Segmentation fault on scanf? - scanf

Question

I don't understand why the scanf call causes segmentation error. Tried making a separate file with only int number and scanf and it worked no problem. Really confused here need help. I use pointer in the scanf parameter so I'm suspecting the error might come from the interaction between scanf and shared memory somehow?
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <semaphore.h>
int main() {
const char *name = "message";
const char *names = "sem";
const int SIZE = 4096;
const int size = 1;
int number;
int shm_fd1;
int shm_fd2;
char *ptr; // Pointer to shared memory
sem_t *sem;
/* create the shared memory segment */
shm_fd1 = shm_open(name, O_CREAT | O_RDWR, 0666);
shm_fd2 = shm_open(names, O_CREAT | O_RDWR, 0666);
/* configure the size of the shared memory segment */
ftruncate(shm_fd1, SIZE);
ftruncate(shm_fd2, size);
/* initialize semaphore */
if (sem_init(sem, 1, 1) < 0) { // 1 = multiprocess
fprintf(stderr, "ERROR: could not initialize semaphore.\n");
exit(0);
}
/* now map the shared memory segment in the address space of the process */
ptr = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd1, 0);
sem = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd2, 0);
if (ptr == MAP_FAILED) {
printf("Map failed\n");
return -1;
}
if (sem == MAP_FAILED) {
printf("Map failed\n");
return -1;
}
strcpy(ptr, "Hello from the server\n");
/* store and read from the shared memory region */
printf("Message: %s\n", ptr);
printf("0. Quit\n 1. Change message\n 2. Check message\n");
printf("Type an integer: ");
scanf("%d", &number); //confirmed this causes the error
if I remove the line above the program runs no problem. the part alone also works fine apart from this file.
if (number == 1) {
sem_wait(sem);
printf("Enter your message: ");
scanf("%s", ptr);
sem_post(sem);
}
else if (number == 2) {
printf("Message: %s\n", ptr);
}
else {
/*break;*/
}
/* remove the shared memory segment */
if (shm_unlink(name) == -1) {
printf("Error removing %s\n",name);
exit(-1);
}
return 0;
}