Function remove Start List

Question

I am with this function to remove at the beginning, it happens that when I insert numbers as for example: 1 2 3, and then call the function to remove at the beginning and delete the number 2, while the correct one was to delete the number 1, because it is the beginning of the list. Already try everything here, up to my limit and I could not.

int retira_no_inicio (tipo_lista *p)
{
tipo_lista * aux;
//tipo_lista * ant;

if (p -> prox == NULL)
{
    return NULL;
}

aux = p -> prox;
p -> prox = aux -> prox;
return p;
}

Answer 1

Let’s imagine that the list consists of the three values: 1, 2 and 3. The pointer p points to the beginning of the list:

+---+    +---+    +---+
| 1 | -> | 2 | -> | 3 |
+---+    +---+    +---+
  ^
  p

It is verified whether p->prox is NULL, false, therefore, the pointer aux will receive p->prox:

+---+    +---+    +---+
| 1 | -> | 2 | -> | 3 |
+---+    +---+    +---+
  ^        ^
  p       aux

And p->prox receives aux->prox:

       +-------+ 
+---+  | +---+ |  +---+
| 1 | -+ | 2 | +> | 3 |
+---+    +---+    +---+
  ^        ^
  p       aux

Returning p. So the final list is:

+---+    +---+
| 1 | -> | 3 |
+---+    +---+
  ^
  p

To correct, just return the reference of aux, as the list will start at 2, eliminating the value 1. It is good to remember that as you are using dynamic allocation (source), you need to free the memory from the removed node:

tipo_lista* retira_no_inicio(tipo_lista *p)
{
    tipo_lista *aux = p->prox;

    free(p);

    return aux;
}

Note: note that the return of the function MUST be of the type tipo_lista and not int.