Getting Started with QNX Neutrino - QNX Software Systems

More on synchronization
© 2009, QNX Software Systems GmbH & Co. KG.
producer (void *notused)
{
printf ("In producer thread...\n");
while (1) {
// get data from hardware
// we’ll simulate this with a sleep (1)
sleep (1);
printf ("producer: got data from h/w\n");
pthread_mutex_lock (&mutex);
while (data_ready) {
pthread_cond_wait (&condvar, &mutex);
}
data_ready = 1;
pthread_cond_signal (&condvar);
pthread_mutex_unlock (&mutex);
}
}
main ()
{
printf ("Starting consumer/producer example...\n");
// create the producer and consumer threads
pthread_create (NULL, NULL, producer, NULL);
pthread_create (NULL, NULL, consumer, NULL);
}
// let the threads run for a bit
sleep (20);
Signal versus broadcast
Pretty much identical to the sleepon example we just saw, with a few variations (we
also added some printf() functions and a main() so that the program would run!).
Right away, the first thing that we see is a new data type: pthread_cond_t. This is
simply the declaration of the condition variable; we’ve called ours condvar.
Next thing we notice is that the structure of the consumer is identical to that of the
consumer in the previous sleepon example. We’ve replaced the
pthread_sleepon_lock() and pthread_sleepon_unlock() with the standard mutex
versions (pthread_mutex_lock() and pthread_mutex_unlock()). The
pthread_sleepon_wait() was replaced with pthread_cond_wait(). The main difference
is that the sleepon library has a mutex buried deep within it, whereas when we use
condvars, we explicitly pass the mutex. We get a lot more flexibility this way.
Finally, we notice that we’ve got pthread_cond_signal() instead of
pthread_sleepon_signal() (again with the mutex passed explicitly).
In the sleepon section, we promised to talk about the difference between the
pthread_sleepon_signal() and pthread_sleepon_broadcast() functions. In the same
breath, we’ll talk about the difference between the two condvar functions
pthread_cond_signal() and pthread_cond_broadcast().
The short story is this: the “signal” version will wake up only one thread. So, if there
were multiple threads blocked in the “wait” function, and a thread did the “signal,”
then only one of the threads would wake up. Which one? The highest priority one. If
64 Chapter 1 • Processes and Threads April 30, 2009