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[perl #46943] LVALUE refs have an incorrect refcount

From:
Father Chrysostomos
Date:
October 27, 2007 02:51
Subject:
[perl #46943] LVALUE refs have an incorrect refcount
Message ID:
rt-3.6.HEAD-20149-1193447610-1243.46943-75-0@perl.org
# New Ticket Created by  Father Chrysostomos 
# Please include the string:  [perl #46943]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=46943 >


I don't seem to be able to weaken LVALUE refs:
$ perl -MScalar::Util=weaken -le '$_ = \substr$y,0; print; weaken $_;  
print'
LVALUE(0x1801d88)
LVALUE(0x1801d88)

(Although 'isweak' claims it's weak.)

But if I execute the same compiled code more than once:
$ perl -MScalar::Util=weaken -le '$x = \substr$y,0 for 1..2; print  
$x; weaken $x; print $x'
LVALUE(0x1801d4c)
<blank line>

It turns out the the first one has the wrong refcount:
$ perl -le 'for (1..5){ push @@, \substr my $y,0; print  
Internals::SvREFCNT(${$@[-1]}), " $@[-1]"  }'
2 LVALUE(0x1801dd0)
1 LVALUE(0x1801440)
1 LVALUE(0x1801e30)
1 LVALUE(0x1801e54)
1 LVALUE(0x1801e78)

(Without pushing it onto a var outside the block, it reuses the  
reference,
alternating between two of them:)
$ perl -le 'for (1..5){ $x = \substr my $y,0; print  
Internals::SvREFCNT($$x), " $x"  }'
SvREFCNT($$x), " $x"  }'
2 LVALUE(0x1801dd0)
1 LVALUE(0x180129c)
2 LVALUE(0x1801dd0)
1 LVALUE(0x180129c)
2 LVALUE(0x1801dd0)

If I weaken it, it forces it to use the same one again next time:
$ perl -MScalar::Util=weaken -le 'for (1..5){ push @@, \substr my $y, 
0; print Internals::SvREFCNT(${$@[-1]}), " $@[-1]" ; weaken $@[-1] }'
2 LVALUE(0x1801db8)
2 LVALUE(0x1801db8)
2 LVALUE(0x1801db8)
2 LVALUE(0x1801db8)
2 LVALUE(0x1801db8)
$ perl -MScalar::Util=weaken -le 'for (1..5){ push @@, \substr my $y, 
0; print Internals::SvREFCNT(${$@[-1]}), " $@[-1]" ; weaken $@[-1] if  
$_ %2}'
2 LVALUE(0x1801db8)
2 LVALUE(0x1801db8)
1 LVALUE(0x1801440)
1 LVALUE(0x1801440)
1 LVALUE(0x1818e78)

This occurs also in perl 5.8.8.

I hope that's enough information to help solve the problem. I  
encountered the same thing with 'vec', but I haven't looked into it  
in detail. (I wonder what pos() does.)


Father Chrysostomos

---
Flags:
     category=core
     severity=low
---
This perlbug was built using Perl 5.10.0 - Mon Sep 24 10:10:11 PDT 2007
It is being executed now by  Perl 5.10.0 - Sat Oct  6 16:48:24 PDT 2007.

Site configuration information for perl 5.10.0:

Configured by neo at Sat Oct  6 16:48:24 PDT 2007.

Summary of my perl5 (revision 5 version 10 subversion 0 patch 32056)  
configuration:
   Platform:
     osname=darwin, osvers=8.8.0, archname=darwin-2level
     uname='darwin treebeard.local 8.8.0 darwin kernel version 8.8.0:  
fri sep 8 17:18:57 pdt 2006; root:xnu-792.12.6.obj~1release_ppc power  
macintosh powerpc '
     config_args='-de'
     hint=recommended, useposix=true, d_sigaction=define
     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='cc', ccflags ='-fno-common -DPERL_DARWIN -no-cpp-precomp - 
fno-strict-aliasing -pipe -I/usr/local/include',
     optimize='-O3',
     cppflags='-no-cpp-precomp -fno-common -DPERL_DARWIN -no-cpp- 
precomp -fno-strict-aliasing -pipe -I/usr/local/include'
     ccversion='', gccversion='4.0.0 20041026 (Apple Computer, Inc.  
build 4061)', gccosandvers='darwin8'
     intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=4321
     d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
     ivtype='long', ivsize=4, nvtype='double', nvsize=8,  
Off_t='off_t', lseeksize=8
     alignbytes=8, prototype=define
   Linker and Libraries:
     ld='env MACOSX_DEPLOYMENT_TARGET=10.3 cc', ldflags =' -L/usr/ 
local/lib'
     libpth=/usr/local/lib /usr/lib
     libs=-ldbm -ldl -lm -lc
     perllibs=-ldl -lm -lc
     libc=/usr/lib/libc.dylib, so=dylib, useshrplib=false,  
libperl=libperl.a
     gnulibc_version=''
   Dynamic Linking:
     dlsrc=dl_dlopen.xs, dlext=bundle, d_dlsymun=undef, ccdlflags=' '
     cccdlflags=' ', lddlflags=' -bundle -undefined dynamic_lookup -L/ 
usr/local/lib'

Locally applied patches:
     DEVEL

---
@INC for perl 5.10.0:
     /usr/local/lib/perl5/5.9.5/darwin-2level
     /usr/local/lib/perl5/5.9.5
     /usr/local/lib/perl5/site_perl/5.9.5/darwin-2level
     /usr/local/lib/perl5/site_perl/5.9.5
     .

---
Environment for perl 5.10.0:
     DYLD_LIBRARY_PATH (unset)
     HOME=/Users/neo
     LANG (unset)
     LANGUAGE (unset)
     LD_LIBRARY_PATH (unset)
     LOGDIR (unset)
     PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/TeX/bin/powerpc- 
darwin6.8:/usr/local/bin
     PERL_BADLANG (unset)
     SHELL=/bin/bash




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