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Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde!uunet!ncrlnk!ncr-sd!hp-sdd!megatek!eta!hollen
From: hollen@eta.megatek.uucp (Dion Hollenbeck)
Newsgroups: comp.os.os2,comp.lang.c,comp.std.c
Subject: Re: MSC __STDC__ strange value
Message-ID: <841@megatek.UUCP>
Date: 5 Dec 89 15:41:34 GMT
References: <223@bohra.cpg.oz>
Sender: news@megatek.UUCP
Lines: 45

From article <223@bohra.cpg.oz>, by ejp@bohra.cpg.oz (Esmond Pitt):
> For some strange reason, the MSC 5.1 compiler defines __STDC__ as 0, so
> 
> #if __STDC__
> ...
> #endif

This means:  Take the DEFINED macro __STDC__ and test its value and
		return TRUE if non-zero and FALSE if zero

> 
> yields a different result from
> 
> #ifdef __STDC__
> ...
> #endif
> 
> Why?

This means:	Look to see if the macro __STDC__ is defined. If
		it is defined, return TRUE and if it is not defined
		return FALSE.

The difference is the basic definition of #if and #ifdef.  #if
tests the value of a defined macro and #ifdef tests for its
existence.  In all code I have seen (I have lots of source to
packages which I use) written to support several different compilers,
the test always made is the #ifdef test.

By the way,
	#define __STDC__	/* evaluates  __STDC__ = 0  */
while
	#define __STDC__  1	/* evaluates  __STDC__ = 1 */

You will only get the #if test to be true if the macro IS DEFINED
and IS DEFINED TO BE NON-ZERO.

Hope this helps.



	Dion Hollenbeck             (619) 455-5590 x2814
	Megatek Corporation, 9645 Scranton Road, San Diego, CA  92121

        uunet!megatek!hollen       or  hollen@megatek.uucp