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.

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;

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;
}

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.

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");
  }
}

In practice, NULL is a constant equivalent to 0, or "\0".

This is why you can set a string to NULL using:

char *a_string = '\0';

Lessons in this unit:

0:	Introduction
1:	Input and output
2:	Variable scope
3:	Static variables
4:	Global variables
5:	Type definitions
6:	Enumerations
7:	Structures
8:	Command line parameters
9:	Header files
10:	The preprocessor
11:	▶︎ NULL values
12:	Boolean values
13:	Nesting functions
14:	Conversion specifiers
15:	Using quotes
16:	String length
17:	Returning strings
18:	Array length
19:	Looping through arrays
20:	Checking character values
21:	Printing percentage signs
22:	Troubleshooting: Implicit function declarations