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
wmain (Unicode version). In windows application the program starts in
wWinMain. When the program starts, the operating system creates the first thread. Because, Windows is a multitasking operating system.
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
The thread priority controls the thread process priority. The thread priorities are following.
- Above Normal
- Below Normal
We set thread priority with the
CreateThread function. We get or set thread priorities in
SetThreadPriority Win32API functions or
CWinThread’s functions which we call freely in the code. The priority functions return a
C Run Time Library Multithreading
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
_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 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
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
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.
void ThreadProc(void *param);
int val = 0;
printf("\t Thread Demo\n");
printf("Enter the number of threads : ");
val = i;
handle = (HANDLE) _beginthread( ThreadProc,0,&val); WaitForSingleObject(handle,INFINITE);
void ThreadProc(void *param)
printf("%d Thread is Running!\n",h);
The thread waits for completion of another thread using
WaitForSingleObject Win32 API function.
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
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.
m_hThread The current thread handle
m_bAutoDelete Whether the thread has set to auto delete or not
m_nThreadID The current thread ID
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
AfxEndThread are most widely useful functions in MFC thread. We create thread using
CWinThread* AfxBeginThread( AFX_THREADPROC ThreadProc, LPVOID Param,
int nPriority = THREAD_PRIORITY_NORMAL,UINT nStackSize = 0,
DWORD dwCreateFlags = 0, LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL );
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
SetThreadPriority function. We also get the thread priority.
The Thread terminates using
AfxEndThread has a Exit code argument list.
CwinThread *pThread = AfxBeginThread( ThreadFunction, &data);
UINT ThreadFunction(LPVOID param)
DWORD result =0 ;
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.
But, Advanced Windows by Jeffery Ritcher recommends us to use the
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.
Selvam has worked on several technologies like Core Java, VC++, MFC, Windows API and Weblogic server. He takes a lot of interest in reading technical articles and enjoy writing them too. He has been awarded as a Microsoft Community Star in 2004, MVP in 2005-06, SCJP 5.0 in 2009, Microsoft Community Contributor(MCC) 2011.
Web site: http://www15.brinkster.com/selvamselvam/