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 ex

Answer 1

## concattenates symbols. So for example if the value of p is ab, 0x##p would become 0xab.

Answer 2

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

Answer 3

Als and sepp2k give correct answer.

However I would like to add, that this macro seems to be completely unnecessary.

unsigned int value = r0(b,e,a,f);

can be replaced by better and shorter:

unsigned int value = 0xbeaf;