The problem in the article is something like this: int main() { unsigned char foo = 0x80; unsigned int x = foo << 24; } and with ubsan

The problem in the article is something like this: int main() { unsigned char foo = 0x80; unsigned int x = foo << 24; } and with ubsan enabled it says /app/example.c:5:30: runtime error: left shift of 128 by 24 places cannot be represented in type 'int' SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /app/example.c:5:30 in What's non-obvious is also the rules around arithmetic. Types unsigned char and unsigned int are mentioned in my snippet but the arithmetic - the `<<` operator - performed using the type int.