How to use NULL in C

In C, NULL is a macro that represents a null pointer: a pointer that points to nothing. You use it to initialize pointers that don’t have a target yet, and to check if a pointer is safe to use.

Several programming languages make use of the concept of null.

Go has nil, JavaScript has null, Python has None, and so on.

C has NULL.

NULL however is used differently from other languages. In C, NULL is limited to identifying a null pointer. It’s not a general “no value” marker you can assign to an int or a float.

When we initialize a pointer, we might not always know what it points to. That’s when it is useful:

int * p_some_variable = NULL;

Where is NULL defined?

NULL is not available by default: you need to include stdio.h to use it (or if you prefer, stddef.h):

#include <stdio.h>

int main(void) {
  int * p_some_variable = NULL;
}

Other common headers like stdlib.h and string.h define it too, so in practice most programs get it for free.

Otherwise the C compiler will give you an error:

hello.c:3:26: error: use of undeclared identifier
      'NULL'
        int * p_some_variable = NULL;
                                ^
1 error generated.

How to check for a null pointer

You can check if a pointer is a null pointer by comparing it to NULL:

#include <stdio.h>

int main(void) {
  int * p_some_variable = NULL;

  if (p_some_variable == NULL) {
    printf("equal");
  }
}

This check is everywhere in real C code, because many standard library functions return NULL to signal failure. fopen() is a good example. It returns NULL when the file can’t be opened:

#include <stdio.h>

int main(void) {
  FILE *fp = fopen("data.txt", "r");

  if (fp == NULL) {
    printf("could not open data.txt\n");
    return 1;
  }

  fclose(fp);
}

malloc() works the same way: it returns NULL when it can’t allocate memory.

The pitfall: dereferencing NULL

Reading or writing through a null pointer is undefined behavior. On most systems the program crashes with a segmentation fault:

int * p = NULL;
printf("%d\n", *p); /* crash */

The fix is the check we just saw: compare the pointer to NULL before dereferencing it.

NULL is not ‘\0’

Under the hood, NULL is typically defined as ((void *)0) or 0.

Don’t confuse it with '\0', the null character that terminates C strings. They both have the value zero, but NULL is for pointers and '\0' is a char inside a string.

A line like char *a_string = '\0'; compiles, but it does not create an empty string. It sets the pointer to NULL, because '\0' is just the integer zero. An empty string is "", a real (tiny) array containing only the terminator.