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From: ok@cs.mu.oz.au (Richard O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: More NULL questions
Message-ID: <2320@munnari.oz.au>
Date: 7 Oct 89 09:20:24 GMT
References: <5950001@hpldola.HP.COM>
Sender: news@cs.mu.oz.au
Lines: 29

In article <5950001@hpldola.HP.COM>, jg@hpldola.HP.COM (Joe Gilray) writes:
> 1) what is the danger with using
>       struct thing *ptr; ... if (ptr != NULL) ...
>    as Opposed to
>       struct thing *ptr; ... if (ptr != (struct thing *)NULL) ...

None.  You can even use    ... if (ptr) ...

> 2) Is there danger using
>       int a, b; ... my_func(a, NULL, b);
>    as Opposed to
>       int a, b; ... my_func(a, (struct thing *)NULL, b);
>    when
>    a) you are using function prototypes?

No.  If there is a prototype in scope, passing an argument is like
assignment, NULL will be converted to the appropriate pointer type.

>    b) you are NOT using function prototypes (like me)?

Yes.  You will get an int, which may not be the same size as a pointer,
and if it is, may not be the same bit pattern as (struct thing *)NULL.

It isn't just NULL.  With some C compilers, and on some machines,
the cast in
	struct thing *ptr; ...
	if (fwrite((char *)ptr, sizeof *ptr, 1, stream) != 1) ...
is absolutely necessary.  (The cast is (void *) in ANSI C, and is
not needed if you have the prototype of fwrite() in scope.)