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Introduction

A thread is a path of execution. A process requires at least one thread. But, it may contain more then one thread. If the process is closed, all the threads in that process are killed automatically. When we create a thread in an application that is actually a secondary thread. In C or C++ the program entry point is main or wmain (Unicode version). In windows application the program starts in WinMain or wWinMain. When the program starts, the operating system creates the first thread. Because, Windows is a multitucancode.neting operating system.

Thread function

Thread function is like an ordinary function with long void pointer parameter. We can pass any data through void pointer data type.

A simple thread function looks like

ThreadFunction(LPVOID param)
{
    // do somthig

    ......
    .......

    //return value;

}

Thread Priorities

The thread priority controls the thread process priority. The thread priorities are following.

• Highest THREAD_PRIORITY_HIGHEST
• Above Normal THREAD_PRIORITY_ABOVE_NORMAL
• Normal THREAD_PRIORITY_NORMAL
• Below Normal THREAD_PRIORITY_BELOW_NORMAL
• Idle THREAD_PRIORITY_IDLE

We set thread priority with the CreateThread function. We get or set thread priorities in GetThreadPriority and SetThreadPriority Win32API functions or CWinThread’s functions which we call freely in the code. The priority functions return a BOOL value.

C Run Time Library Multithreading

_beginthread
_beginthreadex
_endthread
_endthreadex

All the above c run time library functions are in the process.h header file. Make sure the Microsoft Visual Studio project setting is as multithreaded DLL. The _beginthread and _beginthreadex functions are used to create threads in C run time library. But, these functions have some differences. The _beginthreadex has some add ional parameters like security and thread address. When we create thread using _beginthread we end the thread use _endthread. The _endthread closes thread handle automatically. But, if we use _endthreadex we close the nthread handle using CloseHandle Win32 API function. The C Run Time Library contains the Thread Local Storage (TLS) internally. We can use thread local storage using API or Compiler specific code. The TlsAlloc, TlsFree, TlsGetValue and TlsSetValue are used to store thread specific data. Microsoft recommend that If you use _beginthread functions for C run Time Library you can not use Win32 API like ExitThread or CreateThread. Because, if use that method it might result in deadlocks. _beginthread uses multiple arguments in the thread creation. Our example program is a simple console based application. The user enters number of threads to create and then we execute each thread.

// Secound Thread function
void ThreadProc(void *param);

// First thread
int main()
{

    int n;
    int i;
    int val = 0;
    HANDLE handle;

    printf("\t Thread Demo\n");

    printf("Enter the number of threads : ");
    scanf("%d",&n);


    for(i=1;i<=n;i++)
    {
        val = i;
        handle = (HANDLE) _beginthread( ThreadProc,0,&val); // create thread
        WaitForSingleObject(handle,INFINITE);


    }


    return 0;
}


void ThreadProc(void *param)
{

    int h=*((int*)param);
    printf("%d Thread is Running!\n",h);
    _endthread();

}

The thread waits for completion of another thread using WaitForSingleObject Win32 API function.

MFC Multithreaded

The CWinThread is the base class for all thread operations. MFC supports two types of threads. These are User Interface Thread and Worker thread. The user interface thread is based on windows message. The worker thread runs in the background process. (Examples, search a file in the find window or communicate with web browser in the web server) . The CWinThread supports both worker thread and User interface Threads. But, we will discuss only worker threads.

The MFC class hierarchy

CObject
    CCmdTarget
        CWinThread
            CWinApp

In the above class hierarchy the CWinApp application class is derived from CWinThread. So, if we create a application the thread is also created. If we create a thread, that is a secondary thread. The mother class CObject has some features like, Serialization support, Run time Class information and Debugging support. The derived class CWinThread also has the same features. The most common useful data members and member functions are the following.

Data members

• m_hThread The current thread handle
• m_bAutoDelete Whether the thread has set to auto delete or not
• m_nThreadID The current thread ID

Data Functions:

• CreateThread Start the exec execution of thread
• SuspendThread Increment thread suspend count. When the resume thread occur only the thread resume.
• ResumeThread Resume the thread. Decrements the thread count stack
• SetThreadPriority Set thread priority ( LOW, BELOW LOW or HIGH)
• GetThreadPriority Get the thread Priority

Not all the member functions in MFC as class members. We can access some functions globally also. These functions begin with Afx. The AfxBeginThread, AfxEndThread are most widely useful functions in MFC thread. We create thread using AfxBeginThread function. AfxBeginThread Syntax:

CWinThread* AfxBeginThread( AFX_THREADPROC ThreadProc, LPVOID Param,
      int nPriority = THREAD_PRIORITY_NORMAL,UINT nStackSize = 0,
      DWORD dwCreateFlags = 0, LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL );

The ThreadProc is first Parameter in AfxBeginThread function. We use name of thread function in this parameter. We pass the void pointer arguments in this function. The return type in the function is UINT. The other arguments in AfxBeginThread are optional. The default thread priority is THREAD_PRIORITY_NORMAL. We can change this any time using CwinThread SetThreadPriority function. We also get the thread priority.

The Thread terminates using AfxEndThread. The AfxEndThread has a Exit code argument list.

CwinThread *pThread = AfxBeginThread( ThreadFunction, &data);


UINT ThreadFunction(LPVOID param)
{
    DWORD result =0 ;

    // do somthig

    AfxEndThread(exitCode);
    return result;

}

Win32 Multithread

Win32 Thread created by CreateThread function. The syntax in CreateThread is following

 

HANDLE CreateThread(  LPSECURITY_ATTRIBUTES lpThreadAttributes,
            DWORD dwStackSize, LPTHREAD_START_ROUTINE lpStartAddress,
            LPVOID lpParameter,  DWORD dwCreationFlags, LPDWORD lpThreadId);

We terminate thread using the following methods.

1. use TerminateThread function
2. use ExitThread function
3. use return

But, Advanced Windows by Jeffery Ritcher recommends us to use the return method. TerminateThread or ExitThread function does not clear the Thread Stack properly. The GetThreadTimes is used to find the thread’s run time. The GetCurrentThreadID is to get current thread ID. The Sleep function is to sleep a particular thread for so many milliseconds. Example Sleep (1000) will sleep the thread for 1000 milliseconds. The SwithToThread is to switch to other threads. The SuspendThread is to wait until a call of resume Thread. The WaitForSingleObject is to wait when a particular thread completes its work. The WaitForMultipleObject is used for multiple events. These functions wait for following situations - Change notification, Console input, Event, Job, Mutex ,Process, Semaphore, Thread and Waitable timer.

Benefits of Threads

The multithreaded application uses CPU 100% effectively. When we create a process, it will take more memory space. The multithreaded application shares the same process memory space. Every thread contains stack. So, the thread takes up less memory usage compared to a Process. The process may or may not contain more threads. If you run two or more threads in a process, all the threads share the process address space.

References

• http://www.msdn.microsoft.com/
• Programming Windows With MFC by Jeff Prosise
• Advanced Windows by Jeffery Ritcher