Pointer and Integer issues in C

Function is waiting for a pointer but I want to return an integer in the function lookup (i can't modify the return type of the function), what can i do to solve this problem ?

If the function returns void * and you cannot change that, then returning an integer simply is not an option. You could return an integer converted to a pointer (and compare the return value against a similar value), but unless your hash table supports null values, a null pointer would make a better failure code:

void * hashtable_lookup(HashTable * table, void *data) {
    // look up data ...

    // lookup failed
    return NULL;
}

// ...

        if (hashtable_lookup(*ptable, data) != NULL) {
            // data is already present in the hash table
            return -1;
        }

But if a null pointer would be valid data for the table, then

void * hashtable_lookup(HashTable * table, void *data) {
    // look up data ...

    // lookup failed
    return (void *) -1;
}

// ...

        if (hashtable_lookup(*ptable, data) != (void *) -1) {
            // data is already present in the hash table
            return -1;
        }

First warning is because you are comparing a void* to an int. The second one is because you're returning an int instead of a pointer.

You could add casts here and there to remove the warnings (if(hashtable_lookup(*ptable, data) != (void*)(-1)){ and return (void*)(-1);, but it would be harder to read and more error-prone since you usually check pointers to not be NULL, but not to be different than "-1".

Typically, you would return a NULL pointer to indicate that the function failed. Also, you might have if(hashtable_lookup(*ptable, data) != -1){ wrong since it looks like when hasthable_lookup() returns -1 it failed.

I think this is what you are looking for.

int hashtable_insert(HashTable ** ptable, void *data, void (*delete) (void *))
{
    if(hashtable_lookup(*ptable, data) == NULL){
        return -1;
    }

    data = malloc(sizeof(size_t));
    list_append((*ptable)->list, data, (*ptable)->size);
    _hashtable_resize(ptable);

    return 0;
}

void * hashtable_lookup(HashTable * table, void *data)
{
    for(int i = 0; i < (table)->length; i++){
        if((table)->list[i] == data){
            return data;
        }
    }
    return NULL;
}