Question

Question says it all.

If I have this for a 96-bit field:

uint32_t flags[3]; //(thanks @jalf!)

How do I best go about accessing this, given that my subfields therein may lie over the 32-bit boundaries (eg. a field that runs from bit 29 to 35)?

I need my accesses to be as fast as possible, so I’d rather not iterate over them as 32-bit elements of an array.

Answer 1

[This answer is valid for C (and by extension, for C++ as well).]

The platform-independent way is to apply bit-masks and bit-shifts as appropriate.

So to get your field from 29 to 35 (inclusive):

  (flags[1] & 0xF)        << 3
| (flags[0] & 0xE0000000) >> 29  // The bitmask here isn't really needed, but I like symmetry!

Obviously, you could write a class/function/macro to automate this.