Notes

/******************************************************************************
* FILE: condvar.c
* DESCRIPTION:
* Example code for using Pthreads condition variables. The main thread
* creates three threads. Two of those threads increment a "count" variable,
* while the third thread watches the value of "count". When "count"
* reaches a predefined limit, the waiting thread is signaled by one of the
* incrementing threads. The waiting thread "awakens" and then modifies
* count. The program continues until the incrementing threads reach
* TCOUNT. The main program prints the final value of count.
* SOURCE: Adapted from example code in "Pthreads Programming", B. Nichols
* et al. O'Reilly and Associates.
* LAST REVISED: 03/07/17 Blaise Barney
******************************************************************************/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NUM_THREADS 3
#define TCOUNT 10
#define COUNT_LIMIT 12

int count = 0;
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t)
{
int i;
long my_id = (long)t;

for (i=0; i < TCOUNT; i++) {
pthread_mutex_lock(&count_mutex);
count++;

/*
Check the value of count and signal waiting thread when condition is
reached. Note that this occurs while mutex is locked.
*/
if (count == COUNT_LIMIT) {
printf("inc_count(): thread %ld, count = %d Threshold reached. ",
my_id, count);
pthread_cond_signal(&count_threshold_cv);
printf("Just sent signal.n");
}
printf("inc_count(): thread %ld, count = %d, unlocking mutexn",
my_id, count);
pthread_mutex_unlock(&count_mutex);

/* Do some work so threads can alternate on mutex lock */
sleep(1);
}
pthread_exit(NULL);
}

void *watch_count(void *t)
{
long my_id = (long)t;

printf("Starting watch_count(): thread %ldn", my_id);

/*
Lock mutex and wait for signal. Note that the pthread_cond_wait routine
will automatically and atomically unlock mutex while it waits.
Also, note that if COUNT_LIMIT is reached before this routine is run by
the waiting thread, the loop will be skipped to prevent pthread_cond_wait
from never returning.
*/
pthread_mutex_lock(&count_mutex);
while (count < COUNT_LIMIT) {
printf("watch_count(): thread %ld Count= %d. Going into wait...n", my_id,count);
pthread_cond_wait(&count_threshold_cv, &count_mutex);
printf("watch_count(): thread %ld Condition signal received. Count= %dn", my_id,count);
}
printf("watch_count(): thread %ld Updating the value of count...n", my_id);
count += 125;
printf("watch_count(): thread %ld count now = %d.n", my_id, count);
printf("watch_count(): thread %ld Unlocking mutex.n", my_id);
pthread_mutex_unlock(&count_mutex);
pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
int i, rc;
long t1=1, t2=2, t3=3;
pthread_t threads[3];
pthread_attr_t attr;

/* Initialize mutex and condition variable objects */
pthread_mutex_init(&count_mutex, NULL);
pthread_cond_init (&count_threshold_cv, NULL);

/* For portability, explicitly create threads in a joinable state */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_create(&threads[0], &attr, watch_count, (void *)t1);
pthread_create(&threads[1], &attr, inc_count, (void *)t2);
pthread_create(&threads[2], &attr, inc_count, (void *)t3);

/* Wait for all threads to complete */
for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
printf ("Main(): Waited and joined with %d threads. Final value of count = %d. Done.n",
NUM_THREADS, count);

/* Clean up and exit */
pthread_attr_destroy(&attr);
pthread_mutex_destroy(&count_mutex);
pthread_cond_destroy(&count_threshold_cv);
pthread_exit (NULL);

}