Re: mysterious memory corruption, very confused

On Jul 6, 2008, at 7:15 PM, Nobuyoshi Nakada wrote:

Hi,

At Mon, 30 Jun 2008 15:47:19 +0900,
Seebs wrote in [ruby-talk:306636]:
It is. There's a loop of
VALUE x;
char **foo = malloc(buncha char *);

for (big list of things) {
x = rb_obj_as_string(y);
foo[i] = GetStringValue(x);
}

It's your bug.

The idiom of using rb_obj_as_string, and then using the value, is
common in
the Ruby source. It works. ... It works *as long as you don't
allocate
anything more before you're done with it*. What ends up happening
is that,
if enough of the objects in question need a new string allocated by
rb_obj_as_string, sooner or later you end up invoking the garbage
collector.
Now, since there's only one x, the garbage collector assumes the
current
rb_obj_as_string() return is in use, *and the others aren't*. So
it might,
if it wants the space, free one... And then the memory gets reused.

Because you drop the references to the created objects. You
have to keep the objects but not only the pointers.

I submitted a more detailed bug report to the ruby-pg project, and
I've
adopted a workaround (possibly very inefficient) involving an array
of VALUE
objects and rb_gc_{un}register_address. It's ugly but it
eliminates the bug.

VALUE x, array;
char **foo = malloc(buncha char *);

for (big list of things) {
x = rb_obj_as_string(y);
rb_ary_push(array, x);
foo[i] = GetStringValue(x);
}

By keeping the values in an automatic variable `array', they
are marked and won't be freed.


I am wondering why the strings (returned from rb_obj_as_string) will
be garbage collected but the array will not be garbage collected? Both
have the same local scope, and they are not referenced by any other
ruby object.

Please explain when you have time.

Blessings,
TwP

.

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