How to #define variable in C

Question

I have a file called config.h with the following...

#define GL_DOOM

Then I have the following in another file m_misc.c...

#include "config.h"
...
#if ((defined GL_DOOM) && (defined _MSC_VER))
LOGD("Using glboom-plus.cfg");
#define BOOM_CFG "glboom-plus.cfg"
#else
LOGD("Using prboom-plus.cfg");
#define BOOM_CFG "prboom-plus.cfg"
#endif

But it says...

05-02 14:40:24.789: D/Doom(2966): Using prboom-plus.cfg

What is the deal here? I am new to C so what am I missing?

&& (defined _MSC_VER) <- _MSC_VER seems to not be defined. — Daniel Fischer
– Daniel Fischer, Commented May 2, 2013 at 14:47
Is the line #define GL_DOOM in config.h getting parsed? For example, are there #if's or #ifdef's around it? — Jim Cote
– Jim Cote, Commented May 2, 2013 at 14:49
@DanielFischer seems the right answer why you did not put it as an answer ? — MOHAMED
– MOHAMED, Commented May 2, 2013 at 14:49
As for compiler I am using the gcc compiler on Android NDK (So bionic) — Jackie
– Jackie, Commented May 2, 2013 at 15:24

Bill Lynch · Accepted Answer · 2013-05-02 14:58:13Z

4

Let's take the following code:

#define GL_DOOM
#define _MSC_VER

#if ((defined GL_DOOM) && (defined _MSC_VER))
LOGD("Using glboom-plus.cfg");
#else
LOGD("Using prboom-plus.cfg");
#endif

I can compile that code with g++ -E which will output the result of the preprocessor. Let's look at that output.

# 1 "blah.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "blah.c"

LOGD("Using glboom-plus.cfg");

So, to me, this implies that you probably don't have both GL_DOOM and _MSC_VER defined.

You might verify this with a test that looks something like:

#include "config.h"

#ifndef GL_DOOM
#error GL_DOOM is not defined
#endif

#ifndef _MSC_VER
#error _MSC_VER is not defined
#endif

It's also worth noting something. _MSC_VER is a preprocessor symbol that is defined almost strictly by Microsoft Visual Studio. If you're not compiling with that software, then the expectation would be that it would not be defined.

edited May 2, 2013 at 14:58

answered May 2, 2013 at 14:53

Bill Lynch

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1 Comment

djechlin Over a year ago

+1 for educating on -E and #error to debug preprocessing step.

Daniel Fischer · Accepted Answer · 2013-05-02 14:55:23Z

2

#include "config.h"

that includes the #define GL_DOOM.

...
#if ((defined GL_DOOM) && (defined _MSC_VER))

That checks whether both, GL_DOOM and _MSC_VER are defined.

Since GL_DOOM is defined, either your preprocessor is totally broken, or _MSC_VER is not defined.

answered May 2, 2013 at 14:55

Daniel Fischer

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Comments

CatchMeIfYouTry · Accepted Answer · 2013-05-02 15:29:52Z

0

The check

#if ((defined GL_DOOM) && (defined _MSC_VER))

will only evaluate to true if both the conditions are met. You have not specified whether you have defined _MSC_VER in the header file. That is probably the reason why it is executing the else condition.

If you want to log when either of those are defined in the header file, use the OR operation instead:

#if (defined (GL_DOOM)) || (defined (_MSC_VER))

answered May 2, 2013 at 15:29

CatchMeIfYouTry

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