in reply to Re^6: semi-panic: attempt to dup freed string?
in thread semi-panic: attempt to dup freed string?

Since you cannot change the ownership of an SV from one interpreter to another, this creates the SV in the receiver. 

#! perl -slw
use strict;

package O;
#use Inline qw( FORCE NOISY NOCLEAN );
use Inline C => <<'END_C', NAME => 'xso';

typedef struct {
    SV *sv;
} O;

void *mem( size_t size ) {
    void *p;
    Newx( p, size, char );
    return p;
}

SV *new( char *package ) {
    O *o = (O*)mem( sizeof( O ) );

    SV *oh = newSVuv( (UV)o );
    SV *rv = newRV( oh );

    o->sv = newSV(0);

    sv_bless( rv, gv_stashpv( package, 0 ) );
    SvREADONLY_on( oh );
    SvREADONLY_on( rv );
    printf( "Creating oh:%p o:%p o->sv:%p\n", oh, o, o->sv );
    return rv;
}

int set( SV *rv, SV *in ) {
    SV *oh = SvRV( rv );
    O *o = (O*)SvUV( oh );

    sv_setsv( o->sv, in );
    printf( "Setting oh:%p o:%p o->sv:%p\n", oh, o, o->sv );
    return 1;
}

SV *get( SV *rv ) {
    SV *oh = SvRV( rv );
    O *o = (O*)SvUV( oh );

    printf( "Getting from oh:%p o:%p o->sv:%p\n", oh, o, o->sv );
    return newSVsv( o->sv );
}

void DESTROY( SV *rv ) {
    SV *oh = SvRV( rv );
    O *o = (O*)SvUV( oh );

    printf( "Destroying oh:%p o:%p o->sv:%p\n", oh, o, o->sv );

    /* TODO: We can only destroy oh and o in the original.
    SvREFCNT_dec( o->sv );
    free(o);
    */
}

END_C

package main;
use threads;
use Devel::Peek;

my $o = O->new();

$o->set( "abcde" );
print $o->get();

print "\nthreaded\n";

async {
    $o->set( "12345" );
}->join;

#<>;
print $o->get();
#<>;

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Re^8: semi-panic: attempt to dup freed string?
by BrowserUk (Patriarch) on Mar 25, 2011 at 19:56 UTC

    Okay. Ignore the previous reply. The penny has dropped.

    I can't store a reference to an SV allocated by perl inside my struct, because when a thread ends, the referenced SV will be GC'd and I'll end up holding a reference to a freed scalar.

    Your fix is to allocate an SV yourself, and assign a reference to it into the struct. And the set() copies teh contents of the inbound SV, not assign a reference to it.

    The code in the DESTROY() method is there to clean up the SV you allocated, but the TODO is because you haven't found, (or have, but haven't yet implemented), a mechanism to decide when to free the struct and it's contents. I think I see a way to do that.

    I think this gives me what I need. For that, and your time, I thank you.

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

      Couldn't have said it better :)

      For the memory problem, a simple solution might be to have two classes. 

      my $buffer = Buffer->new();     # Creates a buffer and blessed SvUV.

my $shared = $buffer->share();  # Creates a blessed SvUV.
async {
   $shared->set(12345);
}->join();
undef $shared;  # Does nothing.

say $buffer->get();
undef $buffer;  # Frees the buffer.

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[d/l]
Re^8: semi-panic: attempt to dup freed string?
by BrowserUk (Patriarch) on Mar 25, 2011 at 19:30 UTC
    this creates the SV in the receiver.

    Sorry, but could you point out which piece(s) of code are doing that? And what you mean by "in the reciever"?

    Also, is the code in the DESTROY method doing anything useful?

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

      By receiver, I meant the Perl interpreter calling get.

      The code (o->sv = newSV(0);) is in new since new is called in the same Perl interpreter as get.

      My DESTROY doesn't do anything useful, but it needs to free the memory new allocated.

[reply]
[d/l]
[select]

        This doesn't detract from the fact that what you've given me works! But I'm still trying to tie down exactly why it works.

        The code (o->sv = newSV(0);) is in new since new is called in the same Perl interpreter as get.

        I'm calling get() in two threads, but new() only in one?

        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

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