dangerous behavior from local #850

p5pRT · 1999-11-14T14:48:23Z

Migrated from rt.perl.org#1778 (status was 'resolved')

Searchable as RT1778$

p5pRT · 1999-11-14T14:48:23Z

From pimlott@idiomtech.com

A coworker accidentally did:

{
local($/ = undef);
...
}

This did not localize $/ . I don't know what it did do. There was no
warning. Could I get a warning out of it?

Thanks,
Andrew

Perl Info



Site configuration information for perl 5.00503:

Configured by torin at Wed Sep 22 00:18:38 PDT 1999.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=linux, osvers=2.0.36, archname=i386-linux
    uname='linux perv 2.0.36 #2 wed nov 18 03:00:48 pst 1998 i686 unknown '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O2', gccversion=2.95.1 19990809 (prerelease)
    cppflags='-Dbool=char -DHAS_BOOL -D_REENTRANT -DDEBIAN -I/usr/local/include'
    ccflags ='-Dbool=char -DHAS_BOOL -D_REENTRANT -DDEBIAN -I/usr/local/include'
    stdchar='char', d_stdstdio=undef, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -lndbm -lgdbm -ldbm -ldb -ldl -lm -lc -lposix -lcrypt
    libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    


@INC for perl 5.00503:
    /home/pimlott/idiom/perl
    /usr/lib/perl5/5.005/i386-linux
    /usr/lib/perl5/5.005
    /usr/local/lib/site_perl/i386-linux
    /usr/local/lib/site_perl
    /usr/lib/perl5
    .


Environment for perl 5.00503:
    HOME=/home/pimlott
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/pimlott/bin:/usr/sbin:/sbin:/home/pimlott/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games
    PERL5LIB=/home/pimlott/idiom/perl
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

p5pRT · 1999-11-15T19:01:56Z

From [Unknown Contact. See original ticket]

On Sun, 14 Nov 1999, Andrew Pimlott wrote:

A coworker accidentally did:

{
local($/ = undef);
...
}

This did not localize $/ . I don't know what it did do.

It looks to me as if it did localize $/ -- but only _after_ storing undef
into it. Oops!

There was no warning. Could I get a warning out of it?

Or, better yet, could perl simply Do The Right Thing?

--
Tom Phoenix Perl Training and Hacking Esperanto
Randal Schwartz Case: http://www.rahul.net/jeffrey/ovs/

p5pRT · 1999-11-15T19:10:04Z

From @tamias

On Mon, Nov 15, 1999 at 07:01:44PM -0800, Tom Phoenix wrote:

On Sun, 14 Nov 1999, Andrew Pimlott wrote:

A coworker accidentally did:

{
local($/ = undef);
...
}

This did not localize $/ . I don't know what it did do.

It looks to me as if it did localize $/ -- but only _after_ storing undef
into it. Oops!

That was my guess, too, but further tests don't seem to be consistent with
that conclusion:

#!perl -w

$x = 'a';

print "1: $x\n";

{
local($x = undef);
print "2: $x\n";
$x = 'b';
print "3: $x\n";
}

print "4: $x\n";

1: a
Use of uninitialized value at foo line 9.
2:
3: b
4: b

--

If $x were being localized at all, then 4 would be 'a', rather than 'b'.

Ronald

p5pRT · 1999-11-15T19:14:37Z

From [Unknown Contact. See original ticket]

On Mon, Nov 15, 1999 at 07:01:44PM -0800, Tom Phoenix wrote:

On Sun, 14 Nov 1999, Andrew Pimlott wrote:

A coworker accidentally did:

{
local($/ = undef);
...
}

This did not localize $/ . I don't know what it did do.

It looks to me as if it did localize $/ -- but only _after_ storing undef
into it. Oops!

That made so much sense when someone else pointed it out, I felt like an
idiot. Tragically, it is not so. Try this:

#!/usr/bin/perl -w

{
local($/ = undef);
$/ = "0";
}
print scalar <>;
__END__

You will find that the input separator is "0". $/ is never localized.
Beats me what is.

Andrew

p5pRT · 1999-11-16T00:27:39Z

From [Unknown Contact. See original ticket]

Tom Phoenix <rootbeer@redcat.com> writes:

On Sun, 14 Nov 1999, Andrew Pimlott wrote:

A coworker accidentally did:

{
local($/ = undef);
...
}

This did not localize $/ . I don't know what it did do.

It looks to me as if it did localize $/ -- but only _after_ storing undef
into it. Oops!

That is exactly what it says ;-)

There was no warning. Could I get a warning out of it?

Or, better yet, could perl simply Do The Right Thing?

I think it is doing the right thing.

--
Nick Ing-Simmons <nik@tiuk.ti.com>
Via, but not speaking for: Texas Instruments Ltd.

p5pRT · 1999-11-16T02:36:11Z

From [Unknown Contact. See original ticket]

From: Ronald J Kimball [mailto:rjk@linguist.dartmouth.edu]

It looks to me as if it did localize $/ -- but only _after_
storing undef into it. Oops!

That was my guess, too, but further tests don't seem to be
consistent with that conclusion:

Could it be that Perl is
1. Storing undef into $/
2. Looking at the local()
3. "Realising" that, as $/ is undefined, it is a new variable
and so localising is "unnecessary", therefore not doing it.

Ie, could it be an optimiser bug?

Paul.

p5pRT · 1999-11-16T05:59:50Z

From [Unknown Contact. See original ticket]

"Moore, Paul" <Paul.Moore@uk.origin-it.com> wrote

3. "Realising" that, as $/ is undefined, it is a new variable
and so localising is "unnecessary", therefore not doing it.

Ie, could it be an optimiser bug?

It certainly would be a bug if the optimiser did that. But experiment
shows that it doesn't confuse the concepts "undefined" and "uninitialised":

% perl -w
$x = 13;
{ undef $x;
local $x = undef;
$x = 12;
};
print $x;
__END__
Use of uninitialized value at - line 6.

Mike Guy

p5pRT · 1999-11-16T06:04:29Z

From [Unknown Contact. See original ticket]

Nick Ing-Simmons <nik@tiuk.ti.com> wrote

I think it is doing the right thing.

It has been pointed out that the construct apparently isn't localising
anything.

It can't be right for a local() not to localise anything *and* give
no warning.

And as another data point, I note that

local (undef);

is valid Perl. I suspect that '$/ = undef' is being evaluated as
'undef' and passed as argument to local().

Mike Guy

p5pRT · 1999-11-16T06:17:49Z

From [Unknown Contact. See original ticket]

Watch this:

Top x= 0 y=stone &x=080da578 &y=080da59c
Inside Before x=10 y=fred &x=080d3f8c &y=080d3e9c
Inside x=20 y=barney &x=080da548 &y=080d3e9c
Outside x=10 y=barney &x=080d3f8c &y=080d3e9c
Way Outside x= 0 y=stone &x=080da578 &y=080da59c

That's produced by the attached program. Notice that the
address of y is not altered inside, whereas that of x is.

--tom

$x = '0';
$y = "stone";

sub show {
printf "%-20s x=%2d y=%-6s &x=%08x &y=%08x\n",
shift, $x, $y, \$x, \$y;
}

show("Top");

{
local $x = 10;
local $y = "fred";

show("Inside Before");

{
local $x = 20;
local($y = "barney");
show("Inside");
}
show("Outside");
}

show("Way Outside");

p5pRT · 2000-12-13T08:33:46Z

From [Unknown Contact. See original ticket]

This bug appears to still be active. There is some discussion as to
whether it is a bug or not.

A coworker accidentally did:

{
local($/ = undef);
...
}

This did not localize $/ . I don't know what it did do. There was no
warning. Could I get a warning out of it?

-spp

p5pRT · 2001-03-06T08:18:43Z

From [Unknown Contact. See original ticket]

Another reminder that this bug is still active:

Watch this:

Top x= 0 y=stone &x=080da578 &y=080da59c
Inside Before x=10 y=fred &x=080d3f8c &y=080d3e9c
Inside x=20 y=barney &x=080da548 &y=080d3e9c
Outside x=10 y=barney &x=080d3f8c &y=080d3e9c
Way Outside x= 0 y=stone &x=080da578 &y=080da59c

That's produced by the attached program. Notice that the
address of y is not altered inside, whereas that of x is.

--tom

$x = '0';
$y = "stone";

sub show {
printf "%-20s x=%2d y=%-6s &x=%08x &y=%08x\n",
shift, $x, $y, \$x, \$y;
}

show("Top");

{
local $x = 10;
local $y = "fred";

show("Inside Before");

{
local $x = 20;
local($y = "barney");
show("Inside");
}
show("Outside");
}

show("Way Outside");

p5pRT · 2002-08-07T22:32:19Z

From @gbarr

This is still present in 5.8.0

p5pRT · 2003-05-26T19:54:29Z

From @iabyn

(Just reviewing old Perl bug reports).

In the latest development versions of Perl (which will eventually
be released as 5.8.1 and 5.10.0), the code now gives a warning:

$ ./perl -we 'local($/ = undef)'
Useless localization of scalar assignment at -e line 1.
$

This was added by patch #19588

Regards,

Dave M.

p5pRT · 2003-05-26T19:54:30Z

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

p5pRT closed this as completed May 26, 2003

p5pRT added Severity High distro-Linux type-library labels Oct 18, 2019

p5pRT mentioned this issue Oct 19, 2019

Program terminated with signal 11, Segmentation fault. #12469

Closed

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dangerous behavior from local #850

dangerous behavior from local #850

p5pRT commented Nov 14, 1999

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p5pRT commented May 26, 2003

p5pRT commented May 26, 2003

dangerous behavior from local #850

dangerous behavior from local #850

Comments

p5pRT commented Nov 14, 1999

p5pRT commented Nov 14, 1999

From pimlott@idiomtech.com

p5pRT commented Nov 15, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Nov 15, 1999

From @tamias

p5pRT commented Nov 15, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Nov 16, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Nov 16, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Nov 16, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Nov 16, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Nov 16, 1999

From [Unknown Contact. See original ticket]

p5pRT commented Dec 13, 2000

From [Unknown Contact. See original ticket]

p5pRT commented Mar 6, 2001

From [Unknown Contact. See original ticket]

p5pRT commented Aug 7, 2002

From @gbarr

p5pRT commented May 26, 2003

From @iabyn

p5pRT commented May 26, 2003