Let's define our own NULL

[William E. Davidsen Jr]

In article <1090 at holos0.UUCP> lbr at holos0.UUCP (Len Reed) writes:

| > argument lists terminating with NULL, since *only* assignment will
|                                                ^^^^^^^^^^^^^^^^
|    What about initialization?  Surely this works:
| 		 char far * cfptr = 0;
| 
| > translate zero to a pointer. Look at the exec family to see where NULL
| > is passed where a pointer is expected.

  Well, I might have used the the "assignment operator," but I think you
see what I mean.

| On the PC family I'm using Microsoft 5.x and SCO Xenix.  I use
| function prototypes with typed arguments; I don't see why "0" won't work
| in all cases.  The compilers cast "0" to the proper (far or near) null
| value.  Do you have an example using function prototypes with type,
| information, where this would fail?  Is there a compiler that allows
| ((void *)0) but not prototypes?

  I tried both SunOD and Ultrix. Both barfed at the prototype. I don't
have access to a "real BSD" compiler, but I think this might be typical
of BSD.

  The point in case was execl, variable number of args, type char *.
What do you use for the prototype of that, and if VOID is zero, does it
generate a far pointer when you end a list with NULL in large model?

| Even ((void *)0) won't get you out of all mixed-mode pointer problems.

  I totally agree. What I was saying was that it generally *will* get
you out of constant mode problems.

| I like the idea of
| #define NULL ((void *)0)
| though, since much imported code won't use function prototypes.
| Also, ((void *)0 will provide better type checking than 0.  My compilers
| will complain if I use a (void *) where something other than a pointer is
| expected, but they allow 0 to be used (almost?) anywhere.

  A benefit I hadn't considered.

| Some folks use NULL when they should say '\0'; ((void *)0) can
| cause a warning here.  Maybe that's just as well: use 0 or '\0' for
| a null character, but use NULL only for pointers.

  I usually define a symbol, (EOS for end of string) which is '\0'. A
good point, anything which gives the compiler a better chance to catch
errors is a win.

| Microsoft C 5.1 stdio.h defines NULL as 0 or 0L depending upon the model.
| This can cause spurious loss of precision warning messages when using
| explicit "near" pointers in the large model, and is thus inferior
| to ((void *)0).
| 
| All said, I like ((void *)0) and may adopt it.  One additional
| philosophical reason to use it.  (void *) means generic pointer, and
| thus ((void *)0) means generic pointer with value zero, which is
| surely closer to "null pointer" than 0.
| 
| -- 
|     -    Len Reed

  Although I expected flames, almost all of the responses were technical
in nature, and well thought out, such as this one. I did get a few
nastygrams, but they were mainly in the nature of "we are tired of
discussing NULL," rather than disagreement.
-- 
	bill davidsen		(wedu at ge-crd.arpa)
  {uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me