Segfault trying to copy array from main memory to shared memory

Question

  int shmid = shmget(IPC_PRIVATE, sizeof(int)*sizeof(arr), 0666|IPC_CREAT);
  int* shm = (int*) shmat(shmid, (void*)0, 0);

  for(int i=0; i<1; i++){
    *(shm+i) = a