From brucer@gsg-lnx-bld1.cisco.com

This is a bug report for perl from brucer@​gsg-lnx-bld1.cisco.com,
generated with the help of perlbug 1.35 running under perl v5.8.6.

The following program invokes beta once instead of twice.
Extremely non-intuitive.
moving the "#' down one line fixes it.
1) seems like it shouldn't compile.
2) $c is static and keeps its value between invocations if
  one instead of "if (0)" one has "if ($b)"
This is boiled down from a real program, of course.

thanks,
Bruce

--cut--
use strict 'vars';

sub beta ($) {
  my $a = shift;
  print "beta($a)\n";
  "xyz";
}

sub alpha ($) {
  my ( $a, $b ) = @​_;
  print "alpha($a)\n";
# my $c;
  my
  $c = 1 if (0);
  $c = beta ($a) if ( ! defined $c );
}

alpha( 'A');
alpha( 'B');
--cut--
haven't analyzed this one thoroughly​:
--cut 2--
use strict 'vars';

sub beta ($) {
  my $a = shift;
  print "beta($a)\n";
  "xyz";
}

sub alpha ($$) {
  my ( $a, $b ) = @​_;
  print "alpha($a)\n";
# my $c;
  my
  $c = 1 if ($b);
  $c = beta ($a) if ( ! defined $c );
}

alpha( 'A',0);
alpha( 'B',0);
--cut 2--

Flags​:
  category=core
  severity=low

Site configuration information for perl v5.8.6​:

Configured by brucer at Tue Jan 4 17​:17​:23 PST 2005.

Summary of my perl5 (revision 5 version 8 subversion 6) configuration​:
  Platform​:
  osname=linux, osvers=2.4.20-34.7.cisco.1smp, archname=i686-linux
  uname='linux gsg-lnx-bld1.cisco.com 2.4.20-34.7.cisco.1smp #1 smp wed jun 23 15​:03​:42 edt 2004 i686 unknown '
  config_args='-Dcc=gcc -Dprefix=/auto/gsg-sw/inst/i686-pc-linux-gnu -des'
  hint=recommended, useposix=true, d_sigaction=define
  usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef
  useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
  use64bitint=undef use64bitall=undef uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='gcc', ccflags ='-fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/usr/include/gdbm',
  optimize='-O2',
  cppflags='-fno-strict-aliasing -pipe -I/usr/local/include -I/usr/include/gdbm'
  ccversion='', gccversion='2.96 20000731 (Red Hat Linux 7.3 2.96-112)', gccosandvers=''
  intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
  ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
  alignbytes=4, prototype=define
  Linker and Libraries​:
  ld='gcc', ldflags =' -L/usr/local/lib'
  libpth=/usr/local/lib /lib /usr/lib
  libs=-lnsl -lndbm -lgdbm -ldl -lm -lcrypt -lutil -lc
  perllibs=-lnsl -ldl -lm -lcrypt -lutil -lc
  libc=/lib/libc-2.2.5.so, so=so, useshrplib=false, libperl=libperl.a
  gnulibc_version='2.2.5'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
  cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'

Locally applied patches​:
 

@​INC for perl v5.8.6​:
  /auto/gsg-sw/inst/i686-pc-linux-gnu/lib/perl5/5.8.6/i686-linux
  /auto/gsg-sw/inst/i686-pc-linux-gnu/lib/perl5/5.8.6
  /auto/gsg-sw/inst/i686-pc-linux-gnu/lib/perl5/site_perl/5.8.6/i686-linux
  /auto/gsg-sw/inst/i686-pc-linux-gnu/lib/perl5/site_perl/5.8.6
  /auto/gsg-sw/inst/i686-pc-linux-gnu/lib/perl5/site_perl
  .

Environment for perl v5.8.6​:
  HOME=/users/brucer
  LANG=en_US
  LANGUAGE (unset)
  LD_LIBRARY_PATH=
  LOGDIR (unset)
  PATH=/auto/gsg-sw/inst/i686-pc-linux-gnu/bin​:/auto/gsg-sw/inst/share.brucer/sbin​:/auto/gsg-sw/inst/share.brucer/bin​:/usr/java/j2re1.4.1_01/bin​:/users/brucer/share/bin​:/router/bin​:/usr/local/packages/atria/current/contrib​:/usr/cisco/bin​:/usr/atria/bin​:/nfs/ddts/ddtshome/bin​:/bin​:/usr/sbin​:/usr/bin​:/auto/gsg-sw/share​:/usr/openwin/bin​:/usr/local/bin​:/bin​:/usr/bin​:/usr/X11R6/bin
  PERL_BADLANG (unset)
  SHELL=/bin/bash

From @iabyn

On Wed, Sep 07, 2005 at 05​:19​:55PM -0700, brucer @​ gsg-lnx-bld1. cisco. com wrote​:

The following program invokes beta once instead of twice.
Extremely non-intuitive.
moving the "#' down one line fixes it.
1) seems like it shouldn't compile.
2) $c is static and keeps its value between invocations if
one instead of "if (0)" one has "if ($b)"
This is boiled down from a real program, of course.

This is a well-known problem, which is difficult to fix without breaking
backwards compatibility (some people make use of the 'feature' to create
static variables).

The problem boils down to​:

If the 'my $x' is skipped at runtime, then at scope exit the variable isn't
freed, meaning that on next scope entry the variable maintains its
old value.

--
Standards (n). Battle insignia or tribal totems.

The RT System itself - Status changed from 'new' to 'open'

From pfusik@op.pl

The following program invokes beta once instead of twice.
Extremely non-intuitive.

Really? It is effectively​:

$c = beta ($a) if ( ! defined $c ); # the "if" condition is true
$c = beta ($a) if ( ! defined $c ); # the "if" condition is false

1) seems like it shouldn\'t compile.

Why not?

2) $c is static and keeps its value between invocations

Yes, because it is not my-ed.
I've seen the exact explaination that "my" declarations with "if 0"
are no-ops, somewhere in Perl PODs.

@rgs - Status changed from 'open' to 'rejected'

From @schwern

On Wed, Sep 07, 2005 at 05​:19​:55PM -0700, brucer @​ gsg-lnx-bld1. cisco. com wrote​:

The following program invokes beta once instead of twice.
Extremely non-intuitive.
moving the "#' down one line fixes it.
1) seems like it shouldn't compile.
2) $c is static and keeps its value between invocations if
one instead of "if (0)" one has "if ($b)"
This is boiled down from a real program, of course.

The short answer is "don't do that".

C<my $v if 0;> has already been deprecated in the development version of
Perl because its behavior is too confusing. The behavior cannot be
changed because too many people rely on it.

$ bleadperl -wle 'my $v if 0;'
Deprecated use of my() in false conditional at -e line 1.

--
Michael G Schwern schwern@​pobox.com http​://www.pobox.com/~schwern
'All anyone gets in a mirror is themselves,' she said. 'But what you
gets in a good gumbo is everything.'
  -- "Witches Abroad" by Terry Prachett