Malloc in Array of Structures

I have a homework about coding a car saler program. User gives us the datas about cars as a string so I should use strtok and atoi.

Code:

---

struct ilanBilgi {    char sehir[15];
    int fiyat;
    struct ilanTarih;
    char marka[15];
    char model[15];
    int modelYil;
};

---

These are the datas about cars and I should locate memory for 5 cars for the beginning if user keeps entrying data I should locate memory for another 5 so it goes like 5 cars,10 cars,15,20...

What should I do with the malloc functions?

They way I do it is to have two variables, one for the allocated size and another for the used size. When the used size equals the allocated size, it's time to allocate new memory.

Okay thanks for your help but can you show me how should I use the malloc and realloc functions?

Code:

---

struct ilanBilgiVector {
    struct ilanBilgi *data;
    size_t allocated;
};

void init ( struct ilanBilgiVector *v ) {
  v->data = NULL;
  v->allocated = 0;
}

void extend ( struct ilanBilgiVector *v ) {
  size_t new_size = v->allocated * 2;
  if ( new_size == 0 ) new_size = 16;
  struct ilanBilgi *ptr = realloc( v->data, sizeof(*v->data) * new_size );
  if ( ptr ) {
    v->data = ptr;
    v->allocated = new_size;
  } else {
    // v->data is still valid, but there's no more room for more data
  }
}

---