It depends on the architecture. The VAX could be set (on a function-by-function basis) to either ignore 2's complement overflow, or automatically trap. The Intel x86 line can trap, but you have to add the INTO instruction, possibly after each math operation that could overflow. I don't think the Motorola 68k could trap on overflow. The MIPS has two sets of math operations, one that will automatically trap on 2's complement overflow, and a set that won't (and at the time, the C compiler I used only used the non-trap instructions).

That's why the C standard is so weasly with overflow---it varies widely per CPU.