The use of semaphores to solve the critical region problem

int sem_init(sem_t *sem, int pshared, unsigned int value);
The sem_init() function is used to initialize the semaphore's value. The pshared argument must be 0 for semaphores local to a
process. int sem_wait(sem_t * sem);

The sem_wait() function performs the equivalent of the down semaphore operation.
int sem_post(sem_t * sem);
The sem_post() function performs the equivalent of the up semaphore operation.
int sem_destroy(sem_t * sem);
The sem_destroy() function is used to properly deallocate resources alloted to a semaphore. The semaphore in this program is used
as a mutex, a binary semaphore, to implement mutual exclusion between two processes which use a shared resource.

Program

#include <unistd.h> /* Symbolic Constants */
#include <sys/types.h> /* Primitive System Data Types */
#include <errno.h> /* Errors */
#include <stdio.h> /* Input/Output */
#include <stdlib.h> /* General Utilities */
#include <pthread.h> /* POSIX Threads */
#include <string.h> /* String handling */
#include <semaphore.h> /* Semaphore */
/* prototype for thread routine */
void handler ( void *ptr );
/* global vars */
/* semaphores are declared global so they can be accessed
in main() and in thread routine,
here, the semaphore is used as a mutex */
sem_t mutex;
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(&mutex, 0, 1); /* initialize mutex to 1 - binary semaphore */
/* second param = 0 - semaphore is local */
/* Note: you can check if thread has been successfully created by checking return value
of
pthread_create */
pthread_create (&thread_a, NULL, (void *) &handler, (void *) &i[0]);
pthread_create (&thread_b, NULL, (void *) &handler, (void *) &i[1]);
pthread_join(thread_a, NULL);
pthread_join(thread_b, NULL);
sem_destroy(&mutex); /* destroy semaphore */
/* exit */
exit(0);
} /* main() */
void handler ( void *ptr )
{
int x;
x = *((int *) ptr);
printf("Thread %d: Waiting to enter critical region...\n", x);
sem_wait(&mutex); /* down semaphore */
/* START CRITICAL REGION */
printf("Thread %d: Now in critical region...\n", x);
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(&mutex); /* up semaphore */
pthread_exit(0); /* exit thread */
}