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標題: C++多執行緒（二） [打印本頁]

作者: f66666602 時間: 2007-8-14 04:50
標題: C++多執行緒（二）
一簡單實例（來自codeprojct：http://www.codeproject.com/useritems/MultithreadingTutorial.asp）
主執行緒創建2個執行緒t1和t2，創建時2個執行緒就被掛起，後來調用ResumeThread恢復2個執行緒，是其開始執行，調用WaitForSingleObject等待2個執行緒執行完，然後推出主執行緒即結束進程。

/**//* file Main.cpp

* This program is an adaptation of the code Rex Jaeschke showed in

* Listing 1 of his Oct 2005 C/C++ User's Journal article entitled

* "C++/CLI Threading: Part I". I changed it from C++/CLI (managed)

* code to standard C++.

* One hassle is the fact that C++ must employ a free (C) function

* or a static class member function as the thread entry function.

* This program must be compiled with a multi-threaded C run-time

* (/MT for LIBCMT.LIB in a release build or /MTd for LIBCMTD.LIB

* in a debug build).

* John Kopplin 7/2006

#include <stdio.h>

#include <string>
// for STL string class

#include <windows.h>
// for HANDLE

#include <process.h>
// for _beginthread()

using
namespace std;

class ThreadX

{

private:

int loopStart;

int loopEnd;

int dispFrequency;

public:

string threadName;

ThreadX( int startValue、int endValue、int frequency )

{

loopStart = startValue;

loopEnd = endValue;

dispFrequency = frequency;

}

// In C++ you must employ a free (C) function or a static

// class member function as the thread entry-point-function.

// Furthermore、_beginthreadex() demands that the thread

// entry function signature take a single (void*) and returned

// an unsigned.

static unsigned __stdcall ThreadStaticEntryPoint(void
* pThis)

{

ThreadX * pthX = (ThreadX*)pThis; // the tricky cast

pthX->ThreadEntryPoint(); // now call the true entry-point-function

// A thread terminates automatically if it completes execution,

// or it can terminate itself with a call to _endthread().

return
1; // the thread exit code

}

void ThreadEntryPoint()

{

// This is the desired entry-point-function but to get

// here we have to use a 2 step procedure involving

// the ThreadStaticEntryPoint() function.

for (int i = loopStart; i <= loopEnd; ++i)

{

if (i % dispFrequency ==
0)

{

printf( "%s: i = %d\n"、threadName.c_str()、i );

}

printf( "%s thread terminating\n"、threadName.c_str() );

}

};

int main()

{

// All processes get a primary thread automatically. This primary

// thread can generate additional threads. In this program the

// primary thread creates 2 additional threads and all 3 threads

// then run simultaneously without any synchronization. No data

// is shared between the threads.

// We instantiate an object of the ThreadX class. Next we will

// create a thread and specify that the thread is to begin executing

// the function ThreadEntryPoint() on object o1. Once started,

// this thread will execute until that function terminates or

// until the overall process terminates.

ThreadX * o1 =
new ThreadX( 0、1、2000 );

// When developing a multithreaded WIN32-based application with

// Visual C++、you need to use the CRT thread functions to create

// any threads that call CRT functions. Hence to create and terminate

// threads、use _beginthreadex() and _endthreadex() instead of

// the Win32 APIs CreateThread() and EndThread().

// The multithread library LIBCMT.LIB includes the _beginthread()

// and _endthread() functions. The _beginthread() function performs

// initialization without which many C run-time functions will fail.

// You must use _beginthread() instead of CreateThread() in C programs

// built with LIBCMT.LIB if you intend to call C run-time functions.

// Unlike the thread handle returned by _beginthread()、the thread handle

// returned by _beginthreadex() can be used with the synchronization APIs.

HANDLE hth1;

unsigned uiThread1ID;

hth1 = (HANDLE)_beginthreadex( NULL, // security

0, // stack size

ThreadX::ThreadStaticEntryPoint,

o1, // arg list

CREATE_SUSPENDED, // so we can later call ResumeThread()

&uiThread1ID );

if ( hth1 ==
0 )

printf("Failed to create thread 1\n");

DWORD dwExitCode;

GetExitCodeThread( hth1、&dwExitCode ); // should be STILL_ACTIVE = 0x00000103 = 259

printf( "initial thread 1 exit code = %u\n"、dwExitCode );

// The System::Threading::Thread object in C++/CLI has a "Name" property.

// To create the equivalent functionality in C++ I added a public data member

// named threadName.

o1->threadName =
"t1";

ThreadX * o2 =
new ThreadX( -1000000、0、2000 );

HANDLE hth2;

unsigned uiThread2ID;

hth2 = (HANDLE)_beginthreadex( NULL, // security

0, // stack size

ThreadX::ThreadStaticEntryPoint,

o2, // arg list

CREATE_SUSPENDED, // so we can later call ResumeThread()

&uiThread2ID );

if ( hth2 ==
0 )

printf("Failed to create thread 2\n");

GetExitCodeThread( hth2、&dwExitCode ); // should be STILL_ACTIVE = 0x00000103 = 259

printf( "initial thread 2 exit code = %u\n"、dwExitCode );

o2->threadName =
"t2";

// If we hadn't specified CREATE_SUSPENDED in the call to _beginthreadex()

// we wouldn't now need to call ResumeThread().

ResumeThread( hth1 ); // serves the purpose of Jaeschke's t1->Start()

ResumeThread( hth2 );

// In C++/CLI the process continues until the last thread exits.

// That is、the thread's have independent lifetimes. Hence

// Jaeschke's original code was designed to show that the primary

// thread could exit and not influence the other threads.

// However in C++ the process terminates when the primary thread exits

// and when the process terminates all its threads are then terminated.

// Hence if you comment out the following waits、the non-primary

// threads will never get a chance to run.

WaitForSingleObject( hth1、INFINITE );

WaitForSingleObject( hth2、INFINITE );

GetExitCodeThread( hth1、&dwExitCode );

printf( "thread 1 exited with code %u\n"、dwExitCode );

GetExitCodeThread( hth2、&dwExitCode );

printf( "thread 2 exited with code %u\n"、dwExitCode );

// The handle returned by _beginthreadex() has to be closed

// by the caller of _beginthreadex().

CloseHandle( hth1 );

CloseHandle( hth2 );

delete o1;

o1 = NULL;

delete o2;

o2 = NULL;

printf("Primary thread terminating.\n");

}

二解釋
1）如果你正在編寫C/C++代碼，決不應該調用CreateThread。相反，應該使用VisualC++運行期庫函數_beginthreadex，推出也應該使用_endthreadex。如果不使用Microsoft的VisualC++編譯器，你的編譯器供應商有它自己的CreateThred替代函數。不管這個替代函數是什麼，你都必須使用。
2）因為_beginthreadex和_endthreadex是CRT執行緒函數，所以必須注意編譯選項runtimelibaray的選擇，使用MT或MTD。
3)_beginthreadex函數的參數列表與CreateThread函數的參數列表是相同的，但是參數名和類型並不完全相同。這是因為Microsoft的C/C++運行期庫的開發小組認為，C/C++運行期函數不應該對Windows資料類型有任何依賴。_beginthreadex函數也像CreateThread那樣，返回新創建的執行緒的句柄。
下面是關於_beginthreadex的一些要點：
．每個執行緒均獲得由C/C++運行期庫的堆棧分配的自己的tiddata記憶體結構。（tiddata結構位於Mtdll.h檔案中的VisualC++來源碼中）。
．傳遞給_beginthreadex的執行緒函數的地址保存在tiddata記憶體塊中。傳遞給該函數的參數也保存在該資料塊中。
．_beginthreadex確實從內部調用CreateThread，因為這是操作系統瞭解如何創建新執行緒的唯一方法。
．當調用CreatetThread時，它被告知通過調用_threadstartex而不是pfnStartAddr來啟動執行新執行緒。還有，傳遞給執行緒函數的參數是tiddata結構而不是pvParam的地址。
．如果一切順利，就會像CreateThread那樣返回執行緒句柄。如果任何操作失敗了，便返回NULL。
4) _endthreadex的一些要點：
．C運行期庫的_getptd函數內部調用操作系統的TlsGetValue函數，該函數負責檢索調用執行緒的tiddata記憶體塊的地址。
．然後該資料塊被釋放，而操作系統的ExitThread函數被調用，以便真正撤消該執行緒。當然，退出代碼要正確地設置和傳遞。
5)雖然也提供了簡化版的的_beginthread和_endthread，但是可控制性太差，所以一般不使用。
6）執行緒handle因為是內核對象，所以需要在最後closehandle。
7）更多的API：HANDLE GetCurrentProcess();HANDLE GetCurrentThread();DWORDGetCurrentProcessId();DWORD GetCurrentThreadId()。DWORDSetThreadIdealProcessor(HANDLE hThread,DWORD dwIdealProcessor);BOOLSetThreadPriority(HANDLE hThread,int nPriority);BOOLSetPriorityClass(GetCurrentProcess(), IDLE_PRIORITY_CLASS);BOOLGetThreadContext(HANDLE hThread,PCONTEXT pContext);BOOLSwitchToThread();

三注意
1）C++主執行緒的終止，同時也會終止所有主執行緒創建的子執行緒，不管子執行緒有沒有執行完畢。所以上面的代碼中如果不調用WaitForSingleObject，則2個子執行緒t1和t2可能並沒有執行完畢或根本沒有執行。
2）如果某執行緒掛起，然後有調用WaitForSingleObject等待該執行緒，就會導致死鎖。所以上面的代碼如果不調用resumethread，則會死鎖。

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