A sequence point is a language-defined point that serializes execution: evaluations and side effects before it are completed or ordered relative to those after it. Code between sequence points may be evaluated in different orders, interleaved, or governed by other sequencing constraints. A semicolon separating statements provides the simplest example. section 19.2
Sequence points do not necessarily choose an order between expressions; they require that some order exists. Function argument expressions are unsequenced, while function calls can impose sequence points. These relaxed rules support compiler optimization and avoid imposing unnecessary evaluation orders. section 19.2
Two valid outcomes
In printf("...", add(&a, &b), add(&b, &a)), either add call may execute first. Starting with a = 3 and b = 5 therefore permits a = 8, b = 13 or a = 11, b = 8. section 19.2
unsigned add(unsigned* x, unsigned const* y) [[unsequenced]] {
return *x += *y;
}
int main(void) {
unsigned a = 3;
unsigned b = 5;
printf("a = %u, b = %u\n", add(&a, &b), add(&b, &a));
}
Inspect the modifying add function and its two unsequenced uses as printf arguments. section 19.2
a = 8, b = 13
See the output produced when the first add call executes before the second. section 19.2
a = 11, b = 8
See the alternative output produced when the second add call executes first. section 19.2
