Can't understand unsigned int type in C

Question

I'm starting to program in C and I'm having a problem to understand some results I'm getting. I'll paste the code here:

#include <stdio.h>
unsigned int main(void)
{
    unsigned int x = 0;

    printf("%u\n",x-1);
    return 0;
}

The terminal is returning 4.294.967.295, and I'm not getting why. I know that this value is the max value of a unsigned int, but actually I was expecting some warning from the compiler that I would have to use a int type not an unsigned int because the result is negative. Anyway, can someone help me?

Answer 1

When you use an unsigned type all of the bits are used to represent non-negative values ( i.e., >= 0). Think of this as the values wrapping around, so when you decrement below the lower limit (0), you wrap around to the largest value (with 32 bits, 2^32 - 1).

Also, note you did not assign a negative value to variable x, you just provided an expression that subtracted 1 from it. I.e.,

x-1

vs

x = x - 1;

though whether you would have gotten a warning about this would probably depend on the compiler and the warning levels set.