You are seeing the ffffff
because char
is signed on your system. In C, vararg functions such as printf
will promote all integers smaller than int
to int
. Since char
is an integer (8-bit signed integer in your case), your chars are being promoted to int
via sign-extension.
Since c0
and 80
have a leading 1-bit (and are negative as an 8-bit integer), they are being sign-extended while the others in your sample don’t.
char int
c0 -> ffffffc0
80 -> ffffff80
61 -> 00000061
Here’s a solution:
char ch = 0xC0;
printf("%x", ch & 0xff);
This will mask out the upper bits and keep only the lower 8 bits that you want.