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From: guy@sun.uucp (Guy Harris)
Newsgroups: net.lang.c
Subject: Re: if (p), where p is a pointer - PLEASE READ
Message-ID: <2792@sun.uucp>
Date: Thu, 12-Sep-85 05:06:22 EDT
Article-I.D.: sun.2792
Posted: Thu Sep 12 05:06:22 1985
Date-Received: Sat, 14-Sep-85 05:16:10 EDT
References: <118@mit-hector.UUCP>
Organization: Sun Microsystems, Inc.
Lines: 32

> An C idiom I see (and write) frequently is
> 
> 	if (ptr)
> 
> Will this work correctly on a machine where NULL is not 0?  Does it really
> need to say
> 
> 	if (ptr != NULL)

1) "if (X)" means exactly the same thing as "if (X == 0)", for all X
(assuming "if (X == 0)" has a meaning in the first place, i.e. if X is a
struct, forget it).

2) NULL is #defined to be 0, so "if (ptr == NULL)" is equivalent to "if (ptr
== 0)".

So both statements are equivalent.  The compiler is capable of realizing
that the LHS of the comparison is a pointer and the RHS is the constant 0,
and generates code to compare the pointer to a null pointer of the
appropriate type.  The only place where the compiler can't recognize that a
constant 0 is really a null pointer of the appropriate type is in a function
call (i.e., "setbuf(stream, 0)" and "setbuf(stream, NULL)" are wrong; you
have to say "setbuf(stream, (char *)0)" or "setbuf(stream, (char *)NULL)").

ANSI C will permit you to say something like

	void setbuf(FILE *, char *);

in <stdio.h>, in which case the compiler knows that the "0" in "setbuf(0)"
is to be converted to "(char *)0" and will do it for you.

	Guy Harris