Simple POSIX semaphore example.

semaphore.c

#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>

#include <stdio.h>

sem_t mutex;

void* thread(void* msg) {
  while(1) {
    sem_wait(&mutex);
    puts(msg);
    sem_post(&mutex);
    sleep(1);
  }
}

int main(void) {
  pthread_t t0, t1;
  sem_init(&mutex, 0, 1);
  pthread_create(&t0, NULL, thread, "hello");
  pthread_create(&t1, NULL, thread, "world");

  // Yes, I could have just created one thread.
  while(1);
}