用QueryPerformanceCounter实现Windows XP 下微秒级延时

windows的Sleep函数，睡眠线程指定毫秒数，可以用来做毫秒延时。

对于微秒延时，没有一个现成的函数，但是可以通过

QueryPerformanceFrequency

QueryPerformanceCounter

来间接实现。原理就是用循环查询的方式不断调用QueryPerformanceCounter（在Winbase.h中）

LARGE_INTEGER freq;
LARGE_INTEGER start, end;

QueryPerformanceFrequency(&freq);

unsigned int us = 100000;
LONGLONG count = (us * freq.QuadPart) / (1000 * 1000);

QueryPerformanceCounter(&start);
   count = count + startQuadPart ;

do
{
   QueryPerformanceCounter(&end);
}while(end.QuadPart< count);

printf("us %u, elaps %u ms\r\n", us, tpend-tpstart);