Monthly Archives: April 2002

Threads in MFC II: Synchronization Objects

Introduction

In part I, I looked at getting threads communicating with each other. Now let’s look at how we can manage how multiple threads operate on single objects.

Let’s take an example. Suppose we have a global variable (or any variable that is accessible to two or more threads via scope, pointers, references, whatever). Let’s say this variable object is a CStringArray called stringArray
.

Now, let’s suppose our main thread wants to add something to the array. Fine enough. We can do that. Then, let’s throw in a second thread which can somehow access this object. It, too, wants to access stringArray . What would happen if both threads tried to simultaneously write to the first position in the array for example? Or even if one were just reading and the other writing? Well, if there is no synchronization between the two threads, you don’t know what would happen. The result is completely unpredicatable. One thread would write some bytes to memory, while another reads it, and you could have the correct answer or the wrong answer or a mix. Or it could crash. Who knows…

You can’t even assume safety when merely reading an object from two threads. Even if it seems like no bytes are changing, and both threads should get valid results, you have to think about a lower level: A single C++ statement compiles to many assembly or machine language instructions. These instructions directly access the processor, including the registers that keep track of where we are, what data we’re looking at. It’s possible to have one of those registers hold a pointer to the current character in the string, so if you have two threads that rely on that pointer in that register–they are obviously not both going to be correct except in a very rare circumstance.

OK, I think I’ve made the case. How do we control access to objects then?

Windows has a number of synchronization objects that you can use to effectively prevent accidents. MFC encapsulates these into CEvent , CCriticalSection, CMutex , and CSemaphore . To use these, include afxmt.hin your project.

CEvent

Let’s start with these so-called triggers. An event in this context is nothing more than a flag, a trigger. Imagine it as cocking a gun (Reset) and then firing it (Set). You can use events for setting of threads. Here’s how.
Remember how we created a structure that contained all the data we wanted to send the thread? Let’s add a new one. First create a CEvent object in the dialog (or any window or non-window) class called m_event . Now, in our [code lang=”cpp”]THREADINFOSTRUCT [/code], let’s add a pointer to an event:

[code lang=”cpp”]
typedef struct THREADINFOSTRUCT {
…
CEvent* pEvent;
…
} THREADINFOSTRUCT;
[/code]

When we initialize the structure, we must do the assignment:

[code lang=”cpp”]tis->pEvent=&m_event; [/code]

In our thread function, we call:

[code lang=”cpp”]tis->pEvent->Lock(); [/code]

This will “lock” on the event (the same event that is in our dialog class in the main thread). The thread will effectively stop. It will loop inside of CEvent::Lock() until that event is “Set.” Where do you set it? In the main thread. An event is initially reset–cocked. Create the thread. When you want the thread to unblock itself and continue, you call m_event.Set()–fire the gun.

So what are some practical examples? You could lock a thread before you access a global object. In your main thread, when you’re done using that object, you call Set(). You can also use an event to signal a thread to exit (such as if
you hit an abort button in the main thread). To see an example of this usage, look at the demo project I’ve uploaded to the code tool section.

There are two types of events: ones that automatically reset when you set them, and ones that don’t.

You can use a single event to trigger multiple threads, but the event had better be a manual-reset event or only one thread will be triggered at a time.

CCriticalSection

These are pretty simple to use. You simply surround every usage of the shared object by a lock and an unlock command:

[code lang=”cpp”]CCriticalSection cs;
…
cs.Lock();
stringArray.DoSomething();
cs.Unlock();
[/code]
Do that in every thread that uses that object. You must use the same critical section variable to lock the same object. If a thread tries to lock an object that’s already locked, it will just sit there waiting for it to unlock so it can safely access the object.CMutex

A mutex works just like a critical section, but it can also work across different processes. But you don’t want to always use mutexes, because they are slower than critical sections.
You can declare a mutex like this:

[code lang=”cpp”]CMutex m_mutex(FALSE, “MyMutex”); [/code]
The first parameter specifies whether or not the mutex is initially locked or not. The second parameter is the identifier of the mutex so it can be accessed from two different processes.

If you lock a critical section in a thread and then the thread exits without unlocking it, then any other thread waiting on it will be forever blocked. Mutexes, however, will unlock automatically if the thread exits. Mutexes can
also have a time-out value (critical sections can too, but there are some doubts as to whether or not they work–perhaps the bugs are fixed in MFC 7.0).

Otherwise, it works the same:

[code lang=”cpp”]
m_muytex.Lock(60000);//time out in milliseconds
stringArray.DoSomething();
m_mutex.Unlock();
[/code]

CSemaphore

A semaphore is used to limit simultaneous access of a resource to a certain number of threads. Most commonly, this resource is a pool of a certain number of limited resources. If we had ten string arrays, we could set up a semaphore to guard them and let only ten threads at a time access them. Or COM ports, internet connections, or anything else.

It’s declared like this:

[code lang=”cpp”]CSemaphore m_semaphore(10,10); [/code]

The first argument is the initial reference count, while the second is the maximum reference count. Each time we lock the semaphore, it will decrement the reference count by 1, until it reaches zero. If another thread tries to lock the semaphore, then it will just go into a holding pattern until a thread unlocks it.

As with a mutex, you can pass it a time-out value.

It’s used with the same syntax:

[code lang=”cpp”]
m_semaphore.Lock(60000);
stringArray.DoSomething();
m_semaphore.Unlock();
[/code]

Conclusion

These two tutorials, along with the sample projects, should be enough to get you started using threads. There are a couple of other MFC objects and issues that I have yet to cover, so I’ll group all of these into Part III of this
tutorial. These topics include exception-handling and thread-safe classes. Make sure that you examine the documentation of all of these classes: there is more functionality than I could cover in this short tutorial. And if you really want to learn threads, get a good book that covers the Windows kernel (one called Programming Applications for MS Windows comes to mind, published by Microsoft).

The sample project for this tutorial has a time object that it shares between two threads. It’s protected by a critical section. There are also two events: for starting the thread and aborting it. The main thread uses a timer to add the current time to a list box every second, while the thread traces the current time to the debug window and sends a message to the main thread to remove the first time from the list. The thread is only started after the time
hits an even ten-second boundary.

©2004 Ben Watson

Threads in MFC I: Worker Threads

There are two types of threads in MFC. Worker and User Interface. Here, I will discuss how to use a worker thread.First, let’s discuss some multi-threading basics. Each application has what we call a process. Usually, an application has only one process. This process defines all the code and memory space for the application. You can use the Window Task Mananger to view running processes.

You could possibly view a thread as a process within a process. It is an independently (mostly) running sub-process, that the CPU can task and switch to like any other process on the machine.

Under 16-bit Windows, you could have mulitple processes (i.e., many programs running: multi-tasking). However, each application was limited to its one main process. It was multi-tasked, not multi-threaded. With 32-bit Windows, applications could spawn their own threads or sub-processes.
 

Threads have priority levels. The explanation of exactly how Windows manages these in determining how much processor time each receives is a topic you can find in the MSDN literature. Basically, higher priorities receive more time.

When your Win32 program creates a thread, you specify its priority level. By default, it has the same priority as the calling thread.

There are two main issues you must deal with when using threads: 1) Inter-thread communication, and 2) inter-thread object access.

I’ll leave object access for part II of this tutorial.

Communication

The easiest way to communicate among threads in your application is with messages. Since this tutorial deals with worker threads, we’ll restrict this to having the worker thread post messages to the main application thread.
ThreadFunc()

So, now let’s walk through creating a simple worker thread that does nothing but update the progress control in a dialog box.

I’m going to assume you know how to create a dialog box, with a progress control, bound to a member variable in the dialog class. Do that now. You could also create a button that starts the thread.

OK, the first thing you need to do is create the thread’s controlling function. This can either be global or a class member, but I prefer to make it global because this separates the thread from the main process in my mind.

[code lang=”cpp”]UINT MyThreadFunc(LPVOID lParam); [/code]

All MFC thread “controllers” must be declared like that.

To call this function in a thread, we use the following code:

[code lang=”cpp”]
CWinThread *pThread = AfxBeginThread(MyThreadFunc, NULL, THREAD_PRIORITY_NORMAL, 0, 0); [/code]

This creates a separate thread using the MyThreadFunc function, passes a NULL for its one parameter, sets the priority to normal, gives it the same stack size as the calling thread, and starts the thread immediately. If the last parameter here is CREATE_SUSPENDED instead of 0, then the thread is created, but it does not start running until you explicitly tell it to.

This will successfully create a thread, which will run until MyThreadFunc returns. However, the calling thread will not know when that thread is done. Somehow, we have to pass the thread some information about the calling program.
Passing Information To a Thread
A thread controller can only have one parameter–the LPVOID argument. Therefore, it is often convenient to wrap up all the information we want to send the thread into a single struct:
[code lang=”cpp”]
typedef struct THREADINFOSTRUCT {
HWND hWnd;
CString someData;
} THREADINFOSTRUCT; [/code]We can put any data we want in that structure, but one that should always be in there is a handle to the thread’s parent window. This will allow us to communicate with it.

Now, before we start the thread, let’s allocate some space for this structure. If we merely declare it on the stack with

[code lang=”cpp”]THREADINFOSTRUCT tis; [/code]

then as soon as this data goes out of scope, it will be destroyed. So let’s put it on the heap:

[code lang=”cpp”]THREADINFOSTRUCT *tis=new THREADINFOSTRUCT;
tis->hWnd=m_hWnd;
tis->someData=”This is in a thread.”; [/code]

And now we call the same function as before, passing tis:

[code lang=”cpp”]CWinThread *pThread = AfxBeginThread(MyThreadFunc,tis,
THREAD_PRIORITY_NORMAL,0,0); [/code]

OK, now we can pass some information to the thread. How do we let the thread tell the main process what’s going on?

Communicating with Threads
We can communicate to the calling window via a windows messages. First we have to define our own custom messages in our dialog class’s header file:

[code lang=”cpp”]#define WM_USER_THREAD_FINISHED (WM_USER+0x101)
#define WM_USER_THREAD_UPDATE_PROGRESS (WM_USER+0x102) [/code]
We also have to provide handlers for these messages in our dialog class:

[code lang=”cpp”]
afx_msg LRESULT OnThreadFinished(WPARAM wParam, LPARAM lParam);
afx_msg LRESULT OnThreadUpdateProgress(WPARAM wParam, LPARAM lParam); [/code]

All custom message handlers must follow that generic template. But we can interpret the parameters any way we want.

We must also manually update the message map with these two lines:

[code lang=”cpp”]
ON_MESSAGE(WM_USER_THREAD_FINISHED, OnThreadFinished)
ON_MESSAGE(WM_USER_THREAD_UPDATE_PROGRESS, OnThreadUpdateProgress) [/code]

And now we add the function definitions somewhere in our source file:

[code lang=”cpp”]
LRESULT CMyDialog::OnThreadFinished(WPARAM wParam, LPARAM lParam)
{
AfxMessageBox(“Thread has exited”);
return 0;
}LRESULT CMyDialog::OnThreadUpdateProgress(WPARAM wParam, LPARAM lParam)
{
m_progress.SetPos(100*(int)wParam/(int)lParam);
return 0;
}
[/code]
So what should our thread do? In this example, not much:

[code lang=”cpp”]
UINT MyThreadFunc(LPVOID lParam)
{
THREADINFOSTRUCT* tis=(THREADINFOSTRUCT*)lParam;
for (int i=0;i<100;i++) {
PostMessage(tis->hWnd,WM_USER_THREAD_UPDATE_PROGRESS,i,100);
Sleep(100);
}
PostMessage(tis->hWnd,WM_USER_THREAD_FINISHED,0,0);
delete tis;
return 0;
} [/code]

Let’s analyze this. First we typecast the function’s argument into the structure type we passed. Then we just run through a simple loop that sends a message to the main thread to update the progress bar. We sleep for 100 ms just so it doesn’t go too fast that we don’t see it.

Next we send a message saying that our thread is finished.

Finally we delete the pointer to tis; Wait a second! Didn’t we define that in the main thread??? Yes, and it’s perfectly fine to allocate memory in one thread and free it in another. As long as we the programmer keep track of where things are happening. Alternatively, we could have set a class variable to hold that structure, and delete it in the [code lang=”cpp”]OnThreadFinished[/code] functioned. Either way is acceptable.

The function then returns, and the thread ends.

That’s all! It’s so easy! To see a working example project, look in the code tools section.

Of course, we can easily make it more complicated. Part II will talk about some synchronization methods used to control simultaneous access to objects from multiple threads. Now things can start becoming fun…
©2004 Ben Watson