Why is there an unsigned int below zero?

Question

I just tried to lower the unsigned int below 0. To my surprise, this works!

#include<stdio.h>

int main(void)
{
        unsigned int foo = 5;
        foo -= 10;
        printf("%d", foo);
        return 0;
}

Compiled with

clang -Weverything main.c

This program returns

-5

As this post and my personal knowledge say, this is not possible. But why does this work? Am I missing something? Is It Due To Undefined Behavior? Or is it printf? Or something else?

Answer 1

This program uses printfto interpret the value unsigned intas a signed integer. Although this is not a problem when a value is unsigned intalso suitable for int, this behavior is undefined:

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Answer 2

printf() foo . % d % u.

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Answer 3

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