windows下的两个等待技术
第一种： Win32 Sleep()函数
这个函数要求操作系统中止线程动作，直到读过某个指定的时间之后才恢复。能在某个线程结束时(而不是某段时间结束时）被调用。
第二种：busy loop(busy waits)
不断调用GetExitCodeThread(),直到其结果不再是STILL_ACTIVE.
缺点：浪费CPU时间。
绝对不要在Win32中使用busy loop
//busywait.c
/*Domonstrate the effect on performance of using a busy loop.
First call the worker routine with just a function call to get a baseline performance reading
then create a second thread and a busy loop.
*/
#define WIN32_LEAN_AND_MEAN
#include <stdio.h>
#include<stdlib.h>
#include<windows.h>
#include<time.h>
#include "MtVerify.h"
DWORD WINAPI ThreadFunc(LPVOID);
int main()
{
HANDLE hThrd;
DWORD exitCode = 0;
DWORD threadId;
DWORD begin;
DWORD elapsed;
puts("TImiing normal function call.....");
begin = GetTickCount();//示以毫秒为单位的计算机启动后经历的时间间隔。
ThreadFunc(0);
elapsed = GetTickCount() - begin;
printf("Function call took:%d.%.03d seconds\n\n", elapsed / 1000, elapsed % 1000);
puts("Timing thread + busy loop....");
begin = GetTickCount();
MTVERIFY(hThrd = CreateThread(NULL, 0, ThreadFunc, (LPVOID)1, 0, &threadId));
//这个宏内部其实是记录并解释了Win32 GetLastError()的结果。
/*This busy loop chews up lots of CPU time*/
for (;;)
{
GetExitCodeThread(hThrd, &exitCode);
if (exitCode != STILL_ACTIVE)
break;
}
elapsed = GetTickCount() - begin;
printf("Thread+busy loop took: %d.%.03d seconds\n", elapsed / 1000, elapsed % 1000);
MTVERIFY(CloseHandle(hThrd));
return EXIT_SUCCESS;
}
/*Cute little busy work routine that computes the value
of PI using probability.Highly dependent on having a good random number generator (rand is iffy)
*/
DWORD WINAPI ThreadFunc(LPVOID n)
{
int i;
int inside = 0;
double val;
UNREFERENCED_PARAMETER(n);//告诉编译器，已经使用了该变量，不必检测警告！
/*Seed the random-number generator.*/
srand((unsigned)time(NULL));
for (i = 0; i < 1000000; i++)
{
double x = (double)(rand()) / RAND_MAX;
double y = (double)(rand()) / RAND_MAX;
if ((x*x + y*y) <= 1.0)
inside++;
}
val = (double)inside / i;
printf("PI=%.4g\n", val * 4);
return 0;
}
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