semaphores_test.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#define DEBUG 1


static int semID;

void sem_wait(int);
void sem_signal(int);	


int main(int argc, char** argv) {

	semID = semget(2401, 1, 0777 | IPC_CREAT);
	if (semID >= 0) {
		semctl (semID, 0, SETVAL, (int) 1);
		printf("set %d semaphore to 1\n", semID);
		
	} else {
		perror("semget");
	}
	int cpid = fork();
	if(cpid == 0) {
		sem_wait(semID);
		printf("Lock from child for 5 secounds\n");
		sleep(5);
		printf("Child done\n");
		sem_signal(semID);
	} else {
		sleep(1);
		sem_wait(semID);
		printf("Lock from parent for 2 secounds\n");
		sleep(2);
		printf("Parent done\n");
		sem_signal(semID);
		wait(NULL);
		semctl (semID, 0, IPC_RMID, 0);
	}

}

/* Wait operation on semaphore id 
 * decrement semaphore
 */
void sem_wait(int sem_id) {
	
	#if DEBUG
		printf("Semaphore WAIT %d\n", sem_id);
	#endif

	struct sembuf op_wait = {
		.sem_op = -1,
		.sem_flg = SEM_UNDO,
		.sem_num = 0
	};

	if( semop (sem_id, &op_wait, 1) == -1) {
		perror(" semop failure ");
		exit (EXIT_FAILURE);
	}
}

/* Signal operation on semaphore id 
 * increment semaphore
 */
void sem_signal(int sem_id) {
	
	#if DEBUG
		printf("Semaphore SIGNAL %d\n", sem_id);
	#endif

	struct sembuf op_signal = {
		.sem_op = 1,
		.sem_flg = SEM_UNDO,
		.sem_num = 0
	};

	if( semop (sem_id, &op_signal, 1) == -1) {
		perror(" semop failure ");
		exit (EXIT_FAILURE);
	}
}