14.6 Null Pointers

A pointer value can be null, which means it does not point to any object. The cleanest way to get a null pointer is by writing NULL, a standard macro defined in stddef.h. You can also do it by casting 0 to the desired pointer type, as in (char *) 0. (The cast operator performs explicit type conversion; See Explicit Type Conversion.)

You can store a null pointer in any lvalue whose data type is a pointer type:

char *foo;
foo = NULL;

These two, if consecutive, can be combined into a declaration with initializer,

char *foo = NULL;

You can also explicitly cast NULL to the specific pointer type you want—it makes no difference.

char *foo;
foo = (char *) NULL;

To test whether a pointer is null, compare it with zero or NULL, as shown here:

if (p != NULL)
  /* p is not null.  */
  operate (p);

Since testing a pointer for not being null is basic and frequent, all but beginners in C will understand the conditional without need for != NULL:

if (p)
  /* p is not null.  */
  operate (p);