c - unsigned char output of a signed char

Question

I have the following code:

char x = -1;
int y = x;

printf("%u
", x);
printf("%u
", y);

The output is:

4294967295
4294967295

I dont understand why x can get such a value. I know that the maximum value of a unsigned char is 255 and for a signed char 127. How can it be 4294967295?

Answer 1

For functions like printf that use variadic arguments, any integral types smaller than an int (char and short) are implicitly promoted to int. The same is true with floating-point numbers, float is promoted to double.

Hence, your char is being sign-extended to an int with value -1, and since you are printing it as unsigned, in 2's complement you get UINT_MAX.

Edit: as chux notes below, if your char defaulted to unsigned (this depends on your compiler/platform), the answer would be 255 instead. When promotion occurs, the value will be zero-extended instead of sign-extended.