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From: Dong Soo Kim <dsk666 AT cse DOT unsw DOT EDU DOT AU>
Newsgroups: comp.os.msdos.djgpp
Subject: GNU preprocessor problem?
Date: Sat, 23 Feb 2002 14:29:26 +1100
Organization: The University of New South Wales
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Hi all :)

I recently created a macro for error reporting purposes, with different
behaviours for different systems.

But I've hit a little problem with the #ifdef's and #else's

I really don't think it's my mistake .. but could somebody point out what
it is I'm doing wrong?

---- error_macro.h ----
... // #ifndef wrappers

// the macro is defined properly if the following is changed to #ifdef ..
#ifndef _GNU_H_WINDOWS_H

	// for non windows systems ..
	#include <stdio.h>
	#define p(s) { fprintf(stderr, "\nError in line %i of %s", \
		__LINE__, __FILE__); \
		fprintf(stderr, "\nReason : %s", #s); exit(1); }
#else // #ifdef _GNU_H_WINDOWS_H

	// for windows ..
	#define p(s) { MessageBox(NULL, #s, "Fatal Error", MB_OK); \
		exit(1); }
#endif // #else // #ifdef _GNU_H_WINDOWS_H

... // #endif wrappers

---- end error_macro.h ----

---- windows.h ----

#ifndef _GNU_H_WINDOWS_H
#define _GNU_H_WINDOWS_H

... // other includes and stuff

#endif /* _GNU_H_WINDOWS_H */

---- end windows.h ----

---- test.c ----

#include <windows.h>
#include <stdlib.h>
#include <error_macro.h>

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
		LPSTR lpCmdLine, int nCmdShow) {

	p(x)

	return 0;
} // WinMain()

---- end test.c ----

I'm using the gccw32 files.

By my reckoning, _GNU_H_WINDOWS_H is defined when windows.h is included in
test.c, and isn't #undef'ed anywhere else.

The error_macro.h file actually defines the macro for
non-windows (ie. #define p(s) { fprintf .. } ) instead of the windows
macro (ie. #define p(s) { MessageBox .. } ).

Because _GNU_H_WINDOWS_H is defined in windows.h and never #undef'ed, by
the time the pre-processor gets to the p(s) macro definitions it should go
to the #else section .. but it doesn't.

If I change the #ifndef _GNU_H_WINDOWS_H to #ifdef it works properly.
Surely that's not what's should be happening?

Hopefully somebody can point me in the right direction :)

Thanks in advance
DSK