windows下的两个等待函数

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。

另外一种：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));

//这个宏内部事实上是记录并解释了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，不必检測警告。

/*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;

}