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Feb 3, 2012, 3:57 AM
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[perl #109726] PL_sv_undef loses identity
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# New Ticket Created by Zefram
# Please include the string: [perl #109726]
# in the subject line of all future correspondence about this issue.
# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=109726 >
This is a bug report for perl from zefram [at] fysh,
generated with the help of perlbug 1.39 running under perl 5.15.7.
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[Please describe your issue here]
Passing the standard undef object to a subroutine as an argument fails
to pass by reference. Instead the sub sees a fresh *and writable*
undefined scalar:
$ perl -lwe 'BEGIN { *x = \undef; } sub mutate($) { print \$_[0], " second"; $_[0] = 456; } print \$x, " first"; mutate($x); print "done";'
SCALAR(0x8dc6500) first
SCALAR(0x8dc9758) second
done
whereas any other scalar has its identity preserved:
$ perl -lwe 'BEGIN { *x = \123; } sub mutate($) { print \$_[0], " second"; $_[0] = 456; } print \$x, " first"; mutate($x); print "done";'
SCALAR(0x86739f8) first
SCALAR(0x86739f8) second
Modification of a read-only value attempted at -e line 1.
$ perl -lwe 'BEGIN { *x = \!1; } sub mutate($) { print \$_[0], " second"; $_[0] = 456; } print \$x, " first"; mutate($x); print "done";'
SCALAR(0x8d5e510) first
SCALAR(0x8d5e510) second
Modification of a read-only value attempted at -e line 1.
The latter case above is PL_sv_no, which is immortal in the same way as
the misbehaving PL_sv_undef. Devel::Peek::Dump() confirms the identity
of these special scalars. Attempting mutation without the intermediation
of a subroutine call consistently produces the expected failure:
$ perl -lwe 'BEGIN { *x = \undef; } $x = 456; print "done";'
Modification of a read-only value attempted at -e line 1.
$ perl -lwe 'BEGIN { *x = \123; } $x = 456; print "done";'
Modification of a read-only value attempted at -e line 1.
$ perl -lwe 'BEGIN { *x = \!1; } $x = 456; print "done";'
Modification of a read-only value attempted at -e line 1.
[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
category=core
severity=low
---
Site configuration information for perl 5.15.7:
Configured by zefram at Sat Jan 21 18:33:38 GMT 2012.
Summary of my perl5 (revision 5 version 15 subversion 7) configuration:
Platform:
osname=linux, osvers=2.6.32-5-686, archname=i386-linux-thread-multi
uname='linux vigo.rous.org 2.6.32-5-686 #1 smp thu nov 3 04:23:54 utc 2011 i686 gnulinux '
config_args='-des -Darchname=i386-linux -Dcccdlflags=-fPIC -Dccdlflags=-rdynamic -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52 -Dman1ext=1 -Dman3ext=3perl -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
hint=recommended, useposix=true, d_sigaction=define
useithreads=define, usemultiplicity=define
useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
use64bitint=undef, use64bitall=undef, uselongdouble=undef
usemymalloc=n, bincompat5005=undef
Compiler:
cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
optimize='-O2',
cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
ccversion='', gccversion='4.4.5', 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='cc', ldflags =' -fstack-protector -L/usr/local/lib'
libpth=/usr/local/lib /lib/../lib /usr/lib/../lib /lib /usr/lib /usr/lib64
libs=-lnsl -lgdbm -ldl -lm -lcrypt -lutil -lpthread -lc -lgdbm_compat
perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
libc=/lib/libc-2.11.2.so, so=so, useshrplib=true, libperl=libperl.so
gnulibc_version='2.11.2'
Dynamic Linking:
dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/5.15.7/i386-linux-thread-multi/CORE'
cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector'
Locally applied patches:
---
@INC for perl 5.15.7:
/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/site_perl/5.15.7/i386-linux-thread-multi
/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/site_perl/5.15.7
/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/5.15.7/i386-linux-thread-multi
/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/5.15.7
.
---
Environment for perl 5.15.7:
HOME=/home/zefram
LANG (unset)
LANGUAGE (unset)
LD_LIBRARY_PATH (unset)
LOGDIR (unset)
PATH=/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/i686-pc-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
PERL_BADLANG (unset)
SHELL=/usr/bin/zsh
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