eval return value is only sometimes copied

[p5pRT]

Migrated from rt.perl.org#105910 (status was 'resolved')

Searchable as RT105910$

[p5pRT]

From @cpansprout

#!perl -l
print \$x;
$a = sub { \@​_ }->($x);
print \eval ' shift @​$a ';
print \eval ' $x ';

This gives me​:

SCALAR(0x826d50)
SCALAR(0x826d50)
SCALAR(0x826e60)

---

Flags​:
  category=core
  severity=low

---

Site configuration information for perl 5.15.4​:

Configured by sprout at Wed Nov 2 09​:06​:14 PDT 2011.

Summary of my perl5 (revision 5 version 15 subversion 4) configuration​:
  Snapshot of​: f364061
  Platform​:
  osname=darwin, osvers=10.5.0, archname=darwin-thread-multi-2level
  uname='darwin pint.local 10.5.0 darwin kernel version 10.5.0​: fri nov 5 23​:20​:39 pdt 2010; root​:xnu-1504.9.17~1release_i386 i386 '
  config_args='-de -Doptimize=-g -Dusedevel -Duseithreads -Dmad'
  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 ='-fno-common -DPERL_DARWIN -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include',
  optimize='-g',
  cppflags='-fno-common -DPERL_DARWIN -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
  ccversion='', gccversion='4.2.1 (Apple Inc. build 5664)', gccosandvers=''
  intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
  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 =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /usr/lib
  libs=-ldbm -ldl -lm -lutil -lc
  perllibs=-ldl -lm -lutil -lc
  libc=, 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 -fstack-protector'

Locally applied patches​:
 

---

@​INC for perl 5.15.4​:
  /usr/local/lib/perl5/site_perl/5.15.4/darwin-thread-multi-2level
  /usr/local/lib/perl5/site_perl/5.15.4
  /usr/local/lib/perl5/5.15.4/darwin-thread-multi-2level
  /usr/local/lib/perl5/5.15.4
  /usr/local/lib/perl5/site_perl
  .

---

Environment for perl 5.15.4​:
  DYLD_LIBRARY_PATH (unset)
  HOME=/Users/sprout
  LANG=en_US.UTF-8
  LANGUAGE (unset)
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)
  PATH=/usr/bin​:/bin​:/usr/sbin​:/sbin​:/usr/local/bin​:/usr/X11/bin​:/usr/local/bin
  PERL_BADLANG (unset)
  SHELL=/bin/bash

[p5pRT]

From @jkeenan

On Sun Dec 11 13​:07​:43 2011, sprout wrote​:

#!perl -l
print \$x;
$a = sub { \@​_ }->($x);
print \eval ' shift @​$a ';
print \eval ' $x ';

This gives me​:

SCALAR(0x826d50)
SCALAR(0x826d50)
SCALAR(0x826e60)

This bug report is too terse for a mere mortal like me to understand.

Can anyone explicate?

Thank you very much.
Jim Keenan

[p5pRT]

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

[p5pRT]

From @epa

When you eval() a string of code and it returns a value, do you get back that value or a copy of it? The code example shows that in the first eval() call the same scalar $a is being returned, since the SCALAR(0xwhatever) printed is the same as for the original print() call. However, the second eval() call, while the code being evaluated is equivalent, somehow returns a copy of the scalar, printing a different SCALAR(0xsomethingelse). I can reproduce this on 5.18.2 also.

The behaviour should be more consistent​: either always copying the value, or (probably more useful) always returning the value without copying.

[p5pRT]

From @iabyn

On Sun, Dec 11, 2011 at 01​:07​:44PM -0800, Father Chrysostomos wrote​:

# New Ticket Created by Father Chrysostomos
# Please include the string​: [perl #105910]
# in the subject line of all future correspondence about this issue.
# <URL​: https://rt-archive.perl.org/perl5/Ticket/Display.html?id=105910 >

#!perl -l
print \$x;
$a = sub { \@​_ }->($x);
print \eval ' shift @​$a ';
print \eval ' $x ';

This gives me​:

SCALAR(0x826d50)
SCALAR(0x826d50)
SCALAR(0x826e60)

I think that's been fixed by my context stack reworking for 5.24.0.

--
Never work with children, animals, or actors.

[p5pRT]

@iabyn - Status changed from 'open' to 'resolved'