QueryPerformanceCounter实现Windows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QueryPerformanceFrequencyQueryPerformanceCounter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);