C integer overflow behaviour when assigning to larger-width integers

Question

If I execute the following code in C:

#include <stdint.h>

uint16_t a = 4000;
uint16_t b = 8000;

int32_t c = a - b;

printf("%d", c);

It correctly prints '-4000' as the result. However, I'm a little confused: shouldn't there be an arithmetic overflow when subtracting a larger unsigned integer from the other? What casting rules are at play here? This question seems a bit noobish, so any references would be greatly appreciated.

Answer 1

The issue is actually somewhat complicated. Operands of arithmetic expressions are converted using specific rules that you can see in Section 3.2.1.5 of the Standard (C89). In your case, the answer depends on what the type uint16_t is. If it is smaller than int, say short int, then the operands are converted to int and you get -4000, but on a 16-bit system, uint16_t could be unsigned int and conversion to a signed type would not happen automatically.