Difference between '(unsigned)1' and '(unsigned)~0'

Question

What is the difference between (unsigned)~0 and (unsigned)1 . Why is unsigned of ~0 is -1 and unsigned of 1 is 1 ? Does it have something to do with the way unsigned numbers are stored in the memory. Why does an unsigned number give a signed result. It didn't give any overflow error either. I am using GCC compiler:

#include<sdio.h>
main()
{
 unsigned int x=(unsigned)~0; 
 unsigned int y=(unsigned)1; 
 printf("%d\n",x); //prints -1
 printf("%d\n",y); //prints 1
}

Answer 1

Because %d is a signed int specifier. Use %u .

which prints 4294967295 on my machine.

As others mentioned, if you interpret the highest unsigned value as signed, you get -1, see the wikipedia entry for two's complement .

Answer 2

Your system uses two's complement representation of negative numbers. In this representation a binary number composed of all ones represent the biggest negative number -1 .

Since inverting all bits of a zero gives you a number composed of all ones, you get -1 when you re-interpret the number as a signed number by printing it with a %d which expects a signed number, not an unsigned one.

Answer 3

First, in your use of printf you are telling it to print the number as signed ("%d") instead of unsigned ("%u").

Second, you are right in that it has "something to do with the way numbers are stored in memory". An int (signed or unsigned) is not a single bit on your computer, but a collection of k bits. The exact value of k depends on the specifics of your computer architecture, but most likely you have k=32.

For the sake of succinctness, lets assume your ints are 8 bits long, so k=8 (this is most certainly not the case, unless you are working on a very limited embedded system,). In that case (int)0 is actually 00000000, and (int)~0 (which negates all the bits) is 11111111.

Finally, in two's complement (which is the most common binary representation of signed numbers), 11111111 is actually -1. See http://en.wikipedia.org/wiki/Two 's_complement for a description of two's complement.

If you changed your print to use "%u", then it will print a positive integer that represents (2^k-1) where k is the number of bits in an integer (so probably it will print 4294967295).

Answer 4

printf() only knows what type of variable you passed it by what format specifiers you used in your format string. So what's happening here is that you're printing x and y as signed integers, because you used %d in your format string. Try %u instead, and you'll get a result more in line with what you're probably expecting.