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And magazines and books sometimes neglect their responsibility to make sure that the programming practices that they describe are sound. As a programmer, it is important to choose the right tool for the job. It is your responsibility to develop code that not only works now under one particular platform, but that works under all target platforms and system configurations. Your code must follow the rules dictated by the operating system or standards that you are using. Failure to do so can lead to problems in the future as systems and software are upgraded. Recent articles in the Microsoft Systems Journal and Visual Basic Programmer's Journal introduced to Visual Basic programmers the possibility of using the Create. Thread API function to directly support multithreading under Visual Basic. In fact, one reader went so far as to contact me and complain that my Visual Basic Programmer's Guide to the Win. API was fatally flawed because I did not cover this function or demonstrate this technique. This article is in part a response to this reader, and in part a response to other articles written on the subject. This article also serves, in part, as an update to chapter 1. It can run several programs simultaneously, while at the same time playing a compact disk, sending a fax, and transferring a file. Every programmer knows (or should know) that the computer's CPU can only execute one instruction at a time (we'll ignore the existence of multiprocessing machines for the time being). How can a single CPU do multiple tasks? It does this by rapidly switching among the different tasks. The operating system holds all of the programs that are running in memory. It allows the CPU to run each program in turn. Every time it switches between programs, it swaps the internal register values including the instruction pointer and stack pointer. This means that the CPU starts executing instructions at the beginning of the program, and continues following the instructions in the sequence defined by the program until the program terminates. Let's say the program has five instructions: A B C D and E that execute in sequence (no jumps in this example). When an application has a single thread, the instructions will always execute in exactly the same order: A, B, C, D and E. Let's say that instruction D in our sample application had the ability to create a new thread that started at instruction B and ran through the sequence C and E. The first thread would still be A, B, C, D, E, but when D executed a new thread would begin that would execute B, C, E (we don't want to execute D again or we'll get another thread). Exactly what order will the instructions follow in this application? It could be: Thread 1. ABCD. The two threads are completely independent. Why is this a problem? A Multithreading Simulator. Consider the MTDemo project: (You can download the sample code from ftp. Sample. Code/Articles/Thread. The project contains a single code module that contains two global variables as follows: ' MTDemo - Multithreading Demo program. Copyright . All Rights Reserved. Option Explicit. Public Generic. Global. Counter As Long. Public Total. Increments As Long. It contains a single form named frm. MTDemo. 1 which contains the following code: ' MTDemo - Multithreading Demo program. Copyright . All Rights Reserved. Option Explicit. Dim State As Integer. State = 0 - Idle. State = 1 - Loading existing value. State = 2 - Adding 1 to existing value. State = 3 - Storing existing value. State = 4 - Extra delay Dim Accumulator As Long. Const Other. Code. Delay = 1. 0Private Sub Command. The State variable describes the five instructions that this program executes in order. State zero is an idle state. State one loads local variable with the Generic. Global. Counter global variable. State two increments the local variable. State three stores the result into the Generic. Global. Counter variable and increments the Total. Increments variable (which counts the number of times that the Generic. Global. Counter variable has been incremented). State 4 adds an additional delay representing time spent running other instructions in the program. The Update. Display function updates three labels on the form that show the current value of the Generic. Global. Counter variable, the local accumulator, and the total number of increments. Each timer tick represents a CPU cycle on the current thread. If you run the program you'll see that the value of the Generic. Global. Counter variable will always be exactly equal to the Total. Increments variable - - which makes sense, because the Total. Increments variable shows the number of times the thread has incremented the Generic. Global. Counter. But what happens when you click the Command. This new form simulates a second thread. Every now and then, the instructions will line up in such a way that both forms load the same Generic. Global. Counter value, increment it, and store it. In other words - - the variable was incremented twice, but the value only increased by one. If you launch several forms you will quickly see that the number of increments as represented by the Total. Increments variable grows much more rapidly than the Generic. Global. Counter variable. What if the variable represents an object lock count - which keeps track of when an object should be freed? What if it represents a signal that indicates that a resource is in use? This type of problem can lead to resources becoming permanently unavailable to the system, to object being locked internally in memory, or freed prematurely. It can easily lead to application crashes. This example was designed to make the problem easy to see - - but try experimenting with the value of the Other. Code. Delay variable. When the dangerous code is relatively small compared to the entire program, problems will appear less frequently. While this may sound good, the opposite is true. Multithreading problems can be extremely intermittent and difficult to find. This means that multithreading demands careful design up front. Avoiding Multithreading Problems. There are two relatively easy ways to avoid multithreading problems. Avoid all use of global variables. Add synchronization code wherever global variables are used. The first approach is the one used by Visual Basic. When you turn on multithreading in a Visual Basic applications, all global variables become local to a specific thread. This is inherent in the way Visual Basic implements apartment model threading - - more on this later. The original release of Visual Basic 5. This was because they had not figured out at the time a way to make the forms engine thread safe. For example: when you create a form in Visual Basic, VB gives it an implied global variable name (thus if you have a form named Form. Form. 1. method instead of declaring a separate form variable). This type of global variable can cause the kinds of multithreading problems you saw earlier. There were undoubtedly other problems within the forms engine as well - - making a package that complex safe for multithreading can be quite a challenge. With service pack 2, Visual Basic's forms engine was made thread safe. One sign of this is that each thread has its own implied global variable for each form defined in the project. New for Service Pack 2. By making the forms engine thread safe, Service pack 2 made it possible for you to create multithreading client applications using Visual Basic. This is demonstrated in the MTDemo. You can download the sample code from ftp. Sample. Code/Articles/Thread. The application must be defined as an Active. X Exe program with startup set to Sub Main in a code module as follows: ' MTDemo. Multithreading demo program. Copyright . All Rights Reserved. Declare Function Find. Window Lib . The Main routine needs some way of finding out whether this is the first thread of the application because it is executed at the start of every thread. You can't use a global variable to find this out because the Visual Basic apartment model keeps global variables specific to a single thread. In this example the Find. Window API function is used to check if the main form of the example has been loaded. The object must be defined using a class module. In this case, a simple class module is defined as follows: ' MTDemo. Multithreading demo program. Copyright . All Rights Reserved. Private Sub Class. All Rights Reserved. Private Sub cmd. Launch. The form contains two launch buttons, one that uses the New operator, the other that uses the Create. Object operator. If you run the program within the Visual Basic environment, you'll see that the forms are always created in the same thread. This is because the Visual Basic environment only supports a single thread. If you compile the program, you'll see that the Create. Object approach creates both the cls. MTDemo. 2 and its form in a new thread. Why Multithread? Why all the fuss about multithreading if there is so much potential danger involved? Because, in certain situations, multithreading can dramatically improve performance. In some cases it can improve the efficiency of certain synchronization operations such as waiting for an application to terminate. It allows more flexibility in application architecture. For example, Add a long operation to the form in the MTDemo. Private Sub cmd. Long. Op. When you click on the cmd. Long. Op button on any of the forms, you will see that it freezes up operations on all of the other forms. This is because all of the forms are running on a single thread - - and that thread is busy running the long loop. They are running in their own execution thread, and the long loop operation only ties up its own thread. Of course, you probably shouldn't be placing these kinds of long operations in your forms in any case. Here is a brief summary of when multithreading has value: Active. X EXE Server - - no shared resources. When you have an Active. X EXE server that you expect to share among applications, multithreading prevents the applications from interfering with each other. If one application performs a long operation on an object in a single threaded server, the other applications are frozen out waiting for the server to become available. However, there are cases where you may want to use an Active. X EXE server to arbitrate access to a shared resource. An example of this is the stock quote server described in my Developing Active. X Components book. In this case the single thread runs the stock quote server which is shared among all of the applications using the server in turn. Multithreading Client - - Implemented as an Active. X EXE Server. A simple form of this approach is demonstrated in the MTDemo. It is used when an application supports multiple windows that must exit within a single application but work completely independently. Internet browsers are a good example of multithreaded clients, where each browser window runs in its own thread. It is simply a DLL that creates objects that run in the same thread that requests the objects. For example: a multithreaded Active. X control (which is a DLL) creates controls that run in the same thread as the form that contains the control. This can improve efficiency on a multithreaded client such as an Internet browser. Multithreaded Servers DLL or EXEIn a client server architecture, multithreading can improve performance if you have a mix of long and short client requests. Be careful though - - if all of your client requests are of similar length, multithreading can actually slow down the server's average response time! Never assume that the fact that your server is multithreading will necessarily improve performance. The Threading Contract. Believe it or not, all of this has been in the way of introduction. Some of the material reviews information that is covered in far greater depth in my Developing Active. X Components book, other material describes new information for service pack 2. Now, allow me to ask a question that goes directly to the heart of multithreading using COM (the component object model on which all Visual Basic objects, and those in other windows applications that use OLE are based). Given: That multithreading is potentially extremely dangerous and specifically - - that attempting to multithread code that is not designed to support multithreading is likely to cause fatal errors and system crashes. Question: How is it possible that Visual Basic allows you to create objects and use them under both single and multithreaded environments without any regard to whether they are designed for single or multithreaded use? In other words - - How can a multithreaded Visual Basic application use objects that are not designed to be thread safe? How can other multithreaded applications use single threaded Visual Basic objects? In short: how does COM handle threading issues? If you know about COM, you know that it defines the structure of a contract. A COM object agrees to follow certain rules so that it can be used successfully from any application or object that supports COM. Most people think first of the interface part of the contract - - the methods and properties that an object exposes. But you may not be aware of the fact that COM also defines threading as part of the contract. And like any part of the COM contract - - if you break it, you are in very deep trouble. Visual Basic, naturally, hides most of this from you, but in order to understand what follows, you must learn a little bit about the COM threading models. The Single Threading Model: A single threaded server is the simplest type of server to implement. It is also the easiest to understand. In the case of an EXE server, the server runs in a single thread. All objects are created in that thread. All method calls to each object supported by the server must arrive in that thread. But what if the client is running in a different thread? In that case, a proxy object must be created for the server object. This proxy object runs in the client's thread and reflects the methods and properties of the actual object. When a method is called on the proxy object, it performs whatever operations are necessary to switch to the object's thread, then calls the methods on the actual object using the parameters passed to the proxy. Naturally, this is a rather time consuming task - - but it does allow the contract to be followed. This process of switching threads and transferring data from the proxy object, to the actual object and back is called marshalling. It is covered in more depth in chapter 6 in my Developing Active. X Components book. In the case of DLL servers, the single threading model demands that all objects in the server be created and called in the same thread as the first object created by the server. The Apartment Threading Model. Note that apartment model threading as defined by COM does NOT require that each thread have its own set of global variables. That's just how Visual Basic implemented the apartment model. The apartment model states that each object may be created in its own thread, however once an object is created, its methods and properties may only be called by the same thread that created the object. If another thread wants to access methods of the object, it must go through a proxy. This is a relatively easy model to implement. If you eliminate global variables (as Visual Basic does), the apartment model grants you thread safety automatically - - since each object is effectively running in its own thread, and due to the lack of global variables, the different object threads do not interact with each other. The Free Threading Model. The free threading model basically says that all bets are off. Any object can be created in any thread. All methods and properties on any object can be called at any time from any thread. The object accepts full responsibility for handling any necessary synchronization. This is the hardest model to implement successfully, since it demands that the programmer handle all synchronization. In fact, until recently, OLE itself did not support this threading model! However, since marshalling is never required, this is the most efficient threading model. Which model does your server support? How does an application, or Windows itself, know which threading model a server is using? This information is included in the registry. When Visual Basic creates and object, it checks the registry to determine in which cases a proxy object and marshalling are required. It is the client's responsibility to adhere strictly to the threading requirements of each object that it creates. The Create. Thread APINow let's take a look at how the Create. Thread API can be used with Visual Basic. Say you have a class that you want to have running in another thread in order to perform some background operation. A generic class of this type might have the following code (from the MTDemo 3 example): ' Class cls. Background. ' MTDemo 3 - Multithreading example. Copyright . All Rights Reserved. Public Function Do. The. Count(By. Val finalval& ) As Boolean. Dim s As String. s$ = . We could have placed the loop in the object itself, but you'll see shortly that there are sound reasons for designing the object as shown here. The first time the Do. The. Count function is called, a Message. Box appears showing the thread identifier - - that way we can verify the thread in which the code is running. The Message. Box API is used instead of the VB Message. Box command because the API function is known to be thread safe. A second Message. Box is shown when the counting is complete, and an event is raised to indicate that the operation is finished. The background thread is launched using the following code in the frm. MTDemo. 3 form: Private Sub cmd. Create. Free. Each thread has its own stack. This must be an address of a function in a standard module obtained using the Address. Of operator. Details on these flags can be found in Microsoft's online 3. The function returns a handle to the thread. In this case we pass a pointer to the cls. Background object that we wish to use in the new thread. Obj. Ptr retrieves the value of the interface pointer in the qobj variable. After the thread is created, the handle is closed using the Close. Handle function. This does NOT terminate the thread - - the thread continues to run until the Background. Func. Free function exits. However, if we did not close the handle, the thread object would continue to exist even after the Background. Func. Free function exits. We can get away with this because under COM, every interface is based on IUnknown - - so this parameter type is valid regardless of the type of interface originally passed to the function. We must, however, immediately set the param to a specific object type in order to use it. |
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