Where C Meets the Wheel

Ten small C programs that go wrong at the edge of the integer range, each stepped through one ARM instruction at a time with the registers drawn as bits before and after and the N Z C V flags shown. Everything is at 8 bits with int8_t and uint8_t so every bit is on screen; the 32-bit equivalents are named where they differ.

1. The loop that never ends. An unsigned countdown: 0 − 1 is 255, and 255 ≥ 0.

2. 127 + 1. The largest signed value plus one becomes −128, with V set and C clear.

3. 200 into an int8_t. No arithmetic, only a narrowing store: the bits stay 0xC8 and now read as −56.

4. −(−128). Negating the most negative value gives it back, with V set. The abs() trap.

5. Parsing −128. A parser that accumulates positively and negates at the end, and the one input whose positive form never fitted.

6. Detecting a wrap in C. C cannot read the carry flag, so the idiom d > a asks a question only a wrap can answer yes.

7. %d and %u. One register printed under both readings; nothing is computed, the format chooses what the top bit is worth.

8. INT_MIN / −1. The one division that overflows: ARM’s SDIV hands back the same pattern, x86’s IDIV raises an exception.

9. The check that vanished. if (x + 1 > x) compiled with optimisation contains no add and no compare; the compiler assumed the undefined case away. -fwrapv brings it back.

10. 255 + 1, and detecting it. The carry out of the top bit, and the s < a idiom that reveals it from C.

Read it any time after From Adding to Subtracting, whose collapsed “In C” notes these scenarios expand.