http://www.perlmonks.org?node_id=1063097

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

I have gotten a bug report containing a warning message which I am unable to explain. The code is as follows: 
sub _call_hook {
    my $name = shift;
    my $file = shift; # line 123
    my $detach = shift;
    ...

[download]
The warning is: Use of uninitialized value in scalar assignment at /usr/lib/perl5/site_perl/5.14.2/Foo/Bar.pm line 123.

_call_hook is being called as _call_hook($foo, $bar{$something}) where $bar{$something} does not exist.

Obviously, the warning is right, undef is being assigned to a scalar, but this is a pretty common case that doesn't generate a warning under normal conditions.

Could somebody shed some light on the subject? Which specific conditions may be triggering that warning?

Replies are listed 'Best First'.
Re: Unexpected warning
by cdybedahl (Acolyte) on Nov 18, 2013 at 14:48 UTC
    You get that warning on that line if $something is undefined. Although in that case you also ought to get another warning as well, about using an uninitialized value in a hash element on the calling line.
[reply]
      > You get that warning on that line if $something is undefined.

      yep I can reproduce this. =)

      Seems like in the context of the call warnings were disabled.

        DB<132> use warnings; sub tst {my $a=shift, my $b =shift}

  DB<133> %h=()

  DB<134> $bla=""
 => ""

  DB<135> tst $h{$bla}
 => (undef, undef)

  DB<136> undef $bla
 => undef

  DB<137> tst $h{$bla}
Use of uninitialized value in scalar assignment at (eval 81)[multi_per
+l5db.pl:644] line 2.

      [download]

      Cheers Rolf

      ( addicted to the Perl Programming Language)

[reply]
[d/l]
        I can reproduce it myself too now: 
        tst(undef);        # doesn't warn from tst
tst($foo{undef});  # doesn't warn from tst
tst($foo{$undef}); # warns from tst

        [download]
        I would say this is a bug in perl, or I am missing something?
[reply]
[d/l]

        I get no warnings from the code below: 

        use strict;
use warnings;
sub test { my $x = shift; }
my %hash = ( a => "a" );
test $hash{"b"};

        [download]
[reply]
[d/l]
      This should be pretty definitive... 
      use strict;
use warnings;

sub tst {
    warn "entering tst()\n";
    my $arg = shift;  # line 6
    warn "leaving tst()\n\n";
}

my %h=();
tst $h{''};
tst $h{undef};  # line 12
my $bla='';
tst $h{$bla};  # line 14
$bla=undef;
tst $h{$bla};  # line 16
undef $bla;
tst $h{$bla};  # line 19

      [download]
      And here's the results: 
      entering tst()
leaving tst()

entering tst()
leaving tst()

entering tst()
leaving tst()

Use of uninitialized value $bla in hash element at perlmonks-1063097.p
+l line 16.
entering tst()
Use of uninitialized value in scalar assignment at perlmonks-1063097.p
+l line 6.
leaving tst()

Use of uninitialized value $bla in hash element at perlmonks-1063097.p
+l line 18.
entering tst()
Use of uninitialized value in scalar assignment at perlmonks-1063097.p
+l line 6.
leaving tst()

      [download]

      Turning off 'strict' has no effect, while turning off 'warnings' warnings gets rid of the messages.

      I'm actually surprised by a couple of aspects of this. First, I expected $bla=undef; tst $h{$bla} to behave the same as tst $h{undef}; it didn't. Instead, $bla=undef seems to have the same effect as undef $bla. Second, it seems that lines 16 and 19 are generating a value other than a simple undef to pass into the subroutine, otherwise the invocation in line 12 would have also generated an error in line 6.

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[d/l]
[select]

      I get no warnings from this code below: 

      use strict;
use warnings;
my @array;
my $x = shift @array;
my $y = shift @array;

      [download]
[reply]
[d/l]
Re: Unexpected warning
by MidLifeXis (Monsignor) on Nov 18, 2013 at 14:59 UTC

    Is the reporter running under perl -w? Does your module use warnings?

    --MidLifeXis

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[d/l]
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