Use of double hash (##) in C [duplicate]

Answer 1

The important lines are these (also in cissvar.h):

CISSQ_REQUEST_QUEUE(free, CISSQ_FREE);
CISSQ_REQUEST_QUEUE(notify, CISSQ_NOTIFY);

They invoke that macro that you pasted. The "##" operator concatenates two words of code together into a single word, so the code generated (with the macro expansion) for the first line looks something like this:

static __inline void                                                    
ciss_initq_free(struct ciss_softc *sc)                             
{                                                                       
    STAILQ_INIT(&sc->ciss_free);                                    
    CISSQ_INIT(sc, CISSQ_FREE);                                              
}

Answer 2

## makes the preprocessor concatenate the two arguments to the ## operator.