Re: Preprocessor #if confusion

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 6 July 2012 07:22, Marcin S wrote:
> Hello,
> I have a problem with #if preprocessor instructions
> My source file is compiled with -D__TARGET_PROCESSOR=STM32F103RB
>
> and example snippet IN source file:
>
> #if __TARGET_PROCESSOR==SOMETHING_ELSE
>   #error
> #endif
>
>
> And preprocessor always find this expression as TRUE no matter what
> will be on right side.
> Why? And how to make it right

The preprocessor doesn't compare tokens by seeing if they have the same name.

The preprocessor expands it to

#if STM32F103RB == SOMETHING_ELSE

The tokens STM32F103RB and SOMETHING_ELSE are not defined as macros so
are both substituted with the constant 0, so your conditional is 0==0
which is always true.

One way to do it is define the target processor as a macro,
-DSTM32F103RB then test for it being defined:

#ifdef STM32F103RB