C/C++: runtime error: signed integer overflow / null pointer / shift exponent too large (UndefinedBehaviorSanitizer)

Meaning

Undefined behaviour lets the optimiser assume it never happens, so bugs surface only at -O2, on other compilers, or after a compiler upgrade. UBSan reports file:line:col: runtime error: <what>. Fix each report rather than silencing it.

Common causes

• Signed integer overflow (INT_MAX + 1)
• Null pointer dereference or member access through null
• Array index out of bounds or shift by a negative or too large count
• Misaligned access, invalid enum or bool value
• Division by zero or INT_MIN / -1

Detailed fix by platform

Code examples

#include <climits>
#include <iostream>

int add_checked(int a, int b) {
    if ((b > 0 && a > INT_MAX - b) || (b < 0 && a < INT_MIN - b)) {
        throw std::overflow_error("int overflow");       // instead of undefined behaviour
    }
    return a + b;
}

int main() { std::cout << add_checked(INT_MAX, 1) << "\n"; }

How to diagnose

1. Message — Which kind of undefined behaviour?
2. Line — file:line:col from the report?
3. Inputs — Which values trigger it?

More related errors