conditional variable api using semaphores!

conditional_variable.c

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

typedef struct {
	sem_t wait_sem;
	sem_t signal_sem;
	int value;
} cond_var_t;

void cond_var_init (cond_var_t *cv)
{
  sem_init (&cv->wait_sem, 0, 0);
  sem_init (&cv->signal_sem, 0, 0);
  cv->value = 0;
}

void cond_var_wait (cond_var_t *cv)
{
  // Increment the value of the CV, which will allow a waiting thread to proceed
  cv->value++;
  sem_post (&cv->wait_sem);

  // Wait until the CV is signaled
  sem_wait (&cv->signal_sem);

  // Decrement the value of the CV
  cv->value--;
}

void cond_var_signal (cond_var_t *cv)
{
  // If there is a waiting thread, signal it
  if (cv->value > 0)
  {
	sem_wait (&cv->wait_sem);
	sem_post (&cv->signal_sem);
  }
}

void *thread_func (void *arg)
{
  cond_var_t *cv = (cond_var_t *) arg;
  printf ("Thread waiting...\n");
  cond_var_wait (cv);
  printf ("Thread signaled!\n");
  return NULL;
}

int main ()
{
  pthread_t thread;
  cond_var_t cv;

  // Initialize the conditional variable
  cond_var_init (&cv);

  // Start the thread
  pthread_create (&thread, NULL, thread_func, &cv);

  // Wait for a bit to simulate some work being done
  printf ("Main thread sleeping...\n");
  sleep (2);

  // Signal the thread to continue
  printf ("Signaling thread...\n");
  cond_var_signal (&cv);

  // Wait for the thread to finish
  pthread_join (thread, NULL);

  return 0;
}