Question

It is little bit weird. I just play with the unsigned char type and negative values. I have the following code.

 #include <stdio.h>

 int main(int argc, char* agrv[]){
   unsigned char c = -3;
   printf("%d, %u, %d, %u\n", c, c, ~c, ~c);
 }

The output is,

253, 253, -254, 4294967042

I can not figure out the last three values. What does %d and %u really do?

Answer 1

The %d format prints out an int, and %u prints out an unsigned int. All arithmetic on unsigned char values is done by first casting them to int and doing the operations on int values, and so ~c (which is equal to -1 - (int)c) will return a negative int value. An explicit cast would be needed to get the unsigned char result before printing it out (and the call to printf would cast it back to int anyway).