The Null Pointer

Sometimes a pointer has nothing to point to. Imagine a function that searches an array of temperatures for the first frost, a temperature below zero, and returns its address. What should it return when there was no frost all week? For this, C has the value NULL, the null pointer: a pointer that points nowhere.

This function returns an address, so its return type is int *. &numbers[i] is the address of the element with the index i:

int *find_negative(int *numbers, int count) {
    for (int i = 0; i < count; i++) {
        if (numbers[i] < 0) {
            return &numbers[i];
        }
    }
    return NULL;
}

Why does it return an address and not the temperature itself? Because a number can't say "there is none": any number the function returned could be a real temperature. A pointer can, because NULL is not the address of any variable.

Before you use a pointer that can be NULL, check it with if:

#include <stdio.h>

int *find_negative(int *numbers, int count) {
    for (int i = 0; i < count; i++) {
        if (numbers[i] < 0) {
            return &numbers[i];
        }
    }
    return NULL;
}

int main(void) {
    int week[] = {3, 1, -2, 0, -4};
    int *frost = find_negative(week, 5);
    if (frost != NULL) {
        printf("First frost: %d\n", *frost);
    } else {
        printf("No frost\n");
    }
    return 0;
}

This prints:

First frost: -2

Why check? Reading or writing through NULL is a bug, because there is no variable there. On a computer, such a program usually crashes, often with the message Segmentation fault. Here in the browser, reading through NULL stops the program with the error RuntimeError: unreachable, the same error as for an index past the end of an array. It doesn't say what went wrong, so when you see it in a program with pointers, look for a pointer that can be NULL.

NULL is also a good starting value for a pointer that has nothing to point to yet: int *found = NULL;. A pointer without any value, int *p;, is a compile error here as soon as you use it (variable 'p' is uninitialized when used here), like any variable without a value.

One more rule for functions that return a pointer: return only addresses of things that still exist when the function ends, such as the elements of the array it got. A function's local variables disappear when it ends, so never return their addresses.