main.m

#import <objc/runtime.h>
#import <objc/objc-arc.h>
#include <pthread.h>

/* ----- Mimimal Class ----- */
__attribute__((objc_root_class))
@interface Test { id isa; }
+ (Class)class;
+ (id)new;
- (void)dealloc;
- (id)autorelease;
- (id)retain;
- (void)release;
@end

@implementation Test
+ (Class)class        { return self; }
+ (id)new             { return class_createInstance(self, 0); }
- (void)dealloc       { object_dispose(self); }
- (id)autorelease     { return objc_autorelease(self); }
- (id)retain          { return objc_retain(self); }
- (void)release       { objc_release(self); }
- (void)_ARCCompliantRetainRelease {}
@end

/* ----- Test Program ----- */

id ptr = NULL; // Will hold the weak reference.

void *thread_func(void *x_void_ptr) {
    id obj = objc_loadWeakRetained(&ptr);
    return NULL;
}

int main(void) {
    Test *test = [Test new];
    objc_storeWeak(&ptr, test);
    
    pthread_t the_thread;
    if (pthread_create(&the_thread, NULL, thread_func, NULL)) {
        return 1;
    }
    
    [test release];
    test = NULL;
    
    if (pthread_join(the_thread, NULL)) {
        return 2;
    }
    
    return 0;
}