Preprocessor #if confusion

Jonathan Wakely
Fri Jul 6 08:15:00 GMT 2012

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:
>   #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


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

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