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[perl #67838] lvalue substr keeping lexical alive
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From:
Kevin Ryde
Date:
July 24, 2009 05:55
Subject:
[perl #67838] lvalue substr keeping lexical alive
Message ID:
rt-3.6.HEAD-2466-1248394755-35.67838-75-0@perl.org
# New Ticket Created by Kevin Ryde
# Please include the string: [perl #67838]
# in the subject line of all future correspondence about this issue.
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=67838 >
The program foo.pl below prints
SCALAR(0x874b2c0)
where I hoped it would print undef, ie. the lexical scalar $str would be
garbage collected on going out of scope. undef is what I get without
the lvalue substr() assignment, or with a 4-arg substr call.
Some digging around suggests the scratchpad array in foo() holds a
reference to the $str scalar if an lvalue substr is used this way. I
don't know if that's a bug, a feature, or an unavoidable side-effect of
the implementation.
If a feature or unavoidable then take this report as a wish for
something in the docs on the subject, as even perlguts seems very thin
on anything about lvalue scalars.
For what it's worth I struck this in DBI.pm where it does a substr
modify like this and the resulting scalar looks like a memory leak to
Test::Weaken. I think it really is a leak, but only a temporary one
since the next call to foo() or whatever function seems to clear it out.
Of course if a string is very big it'd be bad to have it hanging around
in core beyond what you normally expect to be its scope.
-----------------------------------------------------------------
---
Flags:
category=core
severity=medium
---
Site configuration information for perl 5.10.0:
Configured by Debian Project at Thu Jul 9 09:30:18 UTC 2009.
Summary of my perl5 (revision 5 version 10 subversion 0) configuration:
Platform:
osname=linux, osvers=2.6.30.1-dsa-ia32, archname=i486-linux-gnu-thread-multi
uname='linux murphy 2.6.30.1-dsa-ia32 #1 smp fri jul 3 12:55:10 cest 2009 i686 gnulinux '
config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -Dcccdlflags=-fPIC -Darchname=i486-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.10 -Darchlib=/usr/lib/perl/5.10 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.10.0 -Dsitearch=/usr/local/lib/perl/5.10.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.10.0 -Dd_dosuid -des'
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 -DDEBIAN -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
optimize='-O2 -g',
cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing -pipe -I/usr/local/include'
ccversion='', gccversion='4.3.3', 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 =' -L/usr/local/lib'
libpth=/usr/local/lib /lib /usr/lib /usr/lib64
libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
perllibs=-ldl -lm -lpthread -lc -lcrypt
libc=/lib/libc-2.9.so, so=so, useshrplib=true, libperl=libperl.so.5.10.0
gnulibc_version='2.9'
Dynamic Linking:
dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
cccdlflags='-fPIC', lddlflags='-shared -O2 -g -L/usr/local/lib'
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[perl #67838] lvalue substr keeping lexical alive
by Kevin Ryde