From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
Subject: Re: [9fans] PCC - #if
From: "Russ Cox" <rsc@swtch.com>
Date: Fri,  4 May 2007 17:17:40 -0400
In-Reply-To: <c4167ab1c8e9bbe37e8b8fe0af747a37@proxima.alt.za>
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	term% diff ../../../openldap-2.3.32/include/ac/time.h ../../include/ac
	20c20
	< #if defined(TIME_WITH_SYS_TIME)
	---
	> #if TIME_WITH_SYS_TIME
	23c23
	< #elif defined(HAVE_SYS_TIME_H)
	---
	> #elif HAVE_SYS_TIME_H

The < version is always valid; the > version is often valid.

After

	#define A 1
	#define B 0
	#define C /* nothing! */
	#undef D

These are like #if 1:

	#if defined(A)
	#if defined(B)
	#if defined(C)
	#if A

These are like #if 0:

	#if defined(D)
	#if B
	#if D

and this is invalid:

	#if C

(The rule for variables appearing in #if is that macros
expand and names that make it through macro expansion
turn into zeros, so `#if D' is like `#if 0' but `#if C' is like `#if',
which is missing an expression.)

So if the config.h for this program has done, say,

	#define TIME_WITH_SYS_TIME 1
	#define HAVE_SYS_TIME_H 1

then the two versions you have are equivalent, but
if it has done

	#define TIME_WITH_SYS_TIME
	#define HAVE_SYS_TIME_H

then only the #if defined(...) versions are valid C.
The convention in autoconf etc. is to either
#define X 1 or #undef X for each variable X.

Russ