Bitfield masks in C

Question

Is there a portable way in C to find out the mask for a bit field at compile time?

Ideally, I'd like to be able to atomically clear a field like this:

struct Reference {
    unsigned age : 3;
    unsigned marked : 1;
    unsigned references : 4;
};

struct Reference myRef;
__sync_and_and_fetch(&myRef, age, ~AGE_MASK);

Otherwise I have to take out a lock on the struct, which is more heavyweight than I'd like.

Answer 1

Or, if you really wanted the mask:

union Reference {
  unsigned asWord;
  struct {
    unsigned age : 3;
    unsigned marked : 1;
    unsigned references : 4;
  } asFields;
}

Reference agemask_ref;
agemask_ref.asFields = (typeof(agemask_ref.asFields)){0, -1, -1};
unsigned agemask = agemask_ref.asWord;