Getting Started with QNX Neutrino - QNX Software Systems

More on synchronization
© 2009, QNX Software Systems GmbH & Co. KG.
Sleepons versus condvars
#include
#include
pthread_mutex_t mutex_xy = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cv_xy = PTHREAD_COND_INITIALIZER;
int x, y;
int isprime (int);
thread1 ()
{
for (;;) {
pthread_mutex_lock (&mutex_xy);
while ((x > 7) && (y != 15)) {
pthread_cond_wait (&cv_xy, &mutex_xy);
}
// do something
pthread_mutex_unlock (&mutex_xy);
}
}
thread2 ()
{
for (;;) {
pthread_mutex_lock (&mutex_xy);
while (!isprime (x)) {
pthread_cond_wait (&cv_xy, &mutex_xy);
}
// do something
pthread_mutex_unlock (&mutex_xy);
}
}
thread3 ()
{
for (;;) {
pthread_mutex_lock (&mutex_xy);
while (x != y) {
pthread_cond_wait (&cv_xy, &mutex_xy);
}
// do something
pthread_mutex_unlock (&mutex_xy);
}
}
In these cases, waking up one thread isn’t going to cut it! We must wake up all three
threads and have each of them check to see if its predicate has been satisfied or not.
This nicely reflects the second case in our question above (“Why are these threads
waiting?”). Since the threads are all waiting on different conditions (thread1() is
waiting for x to be less than or equal to 7 or y to be 15, thread2() is waiting for x to be a
prime number, and thread3() is waiting for x to be equal to y), we have no choice but
to wake them all.
Sleepons have one principal advantage over condvars. Suppose that you want to
synchronize many objects. With condvars, you’d typically associate one condvar per
object. Therefore, if you had M objects, you’d most likely have M condvars. With
66 Chapter 1 • Processes and Threads April 30, 2009