CE 155 - Semaphore Example (Home)

This program demonstrates 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.

To compile and link the program:

gcc -o sem-ex sem-ex.c -Wall -Werror -lpthread

sem-ex.c (download)

/* Includes */
#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 */  
} /* 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 */
    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);
    printf("Thread %d: New Counter Value: %d\n", x, counter);
    printf("Thread %d: Exiting critical region...\n", x);
    sem_post(&mutex);       /* up semaphore */
    pthread_exit(0); /* exit thread */