CS360 Final -- December 10, 2002.

CS360 Final -- December 10, 2002

Answers to Question 3

pthread_mutex_lock(mutex) works on a data structure called a mutex as follows. When the mutex is initialized, it is unlocked. When a thread calls pthread_mutex_lock(), if the mutex is unlocked, then it becomes locked, and pthread_mutex_lock() returns. If the mutex is locked, then pthread_mutex_lock() blocks until it is unlocked, and until pthread_mutex_lock() acquires the lock. Only one thread may acquire the lock at any one time.
pthread_cond_wait(cond, mutex) works on two data structures -- a lock and a condition variable. When pthread_cond_wait() is called, the calling thread must own the mutex. The thread then atomically blocks and releases the mutex. When another thread calls pthread_cond_signal(cond), one thread that is blocking on the condition variable is selected to be unblocked. That thread will return from the pthread_cond_wait() call when it can reacquire the mutex. Note -- that means that when a thread calls pthread_cond_wait(), it must wait for two events to occur before it may unblock: first, another thread must call pthread_cond_signal(), and then it must reacquire the mutex.
select(w, rfd, NULL, NULL, NULL): This usage of select() takes as input a collection of read-only file descriptors (specified by a bit-set rfd, and a size delimeter w). It returns when at least one of them has something ready to read -- meaning that read() will not block when called on that file descriptor. Rfd is modified to specify which of these file descriptors are ready, and the return value of select() specifies how many file descriptors are ready to read.

Grading: 8 Points

pthread_mutex_lock(mutex): 2 points. 1 point for what happens on a unlocked mutex, and 1 point for what happens on a locked mutex.
pthread_cond_wait(cond, mutex): 3 points. 1 point for blocking and releasing the mutex, 1 point for needing the signal call, 1 point for reacquiring the mutex.
select(w, rfd, NULL, NULL, NULL): 3 points: 1 point for specifying what the input is, 1 point for specifying when select returns, and 1 point for specifying the output.