Create two threads t1 and t2. t1 and t2 share an integer data counter. the job of t1 is to increase the value of counter by 1 when t1 is scheduled. the job of t2 is to decrease the value of counter by 1 when t2 is scheduled. because counter is a critical section, you need to use semaphore to implement mutual exclusion between t1 and t2. here is the api you may use: sem_init: initialize an unnamed semaphore sem_wait: lock a semaphore, which is equivalent to the p operation. sem_post: unlock a semaphore, which is equivalent to the v operation. sem_destroy: destroy an unnamed semaphore. here is the basic structure of the program: int counter; //shared between t1 and t2