Subj : Re: Condition variables & mutexes
To   : comp.programming.threads
From : Torsten Robitzki
Date : Tue Jan 25 2005 07:16 pm

Ian Pilcher wrote:

> I'm making my first attempt to use a condition variable, and I'm
> struggling with how exactly I'm supposed to use the associated mutex.
> 
> I finally got a simple example program to work (not hang):
> 
> #include <stdlib.h>
> #include <stdio.h>
> 
> #include <pthread.h>
> 
> static pthread_cond_t    cond = PTHREAD_COND_INITIALIZER;
> static pthread_mutex_t    mutex = PTHREAD_MUTEX_INITIALIZER;
> 
> static void *thread(void *arg)
> {
>     puts("thread: started");
>     pthread_mutex_lock(&mutex);
>     puts("thread: mutex locked");
>     pthread_cond_signal(&cond);
>     puts("thread: condition signalled");
>     pthread_mutex_unlock(&mutex);
>     puts("thread: mutex unlocked");
> 
>     return NULL;
> }
> 
> int main(void)
> {
>     pthread_t    tid;
> 
>     puts("main: started");
>     pthread_mutex_lock(&mutex);
>     puts("main: mutex locked");
>     pthread_create(&tid, NULL, thread, NULL);
>     puts("main: thread created");
>     pthread_cond_wait(&cond, &mutex);
>     puts("main: condition signalled");
>     pthread_join(tid, NULL);
>     puts("main: thread joined");
> 
>     return 0;
> }
> 
> Is this basically correct, or is my use of the mutex "overkill"?

As you don't use the mutex to prevent the threads from accessing shared 
data, it's probably "overkill" ;-). With a condition variable you can 
wait for a user defined predicate to become true. And you should wait 
for the predicate in a loop testing the predicate to become true. If you 
change the shared data you should do it with the mutex at hand and 
signal the condition variable if the predicate became true due to your 
change. This would result in something like this:

bool predicate = false;
static void *thread(void *arg)
{
     pthread_mutex_lock(&mutex);
     predicate = true;
     pthread_mutex_unlock(&mutex);

     pthread_cond_signal(&cond);

     return NULL;
}

int main(void)
{
     pthread_t    tid;

     pthread_create(&tid, NULL, thread, NULL);

     pthread_mutex_lock(&mutex);
     while ( !predicate )
       pthread_cond_wait(&cond, &mutex);
     pthread_mutex_unlock(&mutex);

     pthread_join(tid, NULL);
     return 0;
}

but it's hard to see, why you don't just join the thread.

regards
Torsten

.