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Id: 123292
Status: rejected
Priority: 0/
Queue: perl5

Owner: Nobody
Requestors: thospel <perl-bug [at] ton.iguana.be>
Cc:
AdminCc:

Operating System: Linux
PatchStatus: (no value)
Severity: Wishlist
Type: core
Perl Version: 5.20.1
Fixed In: (no value)



Subject: hash dereference of undefined value
From: perl-bug [...] ton.iguana.be
To: perlbug [...] perl.org
Date: Mon, 24 Nov 2014 14:22:11 +0100 (CET)
CC: perl-bug [...] ton.iguana.be
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This is a bug report for perl from perl-bug@ton.iguana.be, generated with the help of perlbug 1.40 running under perl 5.20.1. ----------------------------------------------------------------- [Please describe your issue here] perl -wle 'use strict; print if %$a; print $a' Can't use an undefined value as a HASH reference at -e line 1. perl -wle 'use strict; print if keys %$a; print $a' HASH(0x1da8e78) Until I stumbled into this I thought that both ways of testing if a HASH has elements were equivalent. I'd prefer it if that thought would become true. [Please do not change anything below this line] ----------------------------------------------------------------- --- Flags: category=core severity=wishlist --- Site configuration information for perl 5.20.1: Configured by Debian Project at Fri Oct 10 14:16:26 UTC 2014. Summary of my perl5 (revision 5 version 20 subversion 1) configuration: Platform: osname=linux, osvers=3.2.0-37-generic, archname=x86_64-linux-gnu-thread-multi uname='linux batsu 3.2.0-37-generic #58-ubuntu smp thu jan 24 15:28:10 utc 2013 x86_64 x86_64 x86_64 gnulinux ' config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.20 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.20 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.20 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.20.1 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.20.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.20.1 -des' hint=recommended, useposix=true, d_sigaction=define useithreads=define, usemultiplicity=define use64bitint=define, use64bitall=define, uselongdouble=undef usemymalloc=n, bincompat5005=undef Compiler: cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -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 -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include' ccversion='', gccversion='4.9.1', gccosandvers='' intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678 d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16 ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8 alignbytes=8, prototype=define Linker and Libraries: ld='cc', ldflags =' -fstack-protector -L/usr/local/lib' libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt perllibs=-ldl -lm -lpthread -lc -lcrypt libc=libc-2.19.so, so=so, useshrplib=true, libperl=libperl.so.5.20 gnulibc_version='2.19' Dynamic Linking: dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E' cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib -fstack-protector' Locally applied patches: DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN. DEBPKG:debian/db_file_ver - http://bugs.debian.org/340047 Remove overly restrictive DB_File version check. DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information. DEBPKG:debian/enc2xs_inc - http://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories. DEBPKG:debian/errno_ver - http://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes. DEBPKG:debian/libperl_embed_doc - http://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking DEBPKG:fixes/respect_umask - Respect umask during installation DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor DEBPKG:debian/prefix_changes - Fiddle with *PREFIX and variables written to the makefile DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets. DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor. DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy. DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable. DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian DEBPKG:debian/module_build_man_extensions - http://bugs.debian.org/479460 Adjust Module::Build manual page extensions for the Debian Perl policy DEBPKG:debian/prune_libs - http://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need. DEBPKG:fixes/net_smtp_docs - [rt.cpan.org #36038] http://bugs.debian.org/100195 Document the Net::SMTP 'Port' option DEBPKG:debian/perlivp - http://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local DEBPKG:debian/deprecate-with-apt - http://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules DEBPKG:debian/squelch-locale-warnings - http://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts DEBPKG:debian/skip-upstream-git-tests - Skip tests specific to the upstream Git repository DEBPKG:debian/patchlevel - http://bugs.debian.org/567489 List packaged patches for 5.20.1-1 in patchlevel.h DEBPKG:debian/skip-kfreebsd-crash - http://bugs.debian.org/628493 [perl #96272] Skip a crashing test case in t/op/threads.t on GNU/kFreeBSD DEBPKG:fixes/document_makemaker_ccflags - http://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags} DEBPKG:debian/find_html2text - http://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text DEBPKG:debian/perl5db-x-terminal-emulator.patch - http://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl DEBPKG:debian/cpan-missing-site-dirs - http://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] http://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected DEBPKG:debian/regen-skip - Skip a regeneration check in unrelated git repositories DEBPKG:fixes/regcomp-mips-optim - [perl #122817] http://bugs.debian.org/754054 Downgrade the optimization of regcomp.c on mips and mipsel due to a gcc-4.9 bug DEBPKG:debian/makemaker-pasthru - http://bugs.debian.org/758471 Pass LD settings through to subdirectories DEBPKG:fixes/perldoc-less-R - [rt.cpan.org #98636] http://bugs.debian.org/758689 Tell the 'less' pager to allow terminal escape sequences DEBPKG:fixes/hurd_test_todo_socket.t - [perl #122657] http://bugs.debian.org/758718 Disable failing GNU/Hurd test in t/io/socket.t DEBPKG:fixes/pod_man_reproducible_date - http://bugs.debian.org/759405 Support POD_MAN_DATE in Pod::Man for the left-hand footer DEBPKG:fixes/data_dump_infinite_recurse - [19be3be] don't recurse infinitely in Data::Dumper --- @INC for perl 5.20.1: /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.20.1 /usr/local/share/perl/5.20.1 /usr/lib/x86_64-linux-gnu/perl5/5.20 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.20 /usr/share/perl/5.20 /usr/local/lib/site_perl . --- Environment for perl 5.20.1: HOME=/home/ton LANG=en_US.UTF-8 LANGUAGE (unset) LD_LIBRARY_PATH (unset) LOGDIR (unset) PATH=/home/ton/bin:/home/ton/bin.SampleSetup:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:.:/home/ton/chromiumos/depot_tools:/home/ton/chromiumos/depot_tools PERL_BADLANG (unset) SHELL=/bin/bash
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On Mon Nov 24 05:22:38 2014, thospel wrote: Show quoted text
> > This is a bug report for perl from perl-bug@ton.iguana.be, > generated with the help of perlbug 1.40 running under perl 5.20.1. > > > ----------------------------------------------------------------- > [Please describe your issue here] > > perl -wle 'use strict; print if %$a; print $a' > Can't use an undefined value as a HASH reference at -e line 1. > > perl -wle 'use strict; print if keys %$a; print $a' > HASH(0x1da8e78) > > Until I stumbled into this I thought that both ways of testing if a > HASH has elements were equivalent. > I'd prefer it if that thought would become true. >
I don't think this is a bug in Perl, nor do I think that it should be approved as a feature request. In the first place, your example is somewhat confusing in that '$a' (along with '$b') is a special variable in Perl's sort function. If instead of '$a' you had used, say, '$c', you would have gotten this: ##### $ perl -wle 'use strict; print if keys %$c; print $c' Global symbol "$c" requires explicit package name at -e line 1. Global symbol "$c" requires explicit package name at -e line 1. Execution of -e aborted due to compilation errors. ##### Since strictures are not the real problem here, let's (for once) drop the 'use strict'. Now we get: ##### $ perl -wle 'print if keys %$c; print $c' HASH(0x905148) ##### But all this says is that at the conclusion of this statement a hash exists at that location in memory. It does *not* tell us if that hash has any elements in it or not. The hash has been autovivified by using the 'keys' function, but it doesn't have any elements. We can see that with this: ##### $ perl -wle 'print if keys %$c; print $c; while (($k,$v) = each(%$c)) { print "$k => $v" }' HASH(0x14e8148) ##### No elements in the hash. If your objective is to determine whether a hash has elements or not, you can simply say: ##### # Autovivify an empty hash: $ perl -wle 'keys %$c and print $c' # No output # Ensure that hash has at least 1 element: $ perl -wle '$c = { alpha => 42 }; keys %$c and print $c' HASH(0x107c148) ##### In a boolean context, 'keys %hash' is the way to determine whether a hash has elements or not. In contrast, '%$c' does not cause a hash to be autovivified. The warning you got is an accurate description of what is happening. Show quoted text
> [Please do not change anything below this line] > ----------------------------------------------------------------- > --- > Flags: > category=core > severity=wishlist > --- > Site configuration information for perl 5.20.1: > > Configured by Debian Project at Fri Oct 10 14:16:26 UTC 2014. > > Summary of my perl5 (revision 5 version 20 subversion 1) > configuration: > > Platform: > osname=linux, osvers=3.2.0-37-generic, archname=x86_64-linux-gnu- > thread-multi > uname='linux batsu 3.2.0-37-generic #58-ubuntu smp thu jan 24 > 15:28:10 utc 2013 x86_64 x86_64 x86_64 gnulinux ' > config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN > -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat > -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,- > z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro > -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr > -Dprivlib=/usr/share/perl/5.20 -Darchlib=/usr/lib/x86_64-linux- > gnu/perl/5.20 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 > -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.20 > -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.20.1 > -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.20.1 > -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 > -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 > -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible- > pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil > -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib > -Dlibperl=libperl.so.5.20.1 -des' > hint=recommended, useposix=true, d_sigaction=define > useithreads=define, usemultiplicity=define > use64bitint=define, use64bitall=define, uselongdouble=undef > usemymalloc=n, bincompat5005=undef > Compiler: > cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -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 -fwrapv -fno-strict- > aliasing -pipe -I/usr/local/include' > ccversion='', gccversion='4.9.1', gccosandvers='' > intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678 > d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16 > ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', > lseeksize=8 > alignbytes=8, prototype=define > Linker and Libraries: > ld='cc', ldflags =' -fstack-protector -L/usr/local/lib' > libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include- > fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu > /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib > libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt > perllibs=-ldl -lm -lpthread -lc -lcrypt > libc=libc-2.19.so, so=so, useshrplib=true, libperl=libperl.so.5.20 > gnulibc_version='2.19' > Dynamic Linking: > dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E' > cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib -fstack- > protector' > > Locally applied patches: > DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS > default for modules installed from CPAN. > DEBPKG:debian/db_file_ver - http://bugs.debian.org/340047 Remove > overly restrictive DB_File version check. > DEBPKG:debian/doc_info - Replace generic man(1) instructions with > Debian-specific information. > DEBPKG:debian/enc2xs_inc - http://bugs.debian.org/290336 Tweak > enc2xs to follow symlinks and ignore missing @INC directories. > DEBPKG:debian/errno_ver - http://bugs.debian.org/343351 Remove > Errno version check due to upgrade problems with long-running > processes. > DEBPKG:debian/libperl_embed_doc - http://bugs.debian.org/186778 > Note that libperl-dev package is required for embedded linking > DEBPKG:fixes/respect_umask - Respect umask during installation > DEBPKG:debian/writable_site_dirs - Set umask approproately for > site install directories > DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of > libperl.a under /usr/lib > DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or > perllocal.pod for perl or vendor > DEBPKG:debian/prefix_changes - Fiddle with *PREFIX and variables > written to the makefile > DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to > the binary targets. > DEBPKG:debian/instmodsh_doc - Debian policy doesn't install > .packlist files for core or vendor. > DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH > as per Debian policy. > DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to > /etc/perl/Net as /usr may not be writable. > DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian > DEBPKG:debian/module_build_man_extensions - > http://bugs.debian.org/479460 Adjust Module::Build manual page > extensions for the Debian Perl policy > DEBPKG:debian/prune_libs - http://bugs.debian.org/128355 Prune the > list of libraries wanted to what we actually need. > DEBPKG:fixes/net_smtp_docs - [rt.cpan.org #36038] > http://bugs.debian.org/100195 Document the Net::SMTP 'Port' option > DEBPKG:debian/perlivp - http://bugs.debian.org/510895 Make perlivp > skip include directories in /usr/local > DEBPKG:debian/deprecate-with-apt - http://bugs.debian.org/747628 > Point users to Debian packages of deprecated core modules > DEBPKG:debian/squelch-locale-warnings - > http://bugs.debian.org/508764 Squelch locale warnings in Debian > package maintainer scripts > DEBPKG:debian/skip-upstream-git-tests - Skip tests specific to the > upstream Git repository > DEBPKG:debian/patchlevel - http://bugs.debian.org/567489 List > packaged patches for 5.20.1-1 in patchlevel.h > DEBPKG:debian/skip-kfreebsd-crash - http://bugs.debian.org/628493 > [perl #96272] Skip a crashing test case in t/op/threads.t on > GNU/kFreeBSD > DEBPKG:fixes/document_makemaker_ccflags - > http://bugs.debian.org/628522 [rt.cpan.org #68613] Document that > CCFLAGS should include $Config{ccflags} > DEBPKG:debian/find_html2text - http://bugs.debian.org/640479 > Configure CPAN::Distribution with correct name of html2text > DEBPKG:debian/perl5db-x-terminal-emulator.patch - > http://bugs.debian.org/668490 Invoke x-terminal-emulator rather than > xterm in perl5db.pl > DEBPKG:debian/cpan-missing-site-dirs - > http://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with > nonexisting site dirs if a parent is writable > DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] > http://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' > option not respected > DEBPKG:debian/regen-skip - Skip a regeneration check in unrelated > git repositories > DEBPKG:fixes/regcomp-mips-optim - [perl #122817] > http://bugs.debian.org/754054 Downgrade the optimization of regcomp.c > on mips and mipsel due to a gcc-4.9 bug > DEBPKG:debian/makemaker-pasthru - http://bugs.debian.org/758471 > Pass LD settings through to subdirectories > DEBPKG:fixes/perldoc-less-R - [rt.cpan.org #98636] > http://bugs.debian.org/758689 Tell the 'less' pager to allow terminal > escape sequences > DEBPKG:fixes/hurd_test_todo_socket.t - [perl #122657] > http://bugs.debian.org/758718 Disable failing GNU/Hurd test in > t/io/socket.t > DEBPKG:fixes/pod_man_reproducible_date - > http://bugs.debian.org/759405 Support POD_MAN_DATE in Pod::Man for the > left-hand footer > DEBPKG:fixes/data_dump_infinite_recurse - [19be3be] don't recurse > infinitely in Data::Dumper > > --- > @INC for perl 5.20.1: > /etc/perl > /usr/local/lib/x86_64-linux-gnu/perl/5.20.1 > /usr/local/share/perl/5.20.1 > /usr/lib/x86_64-linux-gnu/perl5/5.20 > /usr/share/perl5 > /usr/lib/x86_64-linux-gnu/perl/5.20 > /usr/share/perl/5.20 > /usr/local/lib/site_perl > . > > --- > Environment for perl 5.20.1: > HOME=/home/ton > LANG=en_US.UTF-8 > LANGUAGE (unset) > LD_LIBRARY_PATH (unset) > LOGDIR (unset) > PATH=/home/ton/bin:/home/ton/bin.SampleSetup:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:.:/home/ton/chromiumos/depot_tools:/home/ton/chromiumos/depot_tools > PERL_BADLANG (unset) > SHELL=/bin/bash
-- James E Keenan (jkeenan@cpan.org)
Date: Tue, 25 Nov 2014 09:59:57 +0100
CC: Perl5 Porteros <perl5-porters [...] perl.org>
Subject: Re: [perl #123292] hash dereference of undefined value
From: demerphq <demerphq [...] gmail.com>
To: Perl RT Bug Tracker <perlbug-followup [...] perl.org>
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On 25 November 2014 at 04:12, James E Keenan via RT <perlbug-followup@perl.org> wrote:
Show quoted text
On Mon Nov 24 05:22:38 2014, thospel wrote:
>
> This is a bug report for perl from perl-bug@ton.iguana.be,
> generated with the help of perlbug 1.40 running under perl 5.20.1.
>
>
> -----------------------------------------------------------------
> [Please describe your issue here]
>
> perl -wle 'use strict; print if %$a; print $a'
> Can't use an undefined value as a HASH reference at -e line 1.
>
> perl -wle 'use strict; print if keys %$a; print $a'
> HASH(0x1da8e78)
>
> Until I stumbled into this I thought that both ways of testing if a
> HASH has elements were equivalent.
> I'd prefer it if that thought would become true.
>

I don't think this is a bug in Perl, nor do I think that it should be approved as a feature request.

I agree.

However not for the reasons you explained. I think Ton is getting confused by the minutae of autovivification of hashrefs, which IMO /is/ a gnarly subject. Basically telling when a dereference operation like %{$hash} is considered to be in LVALUE context, and as such supposed to autovivify, from an RVALUE access where it should die is harder than it should be, and in fact moderately inconsistent.

A raw %$hash where $hash is undef will not autovivify:

perl -wle'use strict; my $hash; my $k = %$hash'
Can't use an undefined value as a HASH reference at -e line 1.

However, when one hash-derefs an undef var in the *arguments* to a normal function it *is* autovivified:

perl -wle'use strict; sub P {print for @_}; my $hash; P(%$hash); print $hash'
HASH(0x1798998)

Which is why keys(%$hash) behaves differently than a simple (%$hash) dereference.

I am under the impression this is because the arguments to a sub call are treated as being in lvalue context due to the requirement of aliasing their contents. In particular this has to work:

perl -wle'use strict; sub P {$_[1]="P";}; my $hash={a=>0}; P(%$hash); print $hash->{a}'
P

Which if I understand things correctly is why %$hash is treated differently than P(%$hash).

So, this is either not-a-bug, or it is a can't-fix/won't-fix due to wide exploitation. On the other hand the behavior is inconsistent, as it does not apply to all built-ins, such as print():

perl -wle'use strict; my $hash; print %$hash'
Can't use an undefined value as a HASH reference at -e line 1.

Anyway, I think one could argue that the inconsistency is a bug somewhere, but the outcome of that would probably be best to fix the places where it doesn't happen, like print, and not the other way around. And I kinda wonder how much trouble we would cause if we made %$undef_var *not* autovivify, but also *not* die (assuming it is a reasonable thing to do implementation wise).

cheers,
Yves



 

Date: Sat, 16 Dec 2017 07:58:50 +0000
To: Perl5 Porteros <perl5-porters [...] perl.org>
Subject: Re: [perl #123292] hash dereference of undefined value
From: Zefram <zefram [...] fysh.org>
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We can't change default autovivification behaviour for backcompat reasons. This ticket should be closed. -zefram


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