1、统计一个无符号整数的二进制表示中1的个数，函数原型是int countbit(unsigned int x);

#include<stdio.h>

int countbit(unsigned int x)
{
	int i = 0;
	int count = 0;
	int mask = 0x00000001;
	for (i = 0; i < 32; i++) {
		if ( 1 == (x >> i & mask))
			count++;
	}
	return count; 
}

int main()
{
	printf ("%d\n",countbit (0x23));
}

#include<stdio.h>

unsigned int multiply(unsigned int x, unsigned int y)
{
	int i;
	int result = 0;
	int mask = 0x00000001;
	for (i = 0; i < 32; i++) {
			if ( 1 == (x >> i & mask))
				result = result + (y << i);
	}
	return result;
}

int main()
{
	printf ("%d\n",multiply (9, 2));
}

3、对一个32位无符号整数做循环右移，函数原型是unsigned int rotate_right(unsigned int x, int n);。所谓循环右移就是把低位移出去的部分再补到高位上去，例如rotate_right(0xdeadbeef, 8)的值应该是0xefdeadbe。

#include<stdio.h>

unsigned int rotate_right(unsigned int x, int n)
{
	int i;
	int mask = 1;
	for (i = 1; i < n; i++) {
		mask = ((mask << 1) | 1);
	}
	unsigned int y = (x >> n) | ((x & mask) << (32 - n));
	return y;
}

int main()
{
	printf ("%x\n",rotate_right(0xdeadbeef, 8));
	return 0;
}