Allocate two objects simultaneously in multi-threading

I’m studying Ruby 1.9.2 source code. Some issues about simultaneity. will call following funtion to allocate space from heap:
static VALUE rb_class_allocate_instance(VALUE klass)
NEWOBJ(obj, struct RObject);
OBJSETUP(obj, klass, T_OBJECT);
return (VALUE)obj;

#define NEWOBJ(obj,type) type obj = (type)rb_newobj()

if (during_gc) {
dont_gc = 1;
during_gc = 0;
rb_bug(“object allocation during garbage collection phase”);

return rb_newobj_from_heap(objspace);


static VALUE
rb_newobj_from_heap(rb_objspace_t *objspace)
VALUE obj;

if ((ruby_gc_stress && !ruby_disable_gc_stress) || !freelist) {

if (!heaps_increment(objspace) && !garbage_collect(objspace)) {
during_gc = 0;

obj = (VALUE)freelist;
freelist = freelist->;

MEMZERO((void*)obj, RVALUE, 1);

return obj;


I doesn’t see any code(in rb_newobj_from_heap), which is to make sure
only one thread can call the function rb_newobj at the same time in
The issue found in many other places. It seems that there can be only
one thread in Ruby. But class Thread can create another thread.

x = {
for a in 1…999
for b in 1…999

Please tell me if some other mechanism solve the simultaneity issue.
Thanks Vince

Hi Vince,

good point, but at the moment this is not an issue, thanks to the
Global Interpreter Lock: only one Ruby thread is ever executed at the
same time. While this is not a great situation, it will at least make
sure that such a problem will never occur.