Thread: stumbled on this:

Let me try to explain what is happening...
Here's code:

Code:

#include <stdio.h>
#include <stdlib.h>


int main(void)
{
        unsigned int var1 = 5;
        char tmp;
        tmp = ~(char)var1;
        unsigned int var2 = tmp;
        
        printf("\nvar1 = %u, var2 = %u, tmp = %d",var1,var2,tmp);
        system("pause");
}

My output:

var 1 = 5, var2= 4294967290, tmp = -6

Why?

OK, on most machines (32 bit) unsigned it is 4 bytes (you can check with sizeof operator) and number 5 is represented like:

00000000 00000000 00000000 00000101

now when you cast it to char (1 byte) you get:

00000101
Now when you complement it you get

11111010

Since you tmp is defined as signed (default signed on most compilers) this is interpreted in this way:
first bit is signed bit if it is 1 tthen nubmer is negative,since it's two's complement number is then
1111010 this must be complemented first
0000101
and 1 is added so finaly

Code:

  0000101
+0000001
-------------
  0000110

So number is -6.

Now, in line:

Code:

 unsigned int var2 = tmp;

you there is a promotion form char to unsigned int.
Since tmp is interprated as negative number because the most significat bit is 1 it will be promoted like:

11111111 11111111 11111111 11111010

and that is var2= 4294967290.

So, basically your question is about char to int promotion. I strongly recommend you reading FAQ. If you want to get result 250 you'll need to use something like this:

Code:

 unsigned int var2 = (unsigned char)(~(char)var1);

Hope this helps.

- Micko