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            編城浪子的博客
            游戲開發、圖形引擎

            最近一段時期準備溫習多線程同步相關內容,主要包括關鍵代碼段和各種內核同步對象。先學習最簡單的同步方法--Critical Section。所謂關鍵代碼段,是指給一段代碼加上鎖,要執行這段代碼,必須拿到一把鑰匙,而且這把鑰匙世界上僅存一把,獨一無二。

            如下代碼所示:

            CRITICAL_SECTION g_cs;

            EnterCriticalSection(&g_cs);

            //add your critical code here

            ....

            LeaveCriticalSection(&g_cs);

            在執行...這段代碼前,必須確保g_cs沒有被其他線程占有,否則線程就在此處掛起。通過這種方式可以保護某個資源,不被多個線程同時訪問。

            以下代碼演示了兩個線程在使用Critical Section和不使用的條件下的輸出結果:

            // stdafx.h : 標準系統包含文件的包含文件,

            // 或是經常使用但不常更改的

            // 特定于項目的包含文件

            #pragma once

            #define WIN32_LEAN_AND_MEAN                                       // Windows 頭中排除極少使用的資料

            #include <stdio.h>

            #include <tchar.h>

            #include <iostream>

             

            // critical_section.cpp : 定義控制臺應用程序的入口點。

            //

             

            #include "stdafx.h"

            #include <Windows.h>

             

            using namespace std;

             

            CRITICAL_SECTION g_cs;

            int g_var = 0;

             

            DWORD WINAPI ThreadProc(LPVOID lpParam)

            {

                                cout << "child thread started!\n";

                                for(int i=0;i<30;i++)

                                {

                                                     //EnterCriticalSection(&g_cs);

                                                     cout << "The " << "child ";

                                                     cout << "thread " << "enter ";

                                                     cout << "the CS!" << "\n";

                                                     g_var++;

                                                     cout << "g_var added by 1: g_var = " << g_var << endl;

                                                     cout << "The " << "child " << "thread " << "leave " << "the CS!" << "\n";

                                                     //LeaveCriticalSection(&g_cs);

                                }

                                cout << "child thread ended!\n";

                                return 0;

            }

             

            int _tmain(int argc, _TCHAR* argv[])

            {

                                cout << "main thread started!\n";

                                InitializeCriticalSection(&g_cs);

                                DWORD tid;

             

                                HANDLE hThread = CreateThread(0, 0, ThreadProc, 0, 0, &tid);

             

                                for(int i=0;i<30;i++)

                                {

                                                     //EnterCriticalSection(&g_cs);

                                                     cout << "The " << "main " << "thread ";

                                                     cout << "enter " << "the CS!" << "\n";

                                                     g_var--;

                                                     cout << "g_var minused by 1: g_var = " << g_var << endl;

                                                     cout << "The " << "main " << "thread ";

                                                     cout << "enter " << "the CS! " << "\n";

                                                     //LeaveCriticalSection(&g_cs);

                                }

             

                                WaitForSingleObject(hThread, INFINITE);

             

                                CloseHandle(hThread);

             

                                DeleteCriticalSection(&g_cs);

             

                                cout << "main thread ended!\n";

                                return 0;

            }

            將如下代碼的注釋去掉即變成了使用Critical Section的實例。

            //EnterCriticalSection(&g_cs);

            //LeaveCriticalSection(&g_cs);

            EnterCriticalSection()的缺點是如果g_cs仍未被其他線程釋放,就一直在此處掛起,如果不想線程掛起,可以用替代函數:

            BOOL WINAPI TryEnterCriticalSection(

              __inout  LPCRITICAL_SECTION lpCriticalSection

            );

            下一章將講述互斥--Mutex的使用。

            posted on 2008-10-30 23:10 zengfancy 閱讀(517) 評論(0)  編輯 收藏 引用
             
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