before calling GetOptions, it will ‘reify’ @_ and make it hold reference counts on its elements, thereby circumventing the bug.  However, I don’t think it will do what you want. Getopt::Long is designed specifically to remove the options it parses from @ARGV, so that all you have left are the command line arguments that come after the options.  For example, if you write a ‘tail’ program in perl, then GetOptions will remove the -f in ‘tail -f foo’ and leave you just the foo, which is what most programs want. If you want to make @_ an alias to @ARGV, so that you can use shift, etc., inside a sub without having to say @ARGV explicitly, you can alias it like this: *_ = \@ARGV; &main; # no parentheses The &foo-style call leaves @_ alone; i.e., whatever array @_ pointed to outside the sub call is visible in the sub, which in this case is the same as @ARGV.

The problem lied in Carp (and beyond that, in DB/Perl core, the infamous RT 77706).It is not necessary to use @ARGV or any predefined variables to trigger it as the following code shows: #! /usr/bin/perl use Carp qw( carp ); my @toto = (qw(a b c d)); sub main {   carp('Hello');   @toto = @toto x 1000;   carp('Bye'); } exit main(@ARGV); $ perl a.pl ...at least it fails on 5.24.1 (but not on 5.20.2!). As I was about to report that on Carp I noticed the problem had gone in 5.28.0 (as of Carp 1.47).The comment is not too reassuring (it does not guarantee 100% protection) so I will use the following extra steps in my code (all tested and sufficient by themselves): 1/ use exit main(@[@ARGV])2/ use GetOptionsFromArray([@ARRAY], \%h, ...) instead of GetOptions(\%h, ...)3/ use a locally patched Carp.pm (called Carp2) while not migrated to 5.28 (or 5.28's). This seems to work in the particular case of @ARGV: replace             @args = @DB::args; with             if (scalar caller ($i+1)) {               @args = @DB::args;             } else {               @args = @ARGV;             } HTH,