From: Simon Strandgaard <0bz63fz3m1qt3001@...>
Date: 2003-07-02T07:52:17+09:00
Subject: rb_gc() and scan stack

I were experimenting with Init_stack, when I discovered a flaw in my mind.

I thought that: local variables on stack would be marked in the
mark-phase and therefore survive the sweep-phase.

But my instance is being destroyed.. why ?

Can someone enligthen me ?


> expand -t4 rb_gc1.c
#include <ruby.h>

void Dummy_Free(void *p) {
    printf("free\n");
    delete [] ((int*)p);
}

VALUE Dummy_Alloc(VALUE self) {
    printf("alloc\n");
    int *p = new int[1];
    return Data_Wrap_Struct(self, 0, Dummy_Free, p);
}

VALUE Dummy_Initialize(VALUE self) {
    int *p;
    printf("initialize\n");
    Data_Get_Struct(self, int, p);
    return self;
}

void Init_dummy() {
    VALUE h = rb_define_class("Dummy", rb_cObject);
    rb_define_alloc_func(h, Dummy_Alloc);
    typedef VALUE (*HOOK)(...);
    rb_define_method(h, "initialize", (HOOK)(&Dummy_Initialize), 0);
}

VALUE tst_new_dummy() {
    return rb_class_new_instance(0, 0, rb_path2class("Dummy"));
}

void test() {
    VALUE instance = tst_new_dummy();
    printf("1\n");
    rb_p(instance);
    // the instance SHOULD survive this step (1_2)
    rb_gc();
    printf("2\n");
    rb_p(instance);
    // the instance should NOT survive this step (2_3)
    instance = Qnil;
    rb_gc();
    printf("3\n");
    rb_p(instance);
}

int main(int argc, char **argv) {
    ruby_init();
    Init_dummy();
    test();
    return 0;
}
>


> ./a.out
alloc
initialize
1
#<Dummy:0x8103b38>
free
2
ruby: [BUG] Segmentation fault
ruby 1.8.0 (2003-06-23) [i386-freebsd5.0]

Abort (core dumped)
>


As you can see the 'free' is occuring between '1' and '2'.

I thought 'free' was suppose to happen between '2' and '3' ?
Expecting output should look like this:

alloc
initialize
1
#<Dummy:0x8103b38>
2
#<Dummy:0x8103b38>
free
3
ruby: [BUG] Segmentation fault
ruby 1.8.0 (2003-06-23) [i386-freebsd5.0]

Abort (core dumped)


--
Simon Strandgaard