Visual Basic 6 Win32 API Tutorial, Page 9
Well, we've covered a lot of the major data type issues that we can run into when using the Win32 calls. But there's still one more that we should address, and that's error handling.
The majority of API calls will inform you in some way, shape, or form if an error occurred (note, however, that this doesn't include memory exceptions). Most of the time, it takes the form of a return value or some parameter that you pass into the procedure. But virtually all of the calls set some internal OS information that you can obtain using just two API calls. Let's review these calls, and then we'll create a VB function that we can use within our main development application.
Declare Function GetLastError Lib "kernel32" _ Alias "GetLastError" () As Long Declare Function FormatMessage Lib "kernel32" _ Alias "FormatMessageA" (ByVal dwFlags As Long, _ lpSource As Any, ByVal dwMessageId As Long, _ ByVal dwLanguageId As Long, ByVal lpBuffer As String, _ ByVal nSize As Long, Arguments As Long) As Long
The GetLastError function simply returns a number that corresponds to the last error that occurred within a Win32 API call. We can then use this value and pass it into FormatMessage to obtain a message that may make more sense than error code 10594. As you can see, that second parameter is already causing me some concern. We'd better look into this further.
If you want, you can use the SetLastError API call to set the error code.I can't think of a reason why you'd need to do this in VB, but it is possible to change this value.
The first parameter, dwFlags, is used to tell the call how it should be used. It's cryptic, I know, but this function gets pretty flexible in a hurry when you start to read the documentation on the call. The only value of dwFlags we're concerned about is FORMAT_MESSAGE_FROM_SYSTEM (equal to 4096), which will tell the call to look up a description from an internal resource. We can ignore the second parameter for our purposes (thank goodness), since Microsoft's documentation tells us this argument is ignored when we set dwFlags equal to 4096. We can pass in the return value from GetLastError to dwMessageId. We don't care about language issues for now, so we'll set dwLanguageId equal to 0. The lpBuffer is another case where we need to pass in a pre-allocated string to the call - the length of the string is passed in through nSize. We can also ignore the Arguments argument as well.
There's a lot more that you can use FormatMessage for, but we just want to use it to obtain error information. If you're curious, hop onto Microsoft's web site and look up the documentation on the call. For now, let's just use what we know about the call to create a prototype API error call:
Function GetWin32ErrorDescription(ErrorCode _ as Long) as String Dim lngRet as Long Dim strAPIError as String ' Preallocate the buffer. strAPIError = String$(2048, " ") ' Now get the formatted message. lngRet = FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, _ ByVal 0&, ErrorCode, 0, strAPIError, Len(strAPIError), 0) ' Reformat the error string. strAPIError = Left$(strAPIError, lngRet) ' Return the error string. GetWin32ErrorDescription = strAPIError End Function
We'll improve upon the function in a moment, but I hope you see the point. Whenever you get an error from a Win32 call, run this function to get an error message. It may help you in debugging your applications.
Which DLL Error Code?
Notice that the GetWin32ErrorDescription uses an argument to get the error code. This was done for a lot of reasons; such as letting us enter in any value to see what the result would be (not the most exciting thing to do in the world, but it might be somewhat educational). However, it was also done to illustrate the behavior of capturing the error code from a DLL. Let's use a very simple but powerful example to illustrate this fact.
Create a new Standard EXE project in VB called MutexTest. Add one label to the form. Here's some more specific information about the project:
Your main form should look something like this:
As with the TestAPIStringReturn project, simplicity is the key with this project. We want to focus in on the API calls and keep the UI to a minimum where possible.
Add the GetWin32ErrorDescription function that we just looked at to the code window for the form. Now we'll need four API declarations in this project along with one form-level variable and two constants. Add them to the form's Declarations section like this:
Private mlngHMutex As Long Private Const ERROR_ALREADY_EXISTS = 183& Private Const FORMAT_MESSAGE_FROM_SYSTEM = 4096 Private Declare Function GetLastError Lib "kernel32" () As Long Private Declare Function CreateMutex Lib "kernel32" Alias _ "CreateMutexA" (ByVal lpMutexAttributes As Long, _ ByVal bInitialOwner As Long, ByVal lpName As String) As Long Private Declare Function CloseHandle Lib "kernel32" _ (ByVal hObject As Long) As Long Private Declare Function FormatMessage Lib "kernel32" _ Alias "FormatMessageA" (ByVal dwFlags As Long, _ lpSource As Any, ByVal dwMessageId As Long, _ ByVal dwLanguageId As Long, ByVal lpBuffer As String, _ ByVal nSize As Long, Arguments As Long) As Long
Note that we didn't use a module this time. For simple projects like this, we really don't need a module to house the API declaration.
Now that we've got the project set up, let's back up for a second so I can explain what this project will actually do!
You may run into a situation in your project development where you want to prevent the user from opening up more than one instance of your application. The global App object does have a property call PrevInstance, which you can use to determine if another instance is running. But I was curious one day, and I started looking into ways to do this myself. As it turns out, you can use a kernel object called a mutex (short for mutual exclusion) to make this a very easy task.
Before we get into the details of the code, I should note that we're using a mutex in a way that it probably wasn't intended (of course, that doesn't necessarily make it wrong, either!). Mutexes are commonly used in multithreaded applications, and that's beyond the scope of this book. However, we'll use one nice little feature of a mutex for our problem at hand: it can be accessed from any process in Windows.
So how does this work? Let's add this function to our form:
Private Function IsPrevAppRunning() As Boolean On Error GoTo error_IsPrevAppRunning Dim lngVBRet As Long mlngHMutex = CreateMutex(0, 0, "MutexTest.frmMutex") lngVBRet = Err.LastDllError If lngVBRet = ERROR_ALREADY_EXISTS Then ' This app is already running. IsPrevAppRunning = True MsgBox GetWin32ErrorDescription(lngVBRet) End If Exit Function error_IsPrevAppRunning: IsPrevAppRunning = False End Function
I'll come back to this function in a moment. For now, assume that this function will return a True if the application is already running and False if it isn't. In the Initialize event of the form, add this code:
Private Sub Form_Initialize() If IsPrevAppRunning = True Then MsgBox "This app is already running.", _ vbOKOnly + vbExclamation, "App Already In Use" Unload Me Set frmMutex = Nothing Else lblMutex.Caption = "Mutex Number = " & CStr(mlngHMutex) End If End Sub
And, in the Terminate event of the form, add this code:
Private Sub Form_Terminate() If mlngHMutex <> 0 Then ' Close the mutex. CloseHandle mlngHMutex End If End Sub
Now let's go through how this all works. The first thing we do is create a mutex using the CreateMutex API call. The first argument is set to 0 since we don't care about the security attributes of this mutex. The second argument is also set to 0, which means that we dont "own" the mutex if we create it successfully. The last parameter is the one that we're concerned about. If we didn't name the mutex, we'd have to identify it by the return value, which is the handle to the mutex. However, a named mutex can be used by the name given. Furthermore, as I hinted at before, mutexes can be used across processes. Therefore, if another instance of our application has already created this mutex, CreateMutex will let us know about it.
The way CreateMutex lets us know of this situation is a bit weird, though. The return value is the handle to the mutex on success (i.e. a non-zero value), so we can't use the return value to let us know if the mutex already exists. However, if the mutex already exists, the GetLastError function would return a value equal to ERROR_ALREADY_EXISTS. In our case, we catch that situation and return a True. Note, though, that we don't use GetLastError - we use Err.LastDLLError. This will be important, as we'll see.
If we haven't created the mutex yet, the app will load up just fine. We should get a window that looks like this:
Of course, we should get rid of the mutex when the application is done. This is why we use CloseHandle on mlngHMutex in the Terminate event of the form.
So what does any of this have to do with GetLastError? Here's the problem. First, compile the project into an executable (make sure the code is native code, not p-code! To check go to Project | Properties and select the Compile tab.). Then run two instances of the program. The first one should load up fine, but the second one will show the following two message boxes:
That's the last you'll hear from the second instance.
Now go back into the code and replace this line from within IsPrevAppRunning:
lngVBRet = Err.LastDllError
lngVBRet = GetLastError
As before, compile the app and run it twice. Now the app shows up both times! What's the deal?
Internally, VB is making Win32 calls all the time in your application. Sometimes, when you make a Win32 call, this may trigger an event within your application that causes VB to make other Win32 calls (we'll address window messages later on in the book). Well, what happens if your initial call causes an error, but the internal Win32 call made by VB clears out the value of the error? You get the situation that we just saw. GetLastError returns a zero, but Err.LastDllError returns the correct error code.
In previous versions of VB, the LastDllError property didn't exist, and strange situations like the one we just saw popped up again and again. Therefore, the VB designers decided to add the LastDllError to the Err object. This property should be used when a Win32 call is made that causes an error, because VB will track your Win32 call and make sure that this property reflects any possible error conditions.
But you need to be quick. Grab the value of LastDllError after any Win32 calls in question, even before you look at the return value! This is the only way to guarantee that any error condition generated by the Win32 call you just made is in LastDllError. If you start changing the size of the form, or adding text to a text box, all bets are off, and you've lost the error code (unless you're really, really, REALLY lucky).
So what's the best situation to be in? Let's leave this discussion of capturing Win32 error codes in VB with this outline. It's no guarantee that we'll report the correct error code all of the time, but this is about as close as we're going to get:
- Grab the value of LastDllError immediately and store it in a variable (call it lngErrVB).
- Grab the value of GetLastError and store it in a variable (call it lngErr32).
- If lngErrVB is nonzero, then use it to report the error condition.
- If it's zero, check lngErr32. Chances are it's probably zero as well, but if it's nonzero, use it to report the error.
The moral of the story? You should always use the value from LastDllError whenever possible. VB is making a conscious effort to intervene on your behalf, so this is your best bet for Win32 error codes.