Double ## in macros

Question

I was looking at some macro use in an Arduino code. I believe what this macro does is it expands into a state that basically sets a pin to either input or output, but I'm a little confused about everything going on in this macro.

I do understand the point behind the do{...} while(0), but I'm sure what the ## are accomplishing. Also, what if anything do the underscores before DDR and PIN do? I would very appreciative of a simple clarification. Thank you!

#define MASK(PIN)  (1 << PIN)

/// set pin as input
#define _SET_INPUT(IO) do {DIO ##  IO ## _DDR &= ~MASK(DIO ## IO ## _PIN); } while (0)
/// set pin as output
#define _SET_OUTPUT(IO) do {DIO ##  IO ## _DDR |=  MASK(DIO ## IO ## _PIN); } while (0)

Answer 1

## is the concatenation operator in macros. It joins two macros or pieces of text together when expanded.

#define _SET_INPUT(IO) do {DIO ##  IO ## _DDR &= ~MASK(DIO ## IO ## _PIN); } while (0)

_SET_INPUT(3);

woukd expand to:

do {DIO3_DDR &= ~MASK(DIO3_PIN); } while (0);

You can read more about how it works here: https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html