Undefined behavior: signed overflow in C

Signed overflow is undefined, so the compiler may assume it never happens.

#include <limits.h>

/* The compiler may assume a + 1 > a always holds. */
int broken_check(int a) {
    return a + 1 > a;
}

/* Correct: ask before the arithmetic happens. */
int safe_check(int a) {
    return a < INT_MAX;
}

unsigned int wraps_defined(unsigned int a) {
    return a + 1u;
}

How it works

1. Unsigned arithmetic wraps and is well defined.
2. Signed overflow lets the optimizer delete your check.
3. So test before the addition, never after it.

Keywords and builtins used here

• broken_check
• int
• return
• safe_check
• unsigned
• wraps_defined

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Step 1 of 3 in Undefined behavior, step 8 of 13 in Unions, bitfields & undefined behavior.

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