pthread sync

If you're keeping track of the thread id's, you might want to try pthread_join.

that is not what I'm looking for

It should be like an interrupt, each device, or thread get a slice of the 20 mils, so all threads must run once, during an interval of 20 milliseconds

and pthread_join, waits until the thread exits, my threads are running still.  so you see I'm looking for a scheduler.
sorry for the multiple comments.

Yes you can use semaphore (mutex) to synchronize the threads as you wanted.
Use pthread_mutex_init to create the mutex, mutex_lock and mutex_unlock to signal and wait for a critical section.

nahumd,
can you point me somewhere where there is an example or a how-to document, other than man pages?

/* This example is copied from */
/* http://www.coe.uncc.edu/~abw/parallel/pthreads/pthreads.html */
/* It illustrates the idea of shared memory programming */

/* Comments: the behavior of this program on Solaris 2.7 is quite different from that of Red Hat Linux 6.2. On Solaris, the program acted incorrectly while the outcome on Red Hat Linux 6.2 was as expected. X.M. */

#include <pthread.h>  
#include <stdio.h>
#define MAX 10
#define MAX_COUNT 15

void * increment(int *id);
void * watch(int *id);

int count =0;
pthread_mutex_t count_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t count_max = PTHREAD_COND_INITIALIZER;
int thread_id[3]  = {0,1,2};

main()
{
        int i;
        /* define the type to be pthread */
       
        pthread_t thread[3];
        /* create 3 threads*/
        pthread_create(&thread[2], NULL, (void *)watch, &thread_id[2]);
        pthread_create(&thread[0], NULL, (void *)increment, &thread_id[0]);
        pthread_create(&thread[1], NULL, (void *)increment, &thread_id[1]);
       
        for(i=0; i< 3 ; i++)
        {
                pthread_join(thread[i], NULL);
        }
}

void * watch(int *id)
{
        /* lock the variable */
        pthread_mutex_lock(&count_mutex);
        while(count <= MAX_COUNT)
        {
                /* using the condition variable for waiting for the event */
                pthread_cond_wait(&count_max, &count_mutex);
                printf("Inside the watch() and the value is %d\n", count);
          fflush(stdout);
        }
        /*unlock the variable*/
        pthread_mutex_unlock(&count_mutex);
}

void * increment(int *id)
{
     int i;
     for(i=0; i< MAX ; i++)
     {
          /* lock the variable */
          pthread_mutex_lock(&count_mutex);
          count++;
          printf("in increment counter by threadof id :%d, and count: %d\n",*id, count);
       fflush(stdout);
          /* for the condition notify the thread */
          pthread_cond_signal(&count_max);
          /*unlock the variable*/
          pthread_mutex_unlock(&count_mutex);
          sleep(rand()%2);
     }
}
This is give you an idea on how to use mutex