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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: "A diagram of C23 basic types"
Date: Thu, 3 Apr 2025 10:04:22 +0200
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On 02/04/2025 20:26, bart wrote:
> On 02/04/2025 18:29, David Brown wrote:
>> On 02/04/2025 17:38, bart wrote:
>>> On 02/04/2025 16:26, Muttley@DastardlyHQ.org wrote:
> 
>>>> Never understood that in C++ never mind C. NULL has worked fine for 
>>>> 50 years.
>>>
>>> And it's been a hack for 50 years. Especially when it is just:
>>>
>>>    #define NULL 0
>>
>>
>> The common definition in C is :
>>
>>      #define NULL ((void*) 0)
>>
>> Some compilers might have an extension, such as gcc's "__null", that 
>> are used instead to allow better static error checking.
>>
>> (In C++, it is often defined to 0, because the rules for implicit 
>> conversions from void* are different in C++.)
>>
>>>
>>> You also need to include some header (which one?) in order to use it. 
>>
>> <stddef.h>, as pretty much any C programmer will know.
> 
> This program:
> 
>    void* p = NULL;
> 
> reports that NULL is undefined, but that can be fixed by including any 
> of stdio.h, stdlib.h or string.h. Those are the first three I tried; 
> there may be others.
> 
> So it is not true that you need include stddef.h, nor obvious that that 
> is where NULL is defined, if you are used to having it available 
> indirectly.
> 

Fair enough - it is correct that there are some other standard headers 
that also define NULL (and/or a few other common identifiers such as 
size_t or wchar_t).  The standard source of these common definitions, 
without pulling in a range of other identifiers, is <stddef.h>.  (That 
is where it is documented in the standards.)

It doesn't matter to C programmers /where/ NULL is defined, or how - it 
matters merely that it is defined when they need it, and what it means.