Re: [PATCH] Trivial warning fix for imap-send.c

From: Linus Torvalds <torvalds@osdl.org>
Date: Mar 12, 2006, 16:57 UTC
Message-ID: <Pine.LNX.4.64.0603120847500.3618@g5.osdl.org>
In-Reply-To: <slrne17urp.fr9.mdw@metalzone.distorted.org.uk>

On Sun, 12 Mar 2006, Mark Wooding wrote:

Show 9 quoted lines

> "Art Haas" <ahaas@airmail.net> wrote:
> 
> > -			execl( "/bin/sh", "sh", "-c", srvc->tunnel, 0 );
> > +			execl( "/bin/sh", "sh", "-c", srvc->tunnel, NULL );
> 
> This is not the right fix.  NULL can be simply a #define for 0 (see
> 6.3.2.3#3 and 7.17).  You need to write (char *)0 or (char *)NULL.  I
> prefer to avoid the macro NULL entirely, since its misleading behaviour
> is precisely what got us into this mess.

It's perfectly fine.

Quite frankly, if you have a 64-bit C compiler that doesn't make "NULL" be "((void *)0)" (or equivalent - some compilers will actually have NULL as an intrisic, because especially if they also support C++, NULL has some really magical properties there), you should switch vendors as quickly as humanly possible.

The "#define NULL 0" practice is still _legal_ C, but that doesn't make it any less broken. It's K&R traditional, but git requires ANSI prototypes and some fancy features from modern compilers, so K&R compilers aren't welcome anyway, and if their headers don't define NULL as a void pointer, their headers are simply _broken_.

So in modern C, using NULL at the end of a varargs array as a pointer is perfectly sane, and the extra cast is just ugly and bowing to bad programming practices and makes no sense to anybody who never saw the horror that is K&R.

It's akin to trying to not using prototypes, or to trying to limit your externally visible names to 7 characters. It was "appropriate" about two decades ago, these days it's just cuddling broken setups that have been broken for a long long time.

Btw, the reason NULL _has_ to be a pointer ("((void *)0)" or otherwise) is simply that if it isn't, you not only won't get reasonable varargs behaviour, you'll also miss real warnings. I've seen broken code like

	int i = NULL;

in my life, and if the compiler doesn't warn about that, then the compiler is BROKEN, and not worth supporting as a programmer.

			Linus