Incomplete example of using pthreads condition signaling.

pthread_cond.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

pthread_t threads[3];

pthread_cond_t cond_a;
pthread_cond_t cond_b;

pthread_mutex_t mutex;

int counter;

void * worker_one();
void * worker_two();
void * worker_three();

int main(int argv, const char ** argc) {
    counter = 0;

    pthread_cond_init(&cond_a, NULL);
    pthread_cond_init(&cond_b, NULL);
    pthread_mutex_init(&mutex, NULL);
    
    pthread_create(&threads[0], NULL, worker_one, NULL);
    pthread_create(&threads[1], NULL, worker_two, NULL);
    pthread_create(&threads[2], NULL, worker_three, NULL);
    
    pthread_join(threads[0], NULL);
    pthread_join(threads[1], NULL);
    pthread_join(threads[2], NULL);

    printf("Value started at %d and ends with %d.\n", 0, counter);

    return 0;
}

void * worker_one() {
    printf("Worker one started.\n");
    
    pthread_mutex_lock(&mutex);
    
    printf("Worker one starting work.\n");

    while (counter < 10000) {
        counter += 7;
    }

    pthread_cond_signal(&cond_a);
       
    printf("Worker one finished work with: %d.\n", counter);

    pthread_mutex_unlock(&mutex);

    pthread_exit(NULL);
}

void * worker_two() {
    printf("Worker two started.\n");
    
    pthread_mutex_lock(&mutex);
    
    pthread_cond_wait(&cond_a, &mutex);
    
    printf("Worker two starting work.\n");

    while (counter > 1000)
        counter -= 3;

    printf("Worker two finished work with: %d.\n", counter);

    pthread_cond_signal(&cond_b);

    pthread_mutex_unlock(&mutex);

    sleep(1);

    pthread_exit(NULL);
}

void * worker_three() {
    printf("Worker three started.\n");
    
    pthread_mutex_lock(&mutex);

    pthread_cond_wait(&cond_b, &mutex);

    printf("Worker three starting work.\n");

    counter /= 2;

    printf("Worker three finished work with: %d.\n", counter);

    pthread_mutex_unlock(&mutex);

    pthread_exit(NULL);
}