有关指向指针并检查它们是否为NULL的问题

Question

I have an array of pointers. I allocate them to all be NULL. I change some of the pointers so that some of them point to an element that is NULL and some of them point to an element. I am iterating through all of the pointers in the array. My question is, how do I check that the actual pointers are NULL and not the elements that they are pointing to are NULL?

I want to be able to distinguish between a NULL pointer and a pointer that points to something NULL. Here is an iteration:

if (ptrptr == NULL) {
    // The actual pointer is NULL, so set it to point to a ptr
    ptrptr = ptr;
} else {
    // The pointer points to SOMETHING, it may be NULL, it may not be, but the ptrptr itself is not NULL
    // Do something
}

What happens is that I set ptrptr to point to ptr, and since ptr is NULL, I am getting NULL for ptrptr even though it points to something.

Answer 1

You need to allocate memory to hold the pointer, and dereference into it.

if (ptrptr == NULL) {
    // The actual pointer is NULL, so set it to point to a ptr
    ptrptr = malloc(sizeof(ptr));
    *ptrptr = ptr;
} else {
    // The pointer points to SOMETHING, it may be NULL, it may not be, but the ptrptr itself is not NULL
    // Do something
}

Answer 2

For example's sake, lets suppose your objects in the end are int s. So ptr is of type int * and ptrptr is of type int** . This means the assignment ptrptr = ptr is WRONG and your compiler should have noticed that and gave you an warning.

For example:

#define N 100

int* my_arr[N]; //My array of pointers;
//initialize this array somewhere...

int **ptrptr;
for(ptrptr = my_arr; ptrptr < my_arr + N; ptrptr++){
    ptr = get_object();
    *ptrptr = ptr; //This is equivalent to my_array[i] = ptr
}