Re: [avr-gcc-list] Check whether two macros are equal to each other

[Georg-Johann Lay]

Karol Babioch schrieb:

> I'm currently trying to check whether two macros are equal to each 
> other. The idea is to generate different code, when they are equal, 
> which basically means that they are connected to the same port.
>
> Basically it looks something like this:
>
> #define LED1_DDR DDRD
> #define LED2_DDR DDRD
> #if LED1_DDR == LED2_DDR
>   // Code
> #endif
>
> But when trying to compile this, I get the following error:
>
> error: operator '*' has no left operand

That won't work.

Compile it with -save-temps and look at the generated precompiled
code in the i file to understand why it's an error.

You can write

if (&LED1_DDR == &LED2_DDR)
{
    // Code
}

and let the optimizer throw away the code if the ports are different.

> Interestingly enough it works just fine, when the values of the macros 
> are not some sort of macro itself, but a literal, e.g. "one". It then 
> just compiles fine.

BTW, also try the following:

#define a one
#define b two

#if a == b
#error
#endif

You cannot compare macros like that.

> Could anyone explain this to me? Is there any way around this (still 
> using macros)?

With macros? No way here.