The C standard requires that NULL be defined in locale.h, stddef.h, stdio.h, stdlib.h, string.h, time.h, and wchar.h.

The C++ standard requires that NULL be defined in the c* header corresponding to each of those.

The C standard is very strict about the names a standard can define--each standard header must define precisely the names the standard requires that header to define. The only other names it can define are those that are reserved for the implementation, such as those starting with an underscore followed by another underscore or a capital letter.

The C++ standard is much more permissive in this respect--including any one standard header can have the same effect as including any or all other standard headers.

From a practical viewpoint, C++ implementations used to take quite a bit of advantage of this permissiveness--that is, including one standard header frequently defined the names from a number of other standard headers. More recent implementations tend to work more like the C standard requires, staying much closer to each header defining only the names required by to be defined by that header. They're still probably not as strict about it as the C standard requires, but much closer than they used to be (as a rule).

NULL is a macro that should be defined in a header . In theory it's possible that some compiler ships without a definition for it (can anyone comment if there's a C standard where this is permitted?), in practice you've simply forgotten to include the right header.

"The correct answer is option 1: stdio.h and stddef.h Key Points NULL is a macro representing a null pointer constant. It is defined as #define NULL ((void*)0) in C. In C language, the NULL macro is defined in both: : Standard Input Output header : Standard Definitions header Additional Information iostream.h: Used in C++ for input-output operations, but not related to defining NULL. string.h: Used for string manipulation functions, does not define NULL. Pre-processor: Though macros are handled by the pre-processor, NULL is defined in header files, not directly by the pre-processor. Hence, the correct answer is: option 1: stdio.h and stddef.h"

The NULL macro is defined in <stddef.h>. (It is also defined in several other headers.) It expands to a null pointer constant and is intended to be used with pointers, not with characters.

There is no standard macro for '\0'. It is equivalent to 0 (character constants have int type), and a macro for it would be of limited use.

The character '\0' is often referred to as a null character, but this is different from the NULL pointer constant.

NULL is defined in the standard C header stddef.h, period.

If you run for example gcc/mingw port in Windows, you can just tell any half-decent IDE to find the declaration of NULL and end up in stddef.h where it says #define NULL ((void *)0).

You can also create a source file like this:

// main.c
#include <windows.h>
int main (void)
{}

Then compile with gcc main.c -H. This will expand all header dependencies, so you'll see which header that includes what other headers. You'll get a whole flood of them and you'll notice that stddef.h is indirectly included at some 2-3 different locations.

Conclusion: NULL is not defined by windows.h or any other windows-specific header that you should be including directly.

If you need to use NULL, then the correct approach is to #include <stddef.h> regardless of OS.