Why does this code cause “EXC_BAD_INSTRUCTION”?

Question

dispatch_semaphore_t aSemaphore = dispatch_semaphore_create(1);        
dispatch_semaphore_wait(aSemaphore, DISPATCH_TIME_FOREVER);
dispatch_release(aSemaphore);
         


        

Answer 1

Somewhat more succinct answer: You are creating the semaphore with the wrong value, it should be zero. Creating it with a value of 1 means you are later releasing a semaphore that's still "in use" and GCD is deliberately generating an illegal instruction in order to help you debug the fact that you have a semaphore with more waiters on it.