Understanding #ifdef in .h files

[Pranay Srivastava]

Hi Harold

On 6/20/14, Harold André <harold.andre at gmx.fr> wrote:
> Hi,
>
> I try to understand how #ifdef in .h files works.
>
> I read Greg Kroah-Hartman's Coding style paper
> http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/mgp00031.html
>
> And as he says, i try to do a simple example but it does not work. I
> try with a small piece of C outside the kernel. I have 3 files.
>
>
> test_ifdef.h:
>

You say here you will define the function else where if TEST_FUNCTION is defined

> 	#ifdef TEST_FUNCTION
> 	void test(int *value);
> 	#else
> 	static inline void test(int *value) { }
> 	#endif
>
> test_ifdef.c:
>
> 	#include "test_ifdef.h"
>
But here you do on and define it any way. You must stick to the rule
you created earlier. If you are defining it here then this must also
be under the test of ifdef TEST_FUNCTION
> 	void test(int *value)
> 	{
> 	    *value += 1;
> 	}
>
> main.c:
>
> 	#include <stdio.h>
> 	#include "test_ifdef.h"
>
> 	int main(int argc, char *argv[])
> 	{
> 	    int i = 3;
> 	
> 	    printf("i = %d\n", i);
>
> 	    test(&i);
>
> 	    printf("i = %d\n", i);
>
> 	    return 0;
> 	}
>
>
> And when i compile:
>
> $ gcc -Wall -g main.c test_ifdef.c -o test_ifdef -DTEST_FUNCTION
> $ ./test_ifdef
> i = 3
> i = 4
> $ gcc -Wall -g main.c test_ifdef.c -o test_ifdef
> test_ifdef.c:14:6: error: redefinition of ‘test’
>  void test(int *value)
>       ^
> In file included from test_ifdef.c:12:0:
> test_ifdef.h:17:20: note: previous definition of ‘test’ was here
>  static inline void test(int *value) { }
>                     ^
> $
>
> I understand why it does not compile. But:
> - How it can work in the kernel code ?
> - Is-it possible to do this in code outside the kernel ?
>
> Thank you.
>
> Harold
>
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-- 
        ---P.K.S