GC Bug? Ruby 1.8.4 core dumps on a very small extension

[Danie Roux]

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Hi all,

Long (and tiring) story short:

I have an extension that is 40 lines* in total, the C code and Ruby
code that exercise it. It core dumps, while I repeatedly call a method
that just returns Qtrue.

All it really does is:

rb_iv_set(self, "@v", 100);

in initialize. If I change that 100 to 99, it doesn't core dump.
Change it back to 100, it core dumps within a few seconds.

Further, the while loop that executes it looks like this:

while true
print ''
b.buzz
end

If I take out the print statement, it keeps on running. Put back the
print statement, and it core dumps.

This is a self-compiled 1.8.4 on Ubuntu i386. A friend confirms that
the same thing happens to him on Gentoo with 1.8.4.

I am attaching the code, hopefully its going to come through.

* It was around 600, and use to core dump every 8 hours or so.
Snipping it down to these 40 lines was pure blind luck, and many 8
hour turnaround iterations.

--
Danie Roux *shuffle* Adore Unix http://blog.acolyte.co.za

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[Ross Bamford]

Hi all,

Long (and tiring) story short:

I have an extension that is 40 lines* in total, the C code and Ruby
code that exercise it. It core dumps, while I repeatedly call a method
that just returns Qtrue.

Please try making this change:

--- buzz-orig.c 2006-02-19 23:01:22.000000000 +0000
+++ buzz.c 2006-02-19 23:01:56.000000000 +0000
@@ -8,7 +8,7 @@

static VALUE initialize(VALUE self)
{
- rb_iv_set(self, "@v", 100);
+ rb_iv_set(self, "@v", INT2FIX(100));

return self;
}

 

[Danie Roux]

Please try making this change:
- rb_iv_set(self, "@v", 100);
+ rb_iv_set(self, "@v", INT2FIX(100));

Ross,

Thank you. Moral of the story: Explicitly cast any C value into a Ruby one.

It still strikes me odd that it would run a couple of hundred
iterations before breaking, though.

 

[Charles Mills]

Ross,

Thank you. Moral of the story: Explicitly cast any C value into a Ruby one.

It still strikes me odd that it would run a couple of hundred
iterations before breaking, though.

Assuming you're using a 32bit machine. 100 % 4 == 0, so Ruby thinks
100 is a pointer to a Ruby object. (Dereferencing 100 causes the core
dump.) 99 % 2 == 1, so 99 looks like a Fixnum to Ruby -- but not 99,
99>>1 I think.

It may take a couple hundred iterations before the GC runs.

-Charlie