Put an `unsigned char` into a `char`

Question

I would like to store an unsigned char into a char by means of a shift. As the two data types have the same length (1 byte on my machine), I would have expected the following code to work:

#include <iostream>
#include <cstring>
#include <cstdio>

using namespace std;

int main () {

        printf ("%d\n", sizeof(char));
        printf ("%d\n", sizeof(unsigned char));

        unsigned char test = 49;
        char testchar = (char) (test - 127);
        printf ("%x\n", testchar);

        return 0;
}

but it doesn't. In particular, I got the following output:

1
1
ffffffb2

that suggests that the char has been casted to int. Does anybody has an explanation and, hopefully, a solution?

Answer 1

%x is a specifier for a 4-byte int. To print one byte char use %hhx.

printf typecasts its arguments according to the format specifiers passed to it.That is why testchar was type promoted to int.

Answer 2

printf is a variable argument function, and as such it's arguments are subject to default promotion rules. For this case, your char is promoted to an int, and in that process is sign extended. A 2's complement int of 4 bytes with the binary pattern 0xffffffb2 is -78. Print it as a char with the %hhx specifier.

See also Which integral promotions do take place when printing a char?