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Attempt to free unreferenced scalar

by tqisjim (Beadle)
on Mar 15, 2013 at 20:27 UTC ( #1023770=perlquestion: print w/ replies, xml ) Need Help??
tqisjim has asked for the wisdom of the Perl Monks concerning the following question:

Same as before. Don't know if it's best to open a new thread. But here goes:

Cpantesters reports that my distro NoSQL::PL2SQL is failing in v5.10 and v5.12.

The errors on CPAN are probably due to the failure that tobyink documented earlier, Attempt to free unreferenced scalar in line 53.

Here's the snipped of code:

for my $v ( values %{ $self->{globals}->{memory} } ) { my $item = NoSQL::PL2SQL::Object::item( $v->[0] ) ; delete $item->[1]->{clone} if $item && $item->[1] ; # +# ERROR IS HERE }

This code is not so straightforward. Line 399 of the same file looks like:

Scalar::Util::weaken( $self->{globals}->{memory}->{$k}->[0] ) ;

So the scalar in question is unreferenced by design.

It's tempting to assume that the error's origin is a bug that was eventually fixed by v5.14. So along with any technical responses from the Monks, it might also be helpful to determine whether I should exclude those versions from my distro. I don't think I have enough knowledge of Perl's internals to consider a workaround.

Thanks again!

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Re: Attempt to free unreferenced scalar
by Khen1950fx (Canon) on Mar 16, 2013 at 17:28 UTC
    FWIW, I tested your module with 5.9, 5.10x, 5.12.0, and 5.16.2---all tests passed.
Re: Attempt to free unreferenced scalar
by ikegami (Pope) on Mar 16, 2013 at 18:55 UTC

    Cpantesters reports that my distro NoSQL::PL2SQL is failing in v5.10 and v5.12.

    Clearly

    The errors on CPAN are probably due to the failure that tobyink documented earlier, Attempt to free unreferenced scalar in line 53.

    Yeah, every failing 5.10 and 5.12 report I checked had this.

    "Attempt to free unreferenced scalar" is caused by over-decrementing a refcount.

    use strict; use warnings; use Inline C => <<'__EOI__'; void testing() { SV* sv = newSV(0); SvREFCNT_dec(sv); /* sv is freed */ SvREFCNT_dec(sv); /* Attempt to free unreferenced scalar */ } __EOI__ testing();

    But I've looked at the extent of your use of XS... It's obviously not a bug in your module.

    So it's either a bug in Perl or a bug in a dependency (with an XS component).

    Do you have a minimal test case?

      I spent about 1/2 hour trying to build a minimal test case. So far, I have not successfully isolated the problem.

      I made a few discoveries, though. According to perldiag, this message is considered a warning. True enough, even though the test script ultimately fails, it continues to run after displaying this warning message.

      Approximately 20% of the time when I run the test script, I get the following output:

      PERL_DL_NONLAZY=1 /home/jim/perl5.12/usr/bin/perl "-MExtUtils::Command +::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t t/NoSQL-PL2SQL.t .. ok t/sqlite.t ........ 25/59 Attempt to free unreferenced scalar: SV 0x8e +3b058 at /home/jim/.cpan/build/NoSQL-PL2SQL-1.20-5swcte/blib/lib/NoSQ +L/PL2SQL/Object.pm line 53, <H> chunk 1. *** glibc detected *** /home/jim/perl5.12/usr/bin/perl: free(): invali +d pointer: 0x08e3c8a0 *** ======= Backtrace: ========= /lib/i386-linux-gnu/libc.so.6(+0x75ee2)[0x40110ee2] /home/jim/perl5.12/usr/bin/perl(Perl_mg_free+0x52)[0x80b5d22] /home/jim/perl5.12/usr/bin/perl(Perl_sv_clear+0x583)[0x80cfc43] /home/jim/perl5.12/usr/bin/perl(Perl_sv_free2+0x36)[0x80d0036] /home/jim/perl5.12/usr/bin/perl(Perl_sv_clear+0x648)[0x80cfd08] /home/jim/perl5.12/usr/bin/perl(Perl_sv_free2+0x36)[0x80d0036] /home/jim/perl5.12/usr/bin/perl(Perl_av_clear+0xd8)[0x80c13d8] /home/jim/perl5.12/usr/bin/perl(Perl_pp_aassign+0x1fb)[0x80c581b] /home/jim/perl5.12/usr/bin/perl(Perl_runops_standard+0x1d)[0x80c2c2d] /home/jim/perl5.12/usr/bin/perl(perl_run+0x2af)[0x80771cf] /home/jim/perl5.12/usr/bin/perl(main+0x10d)[0x805f79d] /lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0x400b44d3] /home/jim/perl5.12/usr/bin/perl[0x805f7d1] ======= Memory map: ======== 08048000-0816f000 r-xp 00000000 08:06 12070401 /home/jim/perl5.12/us +r/bin/perl 0816f000-08170000 r--p 00126000 08:06 12070401 /home/jim/perl5.12/us +r/bin/perl 08170000-08172000 rw-p 00127000 08:06 12070401 /home/jim/perl5.12/us +r/bin/perl 08172000-08173000 rw-p 00000000 00:00 0 088cd000-08ee6000 rw-p 00000000 00:00 0 [heap] 40000000-40020000 r-xp 00000000 08:06 919427 /lib/i386-linux-gnu/l +d-2.15.so 40020000-40021000 r--p 0001f000 08:06 919427 /lib/i386-linux-gnu/l +d-2.15.so 40021000-40022000 rw-p 00020000 08:06 919427 /lib/i386-linux-gnu/l +d-2.15.so 40022000-40023000 r-xp 00000000 00:00 0 [vdso] 40023000-40025000 rw-p 00000000 00:00 0 40025000-40026000 r--p 005e0000 08:06 12592234 /usr/lib/locale/local +e-archive 40026000-4002b000 r-xp 00000000 08:06 3680652 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/List/Util/Util.so 4002b000-4002c000 r--p 00004000 08:06 3680652 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/List/Util/Util.so 4002c000-4002d000 rw-p 00005000 08:06 3680652 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/List/Util/Util.so 4002d000-40031000 r-xp 00000000 08:06 3811631 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/IO/IO.so 40031000-40032000 r--p 00003000 08:06 3811631 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/IO/IO.so 40032000-40033000 rw-p 00004000 08:06 3811631 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/IO/IO.so 40033000-40035000 r-xp 00000000 08:06 3680638 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Fcntl/Fcntl.so 40035000-40036000 ---p 00002000 08:06 3680638 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Fcntl/Fcntl.so 40036000-40037000 r--p 00002000 08:06 3680638 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Fcntl/Fcntl.so 40037000-40038000 rw-p 00003000 08:06 3680638 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Fcntl/Fcntl.so 40038000-4003b000 r-xp 00000000 08:06 920376 /lib/i386-linux-gnu/l +ibdl-2.15.so 4003b000-4003c000 r--p 00002000 08:06 920376 /lib/i386-linux-gnu/l +ibdl-2.15.so 4003c000-4003d000 rw-p 00003000 08:06 920376 /lib/i386-linux-gnu/l +ibdl-2.15.so 4003d000-40067000 r-xp 00000000 08:06 919426 /lib/i386-linux-gnu/l +ibm-2.15.so 40067000-40068000 r--p 00029000 08:06 919426 /lib/i386-linux-gnu/l +ibm-2.15.so 40068000-40069000 rw-p 0002a000 08:06 919426 /lib/i386-linux-gnu/l +ibm-2.15.so 40069000-4006a000 rw-p 00000000 00:00 0 4006a000-40072000 r-xp 00000000 08:06 919416 /lib/i386-linux-gnu/l +ibcrypt-2.15.so 40072000-40073000 r--p 00007000 08:06 919416 /lib/i386-linux-gnu/l +ibcrypt-2.15.so 40073000-40074000 rw-p 00008000 08:06 919416 /lib/i386-linux-gnu/l +ibcrypt-2.15.so 40074000-4009b000 rw-p 00000000 00:00 0 4009b000-4023e000 r-xp 00000000 08:06 919415 /lib/i386-linux-gnu/l +ibc-2.15.so 4023e000-4023f000 ---p 001a3000 08:06 919415 /lib/i386-linux-gnu/l +ibc-2.15.so 4023f000-40241000 r--p 001a3000 08:06 919415 /lib/i386-linux-gnu/l +ibc-2.15.so 40241000-40242000 rw-p 001a5000 08:06 919415 /lib/i386-linux-gnu/l +ibc-2.15.so 40242000-40246000 rw-p 00000000 00:00 0 40246000-40446000 r--p 00000000 08:06 12592234 /usr/lib/locale/local +e-archive 40446000-40455000 r-xp 00000000 08:06 410938 /home/jim/perl5/lib/p +erl5/i686-linux/auto/XML/Parser/Expat/Expat.so 40455000-40456000 r--p 0000f000 08:06 410938 /home/jim/perl5/lib/p +erl5/i686-linux/auto/XML/Parser/Expat/Expat.so 40456000-40457000 rw-p 00010000 08:06 410938 /home/jim/perl5/lib/p +erl5/i686-linux/auto/XML/Parser/Expat/Expat.so 40457000-4047d000 r-xp 00000000 08:06 920205 /lib/i386-linux-gnu/l +ibexpat.so.1.5.2 4047d000-4047e000 ---p 00026000 08:06 920205 /lib/i386-linux-gnu/l +ibexpat.so.1.5.2 4047e000-40480000 r--p 00026000 08:06 920205 /lib/i386-linux-gnu/l +ibexpat.so.1.5.2 40480000-40481000 rw-p 00028000 08:06 920205 /lib/i386-linux-gnu/l +ibexpat.so.1.5.2 40481000-40485000 r-xp 00000000 08:06 3811529 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/File/Glob/Glob.so 40485000-40486000 r--p 00003000 08:06 3811529 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/File/Glob/Glob.so 40486000-40487000 rw-p 00004000 08:06 3811529 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/File/Glob/Glob.so 40487000-404a5000 r-xp 00000000 08:06 3811824 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Compress/Raw/Zlib/Zlib.so 404a5000-404a6000 r--p 0001d000 08:06 3811824 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Compress/Raw/Zlib/Zlib.so 404a6000-404a7000 rw-p 0001e000 08:06 3811824 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Compress/Raw/Zlib/Zlib.so 404a7000-404a9000 r-xp 00000000 08:06 12583135 /home/jim/.cpan/build +/NoSQL-PL2SQL-1.20-5swcte/blib/arch/auto/NoSQL/PL2SQL/PL2SQL.so 404a9000-404aa000 r--p 00001000 08:06 12583135 /home/jim/.cpan/build +/NoSQL-PL2SQL-1.20-5swcte/blib/arch/auto/NoSQL/PL2SQL/PL2SQL.so 404aa000-404ab000 rw-p 00002000 08:06 12583135 /home/jim/.cpan/build +/NoSQL-PL2SQL-1.20-5swcte/blib/arch/auto/NoSQL/PL2SQL/PL2SQL.so 404ab000-404c8000 r-xp 00000000 08:06 13501138 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBI/DBI.so 404c8000-404c9000 r--p 0001c000 08:06 13501138 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBI/DBI.so 404c9000-404ca000 rw-p 0001d000 08:06 13501138 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBI/DBI.so 404ca000-404db000 r-xp 00000000 08:06 3680672 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Storable/Storable.so 404db000-404dc000 r--p 00010000 08:06 3680672 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Storable/Storable.so 404dc000-404dd000 rw-p 00011000 08:06 3680672 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Storable/Storable.so 404dd000-404e1000 r-xp 00000000 08:06 3811512 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Digest/MD5/MD5.so 404e1000-404e2000 r--p 00003000 08:06 3811512 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Digest/MD5/MD5.so 404e2000-404e3000 rw-p 00004000 08:06 3811512 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Digest/MD5/MD5.so 404e3000-404ea000 r-xp 00000000 08:06 3680599 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Data/Dumper/Dumper.so 404ea000-404eb000 r--p 00007000 08:06 3680599 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Data/Dumper/Dumper.so 404eb000-404ec000 rw-p 00008000 08:06 3680599 /usr/local/lib/perl5/ +5.12.5/i686-linux/auto/Data/Dumper/Dumper.so 404ec000-405a9000 r-xp 00000000 08:06 411295 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBD/SQLite/SQLite.so 405a9000-405aa000 ---p 000bd000 08:06 411295 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBD/SQLite/SQLite.so 405aa000-405ab000 r--p 000bd000 08:06 411295 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBD/SQLite/SQLite.so 405ab000-405ac000 rw-p 000be000 08:06 411295 /home/jim/perl5/lib/p +erl5/i686-linux/auto/DBD/SQLite/SQLite.so 405ac000-405ae000 rw-p 00000000 00:00 0 405c1000-405dd000 r-xp 00000000 08:06 920239 /lib/i386-linux-gnu/l +ibgcc_s.so.1 405dd000-405de000 r--p 0001b000 08:06 920239 /lib/i386-linux-gnu/l +ibgcc_s.so.1 405de000-405df000 rw-p 0001c000 08:06 920239 /lib/i386-linux-gnu/l +ibgcc_s.so.1 bfe45000-bfe66000 rw-p 00000000 00:00 0 [stack] t/sqlite.t ........ Failed 15/59 subtests Test Summary Report ------------------- t/sqlite.t (Wstat: 134 Tests: 44 Failed: 0) Non-zero wait status: 134 Parse errors: Bad plan. You planned 59 tests but ran 44. Files=2, Tests=50, 1 wallclock secs ( 0.07 usr 0.01 sys + 0.81 cusr + 0.04 csys = 0.93 CPU) Result: FAIL Failed 1/2 test programs. 0/50 subtests failed. make: *** [test_dynamic] Error 255

      My current theory is that the free() call is failing even when I don't see any output. Is that assumption reasonable?

      Mind you, the code in question is called during destruction. The code is intended to process unreferenced scalars that have already been destroyed. In Perl versions where this distro is successful, these scalars have an undef value that is easily reassigned.

      In the case of errors and warnings, the warning seems to be initiated as soon as the scalar is assigned with another reference.

      Hopefully, with more poking and prodding, I'll have more observations to share soon.

      Thanks!

      Jim

        According to perldiag, this message is considered a warning.

        You shouldn't be dismissing the severity. The code is accessing a freed memory!

        My current theory is that the free() call is failing even when I don't see any output. Is that assumption reasonable?

        Accessing freed memory can lead to a wide variety of behaviour. It's a waste of time looking at this when you already know what leads to this memory corruption (accessing a freed scalar).

        So far, I have not successfully isolated the problem.

        An alternate approach would be to find the commit that fixed the problem. This might give a hint how to work around the problem in the versions before it was fixed.

        There are tools to bisect history to find what a problem started occurring, or in this case, stopped occurring. I don't know how easy it would be to automatically install modules as part of the process.

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