Overflow in bit fields

Question

can I trust that the C compiler does modulo 2^n each time I access a bit field? Or is there any compiler/optimisation where a code like the one below would not print out Overflo

Answer 1

Short answer: yes, you can trust modulo 2^n to happen.

In your program, G.foo++; is in fact equivalent to G.foo = (unsigned int)G.foo + 1.

Unsigned int arithmetic always produces 2^(size of unsigned int in bits) results. The two bits of least weight are then stored in G.foo, producing zero.