// Compile: gcc -o semex1.exe -pthread semex1.c
#include <stdio.h>      
#include <stdlib.h>     
#include <pthread.h>    // Using Posix threads 
#include <semaphore.h>  // Using Posix semaphores

// prototype for thread function
void *handler ( void *ptr );

// global variables 
sem_t binsem;  //The semaphore will be used as a binary semaphore
int counter; /* shared variable */

int main()
{
    int i[2];
    pthread_t thread_a;
    pthread_t thread_b;
    
    i[0] = 0; /* argument to threads */
    i[1] = 1;
    
    sem_init(&binsem, 0, 1); // initialize semaphore to 1,  second param = 0 - semaphore is local */
                                 
    // should check that the thread has been successfully created by checking return value of
    // pthread_create()                                 
    pthread_create (&thread_a, NULL, handler, (void *) &i[0]);
    pthread_create (&thread_b, NULL, handler, (void *) &i[1]);
    
    // wait for all threads to finish before destroying the semaphore
    pthread_join(thread_a, NULL);
    pthread_join(thread_b, NULL);

    sem_destroy(&binsem); // destroy semaphore //
                  
    exit(0);
} 

//Function that each thread executes
void *handler ( void *ptr )
{
    int value, x; 
    x = *((int *) ptr);
    printf("Thread %d: Waiting to enter critical region...\n", x);
    sem_wait(&binsem);  //acquire the semaphore 
    /* START CRITICAL REGION */
    printf("Thread %d: Now in critical region...\n", x);
    sem_getvalue(&binsem, &value); 
    printf("The value of the semaphore is %d\n", value);
    printf("Thread %d: Counter Value: %d\n", x, counter);
    printf("Thread %d: Incrementing Counter...\n", x);
    counter++;
    printf("Thread %d: New Counter Value: %d\n", x, counter);
    printf("Thread %d: Exiting critical region...\n", x);
    /* END CRITICAL REGION */    
    sem_post(&binsem);       /* up semaphore */
    
    pthread_exit(NULL); /* exit thread */
}