Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. More on synchronization
Condition variables
• It’s the status flag between the consumer and the producer that indicates the state of
the system. If it’s set to a 1, it means that data is available for processing; if it’s set
to a 0, it means that no data is available, and the consumer should block.
• It serves as “the place where sleepon synchronization occurs.” More formally, the
address of data_ready is used as a unique identifier, that serves as the rendezvous
object for sleepon locks. We just as easily could have used “(void *) 12345”
instead of “&data_ready” — so long as the identifier is unique and used
consistently, the sleepon library really doesn’t care. Actually, using the address of a
variable in a process is a guaranteed way to generate a process-unique number —
after all, no two variables in a process will have the same address!
We’ll defer the discussion of “What’s the difference between
pthread_sleepon_signal() and pthread_sleepon_broadcast() ” to the discussion of
condition variables next.
Condition variables (or “condvars”) are remarkably similar to the sleepon locks we
just saw above. In fact, sleepon locks are built on top of condvars, which is why we
had a state of CONDVAR in the explanation table for the sleepon example. It bears
repeating that the pthread_cond_wait() function releases the mutex, waits, and then
reacquires the mutex, just like the pthread_sleepon_wait() function did.
Let’s skip the preliminaries and redo the example of the producer and consumer from
the sleepon section, using condvars instead. Then we’ll discuss the calls.
/*
* cp1.c
*/
#include
#include
int data_ready = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condvar = PTHREAD_COND_INITIALIZER;
void *
consumer (void *notused)
{
printf ("In consumer thread...\n");
while (1) {
pthread_mutex_lock (&mutex);
while (!data_ready) {
pthread_cond_wait (&condvar, &mutex);
}
// process data
printf ("consumer: got data from producer\n");
data_ready = 0;
pthread_cond_signal (&condvar);
pthread_mutex_unlock (&mutex);
}
}
void *
April 30, 2009 Chapter 1 • Processes and Threads 63