sem_undo.c

#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/sem.h>
#include <sys/ipc.h>
#include <errno.h>

int test(int semid)
{
	struct sembuf op = {.sem_num = 0, .sem_op = -1, .sem_flg = SEM_UNDO};
	int res = semop(semid, &op, 1);
	if(res != -1){
		sleep(5);
		exit(0);
	}
	return res;
}

int main(int argc, char **argv)
{
	key_t key;
	int semid = -1, pid, status, val;
	key = ftok(argv[0], 131);
	if (key == -1) {
		perror("Can't make key");
		goto out;
	}
	semid = semget(key, 1, 0777 | IPC_CREAT | IPC_EXCL);
	if (semid == -1) {
		perror("Can't get sem");
		goto out;
	}
	if (semctl(semid, 0, SETVAL, 1) == -1) {
		perror("Can't set sem value");
		goto out;
	}
	pid = fork();
	if (pid == -1) {
		perror("Can't fork");
	} else if(pid == 0) {
		exit(test(semid));
	}
	waitpid(pid, &status, 0);
	if ((val = semctl(semid, 0, GETVAL)) == -1) {
		perror("Can't get sem value");
		goto out;
	}
	printf("Semaphore value: %d. Child exit status: %d\n", val, status>>8);
out:
	if(semid != -1){
		semctl(semid, 1, IPC_RMID);
	}
	return 0;
}