Semaphore Example

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 /* Symbolic Constants */
#include /* Primitive System Data Types */
#include /* Errors */
#include /* Input/Output */
#include /* General Utilities */
#include /* POSIX Threads */
#include /* String handling */
#include /* 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 */
}
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 /* Symbolic Constants */
#include /* Primitive System Data Types */
#include /* Errors */
#include /* Input/Output */
#include /* General Utilities */
#include /* POSIX Threads */
#include /* String handling */
#include /* 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 */
}

Links

* Linux Tutorial: POSIX Threads
* Getting Started With POSIX Threads


Links

* Linux Tutorial: POSIX Threads
* Getting Started With POSIX Threads