To two places: stddef.h and stdlib.h :(.

It's interesting to see the different bugs 0, 0L, (char *)0 and (void *)0 expose or hide as definitions of NULL.

0 is fine if sizeof(int) == sizeof(void *). Otherwise varadic function calls break. 0L is the same, but for long. The pointer definitions run into trouble in other contexts since NULL is often incorrectly used as a terminating byte in a string instead of '\0'. (void *) has issues with const pointers, some of which are relevant if you use it in the wrong context.

There were quite spirited debates back in the day for which one was best. They all suck. C++ invented a null pointer symbol because it's type rules were enough different than C to make a universal NULL #define impossible. Is that better or worse? Don't know. It's different. 

Glad to see the null-pointer need not have all zero bits being different than a 0 constant shall be the null-pointer in sufficient detail.

Warner

On Sun, Sep 20, 2020, 5:54 PM Clem Cole <clemc@ccc.com> wrote:

Norman NULL has to be defined and I said that/showed it.   The standard says where.  I was not trying to compile NULL without a definition which I agree it not legal.  If that is what Doug was implying I missed understood him but I note NULL was introduced in Typesetter C /V7 where those compiler s set it to 0 in studio but the ANSI/ISO moved it. 

On Sun, Sep 20, 2020 at 7:03 PM Norman Wilson <norman@oclsc.org> wrote:

Doug McIlroy:



  To put it more strongly. this is not a legal C source file.

          char *s = NULL;

  But this is.

          char *s = 0;



Clem Cole:



   67)The macro NULL is defined in <stddef.h> (and other headers) as a null

   pointer constant; see 7.19.



====



$ cat null.c

char *s = NULL;

$ cat zero.c

char *s = 0;

$



zero.c is a legal C program.  null.c is not.  Create

files exactly as shown and compile them if you don't

believe me.



Prepend `#include <stddef.h>' (or <stdlib.h> or <stdio.h>)

to null.c and it becomes legal, but I think that's Doug's

point: you need an include file.



Personally I prefer to use NULL instead of 0 when spelling

out a null pointer, because I think it's clearer:

        if ((buf = malloc(SIZE)) == NULL)

                error("dammit andrew");

though I am willing to omit it when there's no confusion

about = vs ==:

        if (*p)

                dammit(*p, "andrew");



But that's just a question of style, and Doug's is fine too.



The language does not require the compiler to pre-define

NULL or to recognize it as a keyword; you have to include

an appropriate standard header file.



Norman Wilson

Toronto ON (not 0N nor NULLN)

--

Sent from a handheld expect more typos than usual