Understanding NULL pointer in C

Question

I have found in some code NULL pointer is defined as follows -

#define NULL ((char *)0)

I found these code compiles fine. But I did not understand how this works. Can anyone please explain how 0 is casted to a char pointer?

And is it valid to use it as a FILE pointer making null -

FILE *fp = NULL;

It works...poorly (in fact, I don't think it's allowed). You want ((void *)0) instead. As to how it works: the standard defines a conversion from an integer constant with the value 0 to a pointer, and specifies that the result will always be a null pointer. — Jerry Coffin
– Jerry Coffin, Commented Mar 31, 2015 at 22:30
@Jerry Coffin, yes you are right. While compiling the code I found the code compiles without any error but with the following warning - warning: "NULL" redefined [enabled by default] /usr/lib/gcc/i686-linux-gnu/4.6/include/stddef.h:402:0: note: this is the location of the previous definition But I did not understand how this works. — ShafiH
– ShafiH, Commented Mar 31, 2015 at 22:33
I think this question is already answered here: stackoverflow.com/questions/1296843/… — Mohit
– Mohit, Commented Mar 31, 2015 at 22:35
Once upon a long time ago, that was the normal way to write NULL; we are talking about in the 1980s here, before there was a standard and before void * was uniformly supported. When the C standard was published (1989. 1990), it became less appropriate; it should be a ((void *)0) instead. Maybe your code is old enough that it is using the pre-standard definition. — Jonathan Leffler
– Jonathan Leffler, Commented Mar 31, 2015 at 22:47
((char *)0) is not a valid null pointer constant (in C since the 1989 ANSI standard) and therefore not a valid definition for NULL. Given that definition, FILE *fp = NULL; is a constraint violation, requiring a compile-time diagnostic (and perhaps rejecting the program). — Keith Thompson
– Keith Thompson, Commented Mar 31, 2015 at 22:51

user4291320 · Accepted Answer · 2015-04-01 07:51:14Z

1

The C library Macro NULL is the value of a null pointer constant.It may be defined as ((void*)0), 0 or 0L depending on the compiler vendor. Depending on compiler,declaration of NULL can be

#define NULL ((char *)0)

or

#define NULL 0L

or

#define NULL 0

And is it valid to use it as a FILE pointer making null--> Yes.

answered Apr 1, 2015 at 7:51

user4291320

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