problem with newSViv and changing int value

[Eric Kort]

Unfortunately, when I attempt to recreate the error I am about to describe in a tidy little function, I can't. But within the larger context of my image processing routine, I am encountering the following strangeness:

I am trying to update an element of an AV using av_store, and all is well if I do this:

int newval;
...
for(y = 1; y < iGridHeight - 1; y++) {
for(x = 1; x < iGridWidth - 1; x++) {
newval = 12;
av_store(avGrid, y * iGridWidth + x, newSViv(newval));
}
}

But if I try to increment newval, it dies after 10 iterations (and always after 10 iterations):

int newval=12;
...
for(y = 1; y < iGridHeight - 1; y++) {
for(x = 1; x < iGridWidth - 1; x++) {
newval++;
av_store(avGrid, y * iGridWidth + x, newSViv(newval));
}
}

With this kind of error:

*** glibc detected *** perl: munmap_chunk(): invalid pointer: 0x00007fff7f692410 ***
======= Backtrace: =========
/lib64/libc.so.6[0x2b472ba4739e]
...

Sadly, when I break this out into a little demo function, it runs fine:

void worksFine (SV* sv_ar)
{
AV* av_ar;
int i, newv=2;

av_ar = (AV*)SvRV(sv_ar);

for(i=0; i<1000000; i++) {
newv++;
av_store(av_ar, 1, newSViv(newv));
}
return;
}

Nevertheless, I wonder if you have any insights into what is going wrong (or does the situation dependency of my problem suggest I am doing something evil with my memory management elsewhere?)

Thanks,
Eric
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[Nicholas Clark]

Post by Eric Kort
*** glibc detected *** perl: munmap_chunk(): invalid pointer: 0x00007fff7f692410 ***

As you seem to be on Linux, have you tried using valgrind on the offending
program? The cause might actually be somewhere completely different, and
often it is helpful in finding it.

Nicholas Clark

[Eric Kort]

Sent: Tuesday, January 29, 2008 5:16 AM
To: Kort, Eric
Subject: Re: problem with newSViv and changing int value

Post by Eric Kort
Unfortunately, when I attempt to recreate the error I am about to
describe in a tidy little function, I can't. But within the larger
context of my image processing routine, I am encountering the
*** glibc detected *** perl: munmap_chunk(): invalid pointer: 0x00007fff7f692410 ***

As you seem to be on Linux, have you tried using valgrind on
the offending program? The cause might actually be somewhere
completely different, and often it is helpful in finding it.

I assumed it would be hard/impossible to use valgrind on XS code run by a perl script.

Turned out to be easy (valgrind --leak-check=yes --trace-children=yes perl test.pl)

And yes, the problem was memory mismanagement remote to the place where the problem finally reared its ugly head.

Thanks,
Eric

Nicholas Clark

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