Question

I am new to bitwise operation. I have the basic concepts of AND, OR, XOR, and 2s complement. However I came across following piece of code and am unable to figure out the output.

char c1 = 0xFF; // -1
int shifted = c1 << 8; //-256 (-1 * 256)
printf("%d, %x\n", shifted, shifted);

int myInt;
myInt = 0xFFFFFFE2;
printf("%d\n", myInt);

int i = 0xff;
printf("%d\n", i<<2);

The output is:

-256, ffffff00
-30
1020

Please help me understand what’s going on here!

Answer 1

char c1 = 0xFF; // -1
int shifted = c1 << 8; //-256 (-1 * 256)

c1 is promoted to int for the shift, so it’s still -1, shifting negative ints is implementation-defined, but your implementation seems to do like most and shifts it as if it were an unsigned bit-pattern, so a left-shift by eight places is multiplication by 256.

printf( "%d, %x\n", shifted, shifted );
  -256, ffffff00

as expected. The bit pattern of -256 in two’s complement is 0xFFFFFF00 (32-bit ints).

int myInt;
myInt = 0xFFFFFFE2;

That bit pattern is -30 in two’s complement

printf("%d\n",myInt);

int i = 0xff ;

This is 255, 255*4 = 1020

printf("%d\n", i<<2);

  -30
  1020