unsigned variable is behaving like signed [duplicate]

Question

This is the code,

#include <stdio.h>                                                                                                                     
int main()
{
    unsigned int i = 0xFFFFFFFF;
    if (i == -1)
        printf("signed variable\n");
    else
        printf("unsigned variable\n");
    return 0;
}

This is the output,

signed variable

Why is i's value -1 even it is declared as unsigned? Is it something related to implicit type conversations?

This is the build environment,

Ubuntu 14.04, GCC 4.8.2

Answer 1

The == operator causes its operands to be promoted to a common type according to C's promotion rules. Converting -1 to unsigned yields UINT_MAX.

Answer 2

i's value is 0xFFFFFFFF, which is exactly the same as -1, at least when the later is converted to an unsigned integer. And this is exactly what is happening with the comparison operators:

If both of the operands have arithmetic type, the usual arithmetic conversions are performed. [...]

[N1570 $6.5.9/4]

-1 in two's complement is "all bits set", which is also what 0xFFFFFFFF for an unsigned int (of size 4) is.