Tag Archives: mfc

A Debugging Exercise

A few weeks ago I had an interesting debugging problem. A program we develop had a memory leak in it that Visual Studio was catching when it ended. The trace text was something like this:

Detected memory leaks!
Dumping objects ->
{292300} normal block at 0x05590040, 2771928 bytes long.
Data: <> FF FF FF 00 FF FF FF 00 FF FF FF 00 FF FF FF 00
{292294} normal block at 0x05110040, 2613312 bytes long.
Data: <> FF FF FF 00 FF FF FF 00 FF FF FF 00 FF FF FF 00

Visual Studio 2003 is often pretty good at telling you where a memory leak occured, giving you a source code file and line number of where the memory allocation originated. In the case above, I don’t have that. There are two relatively easy ways of solving this.

The first method is to examine the data that isn’t being freed. In this case, I have two large objects of about 2.5 MB each. That’s pretty big–there aren’t too many (if any) individual objects in the software that are that large. So it could be an array. The data begins with a repeated pattern {FF FF FF 00}. Definitely looks like an array of 4-byte segments. Applying a little knowledge of the application itself–it displays large bitmaps–and we realize that the memory comes from an array of RGB values.

Another way to investigate this is to set a breakpoint on the memory allocation that isn’t being freed. The number in braces {292300} is the memory allocation number in debug mode in Visual C++. In order for this technique to work, you must first ensure that the number is the same each time. In my case, the memory leak would happen if I just opened and closed the program (because an image is always being drawn) without doing anything else.

First, set a breakpoint at the beginning of the program (or any place definitely before the memory leak). Start the program and when it breaks, enter the following into the watch window:

{,,msvcr71d.dll}_crtBreakAlloc

And for the value enter 292300.

If you continue the execution, the program will break on the 292,300th memory allocation. It will stop in the memory allocator, and you can then look at your stack frame and see exactly where the memory allocation is taking place.

MSDN has a great article about this.

In my problem it was for a set of memory blocks being used as working space for resizing and smoothing bitmaps. They were allocated once during execution and thus did not really provide a long-term threat. This leads to a possible third solution:

Ignore. All memory is reclaimed once the program exits. Nobody would ever do that, though…. would they?

…of course not…


Check out my latest book, the essential, in-depth guide to performance for all .NET developers:

Writing High-Performance.NET Code, 2nd Edition by Ben Watson. Available for pre-order:

Curvy: Part 1

I’ve been wanting to learn COM lately, and not knowing where to start I began in some of the MFC books I have. Admittedly, MFC hides quite a bit of COM complexity, but I figured it would be a good place to get my feet wet without getting scared.

My first test program is called Curvy (Actually, Curvy2 since I restarted it with the doc/view architecture). It’s a curve drawing program. You can use the left mouse button to create points on a curve, the right mouse button closes the curve, and you can change the control points and move the curves.

The fun part was implementing copy & paste. It uses the OLE clipboard and puts both its native format and a bitmap on the clipboard.

The other fun part was implementing OLE Drag & Drop. This took me a few times to get quite right, but the results are spectacular. You can drag shapes within a window as well as to other windows. Holding down the ctrl key will copy the shape instead.

The project is in VS .Net 2005 format, but the code will compile under previous versions of VS.

Click here to download.

Next stop for this? Implement a full OLE server so other programs can host Curvy components inside them!


Check out my latest book, the essential, in-depth guide to performance for all .NET developers:

Writing High-Performance.NET Code, 2nd Edition by Ben Watson. Available for pre-order:

Checking if a Directory Exists

I recently had to write a utility that moved hundreds of thousands of files to a new location under a different directory organization. As part of it, I checked to see if the destination directory already existed and if not, created it. At one point I wondered if it would just be faster to try and create it, and if it fails, assume that it already exists (remember, I’m dealing with hundreds of thousands of files here–anything to speed it up is very welcome).

Determining if a directory exists isn’t entirely straightforward. If you use .Net, you can use Directory.Exists(), but that function must use the Win32 API at some point and there is no Win32 API that determines the existence of a directory, so what is it doing?

Ah, but there is an API to get the attributes of a given filename.

[code lang=”cpp”]
BOOL DirectoryExists(const char* dirName)
{
DWORD attribs = ::GetFileAttributesA(dirName);
if (attribs == INVALID_FILE_ATTRIBUTES) {
return false;
}
return (attribs & FILE_ATTRIBUTE_DIRECTORY);
}[/code]

Note that if the function call fails it doesn’t necessarily mean that the directory doesn’t exist–it could be that the device is inaccessible, you don’t have security permissions, or any number of other things. To know for sure what’s going on, you would need to call GetLastError().

So what if you’re creating directories? Why not try to create them no matter what? That’s fine, but is that faster than checking to see if it exists first? Let’s test it.

[code lang=”cpp”]
BOOL CreateDirectory(const char* dirName)
{
return ::CreateDirectoryA(dirName,NULL);
}

for (int i=0;i

CreateDirectory(dirName);

}
[/code]

Results (10,000,000 iterations):
265.227 second(s) total
2.65227e-005 second(s) average iteration

Now let’s try checking first:

[code lang=”cpp”]
for (int i=0;i BOOL bExists = DirectoryExists(dirName);
if (!bExists) {
CreateDirectory(dirName);
}
}
[/code]

Results (10 million iterations):
103.24 second(s) total
1.0324e-005 second(s) average iteration

Over 2 .5 times faster!

Now, my simple test is retrying a single folder over and over, and it never actually creates anything. In my case for the utility I mentioned above, I’m creating far fewer directories than the number of files I’m moving to them (though still in the thousands). In that case, it’s definitely worth my time to check to see if the folder exists before trying to create it.

To me, it appears that unless the number of folders you’re creating is of the same magnitude as the number of files, it definitely makes sense to check first.

This goes to show that you can’t believe anything related to performance until you measure it for your application.


Check out my latest book, the essential, in-depth guide to performance for all .NET developers:

Writing High-Performance.NET Code, 2nd Edition by Ben Watson. Available for pre-order:

Modifying the System Menu in MFC

Introduction

The system menu is a standard feature of every Windows application. It is managed by Windows so normally we don’t have to worry about it at all. However sometimes it is nice to be able to modify that menu according to our own program with things that Windows can’t automatically do for us.

As my main example I will be using a tool I’ve created called BabelOn. This is a C++/MFC program that accesses a web-service to translate text into foreign languages. It uses a toolbox window, so we don’t see the system menu icon in the upper-left corner, however the menu can still be accessed with Ctrl-Space or by right-clicking on the title bar. The menu has been modified to contain two extra commands: About BabelOn, and Exit. Exit is needed because the default action for Close (Alt-F4) has been overridden in the program to hide the window and allow tray access.

Adding Commands

First we need to define a unique variable to represent each menu item. This can be done in the Resource.h file, or in any standard header file. If the commands already exist as part of another standard or context menu, then this step can be skipped because the definitions already exist. However, it’s important to note that even if we use the pre-existing definitions, the message handlers for the commands on the regular menu will NOT be automatically called. System commands are routed differently. So, in the end, it doesn’t matter if we use the existing or define our own.

For our example, we’ll define two:

[code]
#define IDM_ABOUT 16
#define IDM_EXIT 17
[/code]

The IDM just means this is a menu-item ID.

We add these commands in our window’s initializing function ( OnInitDialog(), OnCreate() ). My example is in a dialog class, so this is what the function looks like:

[code lang=”cpp”]
BOOL CBabelOnDlg::OnInitDialog()
{
CDialog::OnInitDialog();
// Add “About…” and “Exit” menu items to system menu.
// Command IDs must be in system range
// the ANDing is because of a bug in Windows 95
ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
ASSERT(IDM_ABOUTBOX < 0xF000);
ASSERT((IDM_EXIT & 0xFFF0) == IDM_EXIT);
ASSERT(IDM_EXIT < 0xF000);
CMenu* pSysMenu = GetSystemMenu(FALSE);
if (pSysMenu != NULL) {
pSysMenu->AppendMenu(MF_STRING,IDM_EXIT,”E&xit Program”);
pSysMenu->AppendMenu(MF_SEPARATOR);
pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, “A&bout BabelOn”);

}
. . .
//other initialization
[/code]

The first thing you should notice is a couple of ASSERT statements for each command. The first one deals with a bug present in Windows 95 and the second ensures the custom command is below the range used by pre-defined system commands. From the MFC documentation: “All predefined Control-menu items have ID numbers greater than 0xF000. If an application adds items to the Control menu, it should use ID numbers less than F000.”

Next we get a pointer to the system menu with the GetSystemMenu. We call it with an argument of FALSE to get the pointer. If we call it with TRUE, then it will reset the menu to its default state.

If the pointer is valid, we call some commands to add to the bottom of the menu, passing the IDs and the string we want to show up when the menu is viewed.

Simple, isn’t it?

Processing Custom Commands

In order to have those commands do anything, we can’t rely on the normal message-handling mechanism, even if we have handlers for the same items in other menus. We have to handle the WM_SYSCOMMAND message in our dialog/window class.

[code lang=”cpp”]
void CBabelOnDlg::OnSysCommand(UINT nID, LPARAM lParam)
{
//trap our own system menu messages
if ((nID & 0xFFF0) == IDM_ABOUTBOX)
{
CAboutDlg dlgAbout; dlgAbout.DoModal();
} else if ((nID & 0xFFF0)==SC_CLOSE)
{
OnClose();
} else if ((nID & 0xFFF0)==IDM_EXIT)
{
::PostQuitMessage(0);
} else {
CDialog::OnSysCommand(nID, lParam);
}
}
[/code]

This is the height of simplicity herself. We compare the code that was passed in with the system to our commands, again ANDing them to account for the Windows 95 bug. Then call whatever code we want to handle it.

If we selected Exit, then we post a message to quit the application.

Take a look at the second test: SC_CLOSE is a predefined menu constant. It’s one normally handled by Windows, but we can still add custom processing to it if we want. In this application, I didn’t want it to exit, but to merely hide the application (with an icon in the tray). So I just call a handler that I wrote that does that. If the ID doesn’t equal anything we want to custom-process, we just pass it on up the hierarchy for default processing.

The IDs for the most common system commands are:

SC_CLOSE – Close the CWnd object.
SC_MAXIMIZE (or SC_ZOOM) – Maximize the CWnd object.
SC_MINIMIZE (or SC_ICON) – Minimize the CWnd object.
SC_MOVE – Move the CWnd object.
SC_RESTORE – Restore window to normal position and size.
SC_SIZE – Size the CWnd object.

There are others in special situations that you can learn about in the documentation for WM_SYSCOMMAND.

Modifying Existing Commands

We just saw that it’s possible to change the default handling of built-in system commands, but it’s also possible to modify existing menu items by changing their text, or to entirely remove them.

To modify the text of a command, use the ModifyMenu() function on pSysMenu in the above example. For example, to change “Close” to “Hide”, I could do this:
[code lang=”cpp”]
pSysMenu->ModifyMenu(SC_CLOSE, MF_BYCOMMAND,IDM_HIDE, “&Hide”);
[/code]

The MF_BYCOMMAND argument tells the function to interpret SC_CLOSE as a command ID. IDM_HIDE is a new command ID, and then comes the text we want to show.

Alternatively, we can call ModifyMenu() on the ith menu item:

[code lang=”cpp”]
pSysMenu->ModifyMenu(0,MF_BYPOSITION,IDM_HIDE,”&Hide”);
[/code]

This changes the first menu item to Hide.

Removing commands

Don’t want a close command at all on the window? Not sure if this is a good idea or not, but you can do it.

Use this method:

[code lang=”cpp”]
pSysMenu->RemoveMenu(SC_CLOSE,MF_BYCOMMMAND); pSysMenu->RemoveMenu(0,MF_BYPOSITION);
[/code]

The first one removes the command associated with SC_CLOSE, while the second removes the first item on the menu.

Conclusion

Modifying the system menu in this way has limited application but it is sometimes useful for small applications and utilities that have a limited menu structure otherwise. It can be useful for commands you want to be easily accessible from the task bar–when a program is is in the background or minimized, right-clicking on it’s icon on the task bar will bring up the system menu.

Something should be said, however, about the wisdom of certain modifications. Removing or modifying functionality that user expects is usually a bad idea. It makes your program less usable and friendly compared to other applications. UI guidelines and standard look and feel exist for a good reason!

Enjoy playing with this!

©2004 Ben Watson


Check out my latest book, the essential, in-depth guide to performance for all .NET developers:

Writing High-Performance.NET Code, 2nd Edition by Ben Watson. Available for pre-order:

Threads in MFC Part III: Exceptions, Suspense, Murder, and Safety

Exceptions

In the previous tutorial, I described the various synchronization objects you can use to control access to shared objects. In most cases, these will work fine, but consider the following situation:

[code lang=”cpp”]
UINT ThreadFunc(LPVOID lParam)
{
::criticalSection.Lock();
::globalData.DoSomething();
SomeFunctionThatThrowsException();
::criticalSection.Unlock();
return 0;
}[/code]

What’s going to happen when that exception gets thrown? The critical section will never be unlocked. If you start the thread again, it will again try to lock it, and finding it already locked, it will sit there forever waiting. Of course, a mutex will unlock when the thread exits, but a critical section won’t. So MFC has a couple of wrapper classes that can incorporate any of the other basic synchronization classes. These are called CSingleLock and CMultiLock.

Here is how they are used:
[code lang=”cpp”]
UINT ThreadFunc(LPVOID lParam)
{
CSingleLock lock(&(::criticalSection));
lock.Lock();
::globalData.DoSomething();
FunctionThatThrowsException();
lock.Unlock();
return 0;
}
[/code]

You merely pass the address of the “real” synchronization object. CSingleLock lock is created on ThreadFunc’s stack, so when an exception is thrown, and that function exits prematurely without a chance to nicely clean up, CSingleLock’s destructor is called, which unlocks the data. This would not happen to criticalSection because, being a global variable, it will not go out of scope and be destroyed when ThreadFunc exits.

CMultiLock

This class allows you to block, or wait, on up to 64 synchornization objects at once. You create an array of references to the objects, and pass this to the constructor. In addition, you can specify whether you want it to unblock when one object unlocks, or when all of them do.

[code lang=”cpp”]
//let’s pretend these are all global objects, or defined other than in the local function
\tCCriticalSection cs;
CMutex mu;
CEvent ev;
CSemaphore sem[3];

CSynObject* objects[6]={&cs, &mu, &ev,
&sem[0], &sem[1], &sem[2]};
CMultiLock mlock(objects,6);
int result=mlock.Lock(INFINITE, FALSE);

[/code]

Notice you can mix synchronization object types. The two parameters I specified (both optional) specify the time-out period and whether to wait for all the objects to unlock before continuing. I saved the return value of Lock() because that is the index into the array of objects of the one that unblocked, in case I want to do special processing.

Killing a Thread

Generally, murder is very messy. You have blood and guts everywhere that certainly don’t clean up after themselves. But sometimes, sadly, it is necessary (no one call the cops–my metaphor is about to end).

If you start a child thread, and for some reason it is just not exiting when you need it to, and you’ve fixed your code, double-checked all your signaling mechanisms, and then and only then you want to kill it, here’s how. When you create the thread, you need to get its handle and save it for later use in your
class.

[code lang=”cpp”]
HANDLE hThread;//handle to thread
[/code]

A handle is only valid while the thread is running. What if we create a thread, start it off running, and it exits immediately for some reason? Back in our main thread, even if the very next statement after creating the thread is to grab its handle, it could very possibly be too late.

So we create a thread suspended! We just don’t even let it get to first base before we allow ourselves to get to the handle. This is a piece of cake, simply change the last parameter we’ve been giving fxBeginThread() from 0 toCREATE_SUSPENDED:

[code lang=”cpp”]
CWinThread* pThread=AfxBeginThread(ThreadFunc,NULL, THREAD_PRIORITY_NORMAL, 0, CREATE_SUSPENDED);
::DuplicateHandle(GetCurrentProcess(), pThread->m_hThread, GetCurrentProcess(), &hThread, 0, FALSE, DUPLICATE_SAME_ACCESS);
pThread->;
ResumeThread();
[/code]

We start the thread suspended, use an API call to duplicate the thread’s handle, saving it to our class variable, and then resuming the thread.

Then, if we want to commit this heinous crime:

[code lang=”cpp”]::TerminateThread(hThread,0);
[/code]

Don’t say I didn’t warn you.

Thread-Safe Classes

Thread-safety refers to the possibility of calling member functions across thread-boundaries. Their are two types of safety: Class-level and Object-level. Class level means that I can create two CStringT objects called a and b, and access each of them in separate threads, but I cannot safely access just a in
two threads. Object safety means that it’s perfectly ok to access a in two or more threads simultaneously. Thread-safety at the object level generally means using synchronization objects to control access to all internal datamembers. So why not make all classes thread-safe at the object level? Because that would just about kill your performance. You can lock objects yourself outside of the actual object (as shown in Part II) to make it safe.

This is not to say that your program will always crash if you try to access a single object from two threads, but it most likely will. Also, you should not generally lock access to MFC member functions or public variables–you don’t know when the MFC framework is going to need access to them. There really isn’t need to lock on a CWnd* object anyway.

Etc

There are many, many details I have neglected to cover in these three tutorials. You can look in the SDK or .NET documentation for more information on such things as pausing/resuming, scheduling, masks in CMultiLock(), or any of the other member functions of the thread classes. If you want to learn about the internal details of Windows, threads and fibers, (plus a lot of other important subjects) check out Programming Applications for Microsoft Windows by Jeffrey Richter.

I have yet to cover so-called user-interface threads (internally, there is no difference–all threads are created equal). Perhaps in a future tutorial…

Threads are a very powerful tool, but they can quickly increase the complexity of your application by an order of magnitude. Use wisely. As always, it takes some experimentation to get the hang of how to go about it. So have fun!

©2004 Ben Watson


Check out my latest book, the essential, in-depth guide to performance for all .NET developers:

Writing High-Performance.NET Code, 2nd Edition by Ben Watson. Available for pre-order:

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


Check out my latest book, the essential, in-depth guide to performance for all .NET developers:

Writing High-Performance.NET Code, 2nd Edition by Ben Watson. Available for pre-order:

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


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