Static GC_stackbottom 

pub unsafe static mut GC_stackbottom: *mut c_char

The cold end (bottom) of user stack. May be set in the client prior to calling any GC_ routines. This avoids some overhead, and potentially some signals that can confuse debuggers. Otherwise the collector attempts to set it automatically. For multi-threaded code, this is the cold end of the stack for the primordial thread. For multi-threaded code, altering GC_stackbottom value directly after the collector initialization has no effect. Portable clients should use GC_set_stackbottom(), GC_get_stack_base(), GC_call_with_gc_active() and GC_register_my_thread() instead.