Subj : Re: Condition variables & mutexes
To   : comp.programming.threads
From : Ian Pilcher
Date : Wed Jan 26 2005 08:53 am

Torsten Robitzki wrote:
> 
> 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:
> 

Thank you so much for the example.  Here is my new (working) code:

#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 volatile int 	thread_is_ready = 0;

static void *thread(void *arg)
{
     pthread_mutex_lock(&mutex);
     thread_is_ready = 1;
     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 (!thread_is_ready)
      	pthread_cond_wait(&cond, &mutex);
     pthread_mutex_unlock(&mutex);
     return 0;
}

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

Well, it's just an example.  I'm trying to roll my own pthread-safe
version of alarm().  My pt_alarm_init() function creates a new thread,
and that thread needs to do a little bit of initialization before
pt_alarm_init() returns.  My plan is to use a condition variable to
achieve this.

Is there a better way?


Thanks!

-- 
========================================================================
Ian Pilcher                                        i.pilcher@comcast.net
========================================================================

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