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$h{k} ||= autovivifies differently from $h{k} = #9903

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p5pRT opened this issue Oct 8, 2009 · 4 comments
Open

$h{k} ||= autovivifies differently from $h{k} = #9903

p5pRT opened this issue Oct 8, 2009 · 4 comments
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distro-Linux type-core

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@p5pRT
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p5pRT commented Oct 8, 2009

Migrated from rt.perl.org#69666 (status was 'open')

Searchable as RT69666$
@p5pRT
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p5pRT commented Oct 8, 2009

From andrew@pimlott.net

Created by andrew@pimlott.net

The following script

  my %h;
  eval { $h{k} = $h{k} || die };
  print exists $h{k} ? "key exists\n" : "key does not exist\n";
  eval { $h{k} ||= die };
  print exists $h{k} ? "key exists\n" : "key does not exist\n";

outputs

  key does not exist
  key exists

This defeats the purpose of the ||= for conditional assignment idiom. I
realize that the left side of ||= can only be evaluated once and then must
be used as both a value and an lvalue, but I can't see why this should
require creating the key.

Andrew

Perl Info 

Flags:
    category=core
    severity=low

Site configuration information for perl 5.10.1:

Configured by Debian Project at Thu Oct  1 14:02:50 UTC 2009.

Summary of my perl5 (revision 5 version 10 subversion 1) configuration:
   
  Platform:
    osname=linux, osvers=2.6.30.8-dsa-ia32, archname=i486-linux-gnu-thread-multi
    uname='linux murphy 2.6.30.8-dsa-ia32 #1 smp thu sep 24 23:51:44 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/5.10.1 -Dsitearch=/usr/local/lib/perl/5.10.1 -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.5.10.1 -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 -fstack-protector -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 -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.3.4', 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 /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.5.10.1
    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 -fstack-protector'

Locally applied patches:
    


@INC for perl 5.10.1:
    /home/andrew/lib/perl
    /home/andrew/local/lib/perl
    /etc/perl
    /usr/local/lib/perl/5.10.1
    /usr/local/share/perl/5.10.1
    /usr/lib/perl5
    /usr/share/perl5
    /usr/lib/perl/5.10
    /usr/share/perl/5.10
    /usr/local/lib/site_perl
    .


Environment for perl 5.10.1:
    HOME=/home/andrew
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LC_COLLATE=C
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/andrew/bin:/usr/sbin:/sbin:/home/andrew/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games
    PERL5LIB=/home/andrew/lib/perl:/home/andrew/local/lib/perl
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

@p5pRT
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p5pRT commented Oct 9, 2009

From @leonerd

On Thu, 08 Oct 2009 12​:18​:26 -0700
"andrew@​pimlott.net (via RT)" <perlbug-followup@​perl.org> wrote​:

The following script

my %h;
eval \{ $h\{k\} = $h\{k\} || die \};
print exists $h\{k\} ? "key exists\\n" : "key does not exist\\n";
eval \{ $h\{k\} ||= die \};
print exists $h\{k\} ? "key exists\\n" : "key does not exist\\n";

outputs

key does not exist
key exists

It may be of further interest to note that the following script

  my %h;
  eval { my $he = \$h{k}; $$he ||= die };
  print exists $h{k} ? "key exists\n" : "key does not exist\n";

outputs

  key exists

which more accurately depicts what the internals of the VM do under this
case.

--
Paul "LeoNerd" Evans

leonerd@​leonerd.org.uk
ICQ# 4135350 | Registered Linux# 179460
http​://www.leonerd.org.uk/

@p5pRT
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p5pRT commented Oct 9, 2009

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

@p5pRT
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p5pRT commented Oct 9, 2009

From @davidnicol

here's a workaround using a nonautovivifying subroutine argument reference​:

perl -wle "sub S { $_[0] ||= die};eval {S $h{k}};print exists $h{k} ?
'E' : 'NE'"
NE

for a quick and backwards-compatible repair, you could make a

  sub TrueOrAssign($$){ $_[0] ||= $_[1] }

to replace the identified key leak in your program.

@xenu xenu removed the affects-5.10 label Nov 19, 2021
@xenu xenu removed the Severity Low label Dec 29, 2021
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