Compiler directive

#if 0
#include <spi.h>

#else
#include <wire.h>
#endif

The above code is part of a prog that basicaly selects between th relevant libraries required.

I do not understand the if though.
Do i have to manualy change the value following if myself or am i missing something.

I think yes.

Normally what I do is use #ifdef and I have somewhere one #define that I use in that conditional compilation.

Where you see that, it is usually because there was some useful or meaningful criterion there previously for conditional compilation, which was removed because something else was changed, or the code was moved to a different platform, where the original criterion became meaningless or inapplicable.

Otherwise, it makes almost no sense. to write the code that way. The first criterion there will always be false, and the second .h file will be included.

If you want to change it, you can manually edit the crterion to something which is meaningful to you, or just delete it.