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Up to now, we have seen that in C++, the null pointer is represented by the keyword nullptr while prior to the C++ 2011 standard, the value 0 was used for the null pointer.

In C, the null pointer is generally represented by the macro

#define NULL ((void *) 0)

That is, NULL is 0 cast as a general address or pointer. Now, by default, NULL is not defined; instead, you must load a library that makes such a definition. The most obvious choice is to include the standard definitions library:

#include <stddef.h>

Another library in which NULL is defined is

#include <stdio.h>

Unfortunately, if you were to try to compile the following program,

#include <stdio.h>

#define NULL ((void *) 0)

int main() {
	int *n = NULL;

	printf( "%p\n", n );

	return 0;
}

you would get a compile-time error:

$ gcc sample.c
sample.c:3:0: warning: "NULL" redefined [enabled by default]
 #define NULL ((void *) 0)
 ^
In file included from /usr/include/_G_config.h:15:0,
                 from /usr/include/libio.h:32,
                 from /usr/include/stdio.h:74,
                 from sample.c:1:
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stddef.h:402:0: note: this is the location of the previous definition
 #define NULL ((void *)0)
 ^

Even though the definition is identical, the compiler will not allow repeated definitions, and therefore you may wish to hedge your definition:

#ifndef NULL
#define NULL ((void *) 0)
#endif

Thus, NULL will only be defined if NULL is not yet defined (ifndef is short for "if not defined").

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