13.2.4 Practical Advice for Signed Overflow Issues

Ideally the safest approach is to avoid signed integer overflow entirely. For example, instead of multiplying two signed integers, you can convert them to unsigned integers, multiply the unsigned values, then test whether the result is in signed range.

Rewriting code in this way will be inconvenient, though, particularly if the signed values might be negative. Also, it may hurt performance. Using unsigned arithmetic to check for overflow is particularly painful to do portably and efficiently when dealing with an integer type like uid_t whose width and signedness vary from platform to platform.

Furthermore, many C applications pervasively assume wraparound behavior and typically it is not easy to find and remove all these assumptions. Hence it is often useful to maintain nonstandard code that assumes wraparound on overflow, instead of rewriting the code. The rest of this section attempts to give practical advice for this situation.

If your code wants to detect signed integer overflow in sum = a + b, it is generally safe to use an expression like (sum < a) != (b < 0).

If your code uses a signed loop index, make sure that the index cannot overflow, along with all signed expressions derived from the index. Here is a contrived example of problematic code with two instances of overflow.

for (i = INT_MAX - 10; i <= INT_MAX; i++)
  if (i + 1 < 0)
    {
      report_overflow ();
      break;
    }

Because of the two overflows, a compiler might optimize away or transform the two comparisons in a way that is incompatible with the wraparound assumption.

If your code uses an expression like (i * 2000) / 1000 and you actually want the multiplication to wrap around on overflow, use unsigned arithmetic to do it, e.g., ((int) (i * 2000u)) / 1000.

If your code assumes wraparound behavior and you want to insulate it against any GCC optimizations that would fail to support that behavior, you should use GCC’s -fwrapv option, which causes signed overflow to wrap around reliably (except for division and remainder, as discussed in the next section).

If you need to port to platforms where signed integer overflow does not reliably wrap around (e.g., due to hardware overflow checking, or to highly aggressive optimizations), you should consider debugging with GCC’s -ftrapv option, which causes signed overflow to raise an exception.