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
/* 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 */ 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 */ }