C/C++ Runtime 多線程函數(shù)
一 簡單實(shí)例（來自codeprojct：http://www.codeproject.com/useritems/MultithreadingTutorial.asp）
主線程創(chuàng)建2個(gè)線程t1和t2，創(chuàng)建時(shí)2個(gè)線程就被掛起，后來調(diào)用ResumeThread恢復(fù)2個(gè)線程，是其開始執(zhí)行，調(diào)用WaitForSingleObject等待2個(gè)線程執(zhí)行完，然后推出主線程即結(jié)束進(jìn)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(yīng)該調(diào)用CreateThread。相反，應(yīng)該使用VisualC++運(yùn)行期庫函數(shù)_beginthreadex，推出也應(yīng)該使用_endthreadex。如果不使用Microsoft的VisualC++編譯器，你的編譯器供應(yīng)商有它自己的CreateThred替代函數(shù)。不管這個(gè)替代函數(shù)是什么，你都必須使用。
2）因?yàn)開beginthreadex和_endthreadex是CRT線程函數(shù)，所以必須注意編譯選項(xiàng)runtimelibaray的選擇，使用MT或MTD。
3) _beginthreadex函數(shù)的參數(shù)列表與CreateThread函數(shù)的參數(shù)列表是相同的，但是參數(shù)名和類型并不完全相同。這是因?yàn)镸icrosoft的C/C++運(yùn)行期庫的開發(fā)小組認(rèn)為，C/C++運(yùn)行期函數(shù)不應(yīng)該對(duì)Windows數(shù)據(jù)類型有任何依賴。_beginthreadex函數(shù)也像CreateThread那樣，返回新創(chuàng)建的線程的句柄。
下面是關(guān)于_beginthreadex的一些要點(diǎn)：
•每個(gè)線程均獲得由C/C++運(yùn)行期庫的堆棧分配的自己的tiddata內(nèi)存結(jié)構(gòu)。（tiddata結(jié)構(gòu)位于Mtdll.h文件中的VisualC++源代碼中）。
•傳遞給_beginthreadex的線程函數(shù)的地址保存在tiddata內(nèi)存塊中。傳遞給該函數(shù)的參數(shù)也保存在該數(shù)據(jù)塊中。
•_beginthreadex確實(shí)從內(nèi)部調(diào)用CreateThread，因?yàn)檫@是操作系統(tǒng)了解如何創(chuàng)建新線程的唯一方法。
•當(dāng)調(diào)用CreatetThread時(shí)，它被告知通過調(diào)用_threadstartex而不是pfnStartAddr來啟動(dòng)執(zhí)行新線程。還有，傳遞給線程函數(shù)的參數(shù)是tiddata結(jié)構(gòu)而不是pvParam的地址。
•如果一切順利，就會(huì)像CreateThread那樣返回線程句柄。如果任何操作失敗了，便返回NULL。
4) _endthreadex的一些要點(diǎn)：
•C運(yùn)行期庫的_getptd函數(shù)內(nèi)部調(diào)用操作系統(tǒng)的TlsGetValue函數(shù)，該函數(shù)負(fù)責(zé)檢索調(diào)用線程的tiddata內(nèi)存塊的地址。
•然后該數(shù)據(jù)塊被釋放，而操作系統(tǒng)的ExitThread函數(shù)被調(diào)用，以便真正撤消該線程。當(dāng)然，退出代碼要正確地設(shè)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）線程handle因?yàn)槭莾?nèi)核對(duì)象，所以需要在最后closehandle。
7）更多的API：HANDLE GetCurrentProcess();HANDLE GetCurrentThread();DWORD GetCurrentProcessId();DWORD GetCurrentThreadId()。DWORD SetThreadIdealProcessor(HANDLE hThread,DWORD dwIdealProcessor);BOOL SetThreadPriority(HANDLE hThread,int nPriority);BOOL SetPriorityClass(GetCurrentProcess(), IDLE_PRIORITY_CLASS);BOOL GetThreadContext(HANDLE hThread,PCONTEXT pContext);BOOL SwitchToThread();
三注意
1）C++主線程的終止，同時(shí)也會(huì)終止所有主線程創(chuàng)建的子線程，不管子線程有沒有執(zhí)行完畢。所以上面的代碼中如果不調(diào)用WaitForSingleObject，則2個(gè)子線程t1和t2可能并沒有執(zhí)行完畢或根本沒有執(zhí)行。
2）如果某線程掛起，然后有調(diào)用WaitForSingleObject等待該線程，就會(huì)導(dǎo)致死鎖。所以上面的代碼如果不調(diào)用resumethread，則會(huì)死鎖。