Ubuntu Semaphore: Segmentation fault (core dumped) - c

Question

I am learning semaphores in C using Ubuntu right now. The professor just throw us this code and ask us to study it and observe. When I compiled I get a warning that ctime(&sem_buf.sem_ctime) returns an int, not a char * but nothing major. When I run it the output is just: Semaphore identifier: 0 Segmentation fault (core dumped). I am very confused as of what went wrong and I have no idea what is going on in this code. Some help would be very much appreciated.
Here is the code:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <semaphore.h>
# define NS 3
union semun {
int val;
struct semid_ds *buf;
ushort *array; // Unsigned short integer.
};
int main(void)
{
int sem_id, sem_value, i;
key_t ipc_key;
struct semid_ds sem_buf;
static ushort sem_array[NS] = {3, 1, 4};
union semun arg;
ipc_key = ftok(".", 'S'); // Creating the key.
/* Create semaphore */
if ((sem_id = semget(ipc_key, NS, IPC_CREAT | 0666)) == -1) {
perror ("semget: IPC | 0666");
exit(1);
}
printf ("Semaphore identifier %d\n", sem_id);
/* Set arg (the union) to the address of the storage location for */
/* returned semid_ds value */
arg.buf = &sem_buf;
if (semctl(sem_id, 0, IPC_STAT, arg) == -1) {
perror ("semctl: IPC_STAT");
exit(2);
}
printf ("Create %s", ctime(&sem_buf.sem_ctime));
/* Set arg (the union) to the address of the initializing vector */
arg.array = sem_array;
if (semctl(sem_id, 0, SETALL, arg) == -1) {
perror("semctl: SETALL");
exit(3);
}
for (i=0; i<NS; ++i) {
if ((sem_value = semctl(sem_id, i, GETVAL, 0)) == -1) {
perror("semctl : GETVAL");
exit(4);
}
printf ("Semaphore %d has value of %d\n",i, sem_value);
}
/*remove semaphore */
if (semctl(sem_id, 0, IPC_RMID, 0) == -1) {
perror ("semctl: IPC_RMID");
exit(5);
}
}

Answer 1

You need to include time.h to the compiler recognize ctime function. The warning is because the compiler don't know ctime is a function and that returns an char*. By default GCC assumes the unknown function returns an int.