c preprocessor - What do two adjacent pound signs mean in a C macro?

Question

I'm taking a look at an application that defines a large set of constant arrays. What really confuses me is the use of two pound signs next to each other in a macro. For example:

#define r0(p,q,r,s) 0x##p##q##r##s

What do those two pound signs mean?

Answer 1

## provides a way to concatenate actual arguments during macro expansion.