Je ne crois pas que ceci soit garantit. Les champs de bits laissent beaucoup de liberté à l'implantation :
n1124 dit
Citation:
6.7.2.1 Structure and union specifiers
....
10 An implementation may allocate any addressable storage unit large enough to hold a bitfield.
If enough space remains, a bit-field that immediately follows another bit-field in a
structure shall be packed into adjacent bits of the same unit. If insufficient space remains,
whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is
implementation-defined. The order of allocation of bit-fields within a unit (high-order to
low-order or low-order to high-order) is implementation-defined. The alignment of the
addressable storage unit is unspecified.