Visual Basic developer's world

Saturday, August 19, 2006
Simulate multithreading with WaitForMultipleObjects (eg. How ICQ monitors connection state)

I have used extensivly the event driven mechanism that Windows provide in manydifferent programming aspects(RDO, ADO, ODBC, Windows Sockets, Winlogon, mutexes, semaphores etc) and usedWaitForSingleObject when i was in need of an event monitor API command.

The WaitForSingleObject is located in kernel32.dll and waits until a specific event object gets signaled or when a time limit is reached. It accepts two parameters; a handle to the event object and a time-out interval.

**The main benefit of this function is that it uses no processor time while waiting for the object state

to become signaled or the time-out interval to elapse.

Hereis the declaration :

Public Declare Function WaitForSingleObject Lib "kernel32" Alias "WaitForSingleObject" _(ByVal hHandle As Long, ByVal dwMilliseconds As Long) As Long

Let's see an example of this command's usage :

In this example we are going to run the Windows calculator. We will open this shelled process and we will monitor the process handle; if it gets 0 then the process was ended.

Public Const WAIT_FAILED = &HFFFFFFFF 'Our WaitForSingleObject failed to wait and returned -1
Public Const WAIT_OBJECT_0 = &H0& 'The waitable object got signaled
Public Const WAIT_ABANDONED = &H80& 'We got out of the waitable object
Public Const WAIT_TIMEOUT = &H102& 'the interval we used, timed out.
Public Const STANDARD_RIGHTS_ALL = &H1F0000 'No special user rights needed to open this process

Public Declare Function OpenProcess Lib "kernel32" (ByVal dwDesiredAccess As Long, ByVal bInheritHandle As Long, ByVal dwProcessId As Long) As Long
Public Declare Function WaitForSingleObject Lib"kernel32" (ByVal hHandle AsLong, ByVal dwMilliseconds AsLong) As Long
Public Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long

Public Sub ShelledAPP()
shProcID As Long
hProcess As Long
WaitRet As Long

shProcID = Shell("calc.exe", vbNormalFocus)
hProcess = OpenProcess(STANDARD_RIGHTS_ALL, False, shProcID)

'This is the proper and optimized way to use the WaitForSingleObject function.

'Isaw many programmers use the INFINITE constant as forthe dwMilliseconds field.

'IfdwMilliseconds is INFINITE, the function's time-out interval neverelapses.

'That'swrong 'cause the program won't refresh thus giving the impression that is a hungapplication.

'InWindows XP specially you might see a popup screen informing you about this.

'The problem also appears when you apply WaitForSingleObject with INFINITE in an application that uses windows.

'Always use a reasonable number of milliseconds and always use DoEvents to refresh the program's message queue

WaitRet = WaitForSingleObject(hProcess, 10) 'wait for 10ms to see if the hProcess was signaled
Select Case


'The first case must always be WAIT_TIMEOUT 'cause it is the most used option

DoEvents 'until the shelled process terminates

MsgBox "Wait failed or abandoned"
Exit Do

WAIT_OBJECT_0 'The object got signaled soinform user and get out of the loop
MsgBox "The shelled application has ended"
Exit Do

End Select

'Close the process handle

CloseHandle(shProcID) 'Close the process id handle

DoEvents 'free any pending messages from the message queue

End Sub

Now what if we had to monitor two or more shelled applications? are we going to use multithreading?

I haven't yet implemented multithreading api in a project of mine but as you most
ultithreading is lethal (basically for those who will implement the CreateThread API function) when used within Visual Basic 6 (or prior).

Crashes, unexpected terminations, exceptions and many other "beautifull" encounters are some of the experiences a programmer can get.

The answer comes from WaitForMultipleObjects API function which is also included in kernel32.dll

Here is the declaration :

Public Declare Function WaitForMultipleObjects Lib "kernel32" Alias "WaitForMultipleObjects"(ByVal nCount As Long, lpHandles As Long, ByVal bWaitAll As Long, ByVal dwMilliseconds AsLong) As Long

it accepts four values :

as the maximum number of events to monitor,

as the array of different event handles (not multiple copies of the same one),

bWaitAll (True/False) True if it must return when the state of all objects is signaled, False if it must return when the state of any one of these objects gets signaled,

as a maximum time-out interval

Like WaitForSingleObject, WaitForMultipleObjects can accept event handles of any of the following object types in
the lpHandles : Change notification, Console input, Waitable timer,Event, Job, Mutex, Process, Semaphore

and Threads. In the following example we are going to try something else than monitoring multiple shelled apps;

Those of you that have ICQ installed, have noticed that "red flower" icon, placed on the system tray.

When you are not connected on the internet, ICQ makes this icon look like inactive.

Now when you connect, it suddently starts to get one by one of it's leaf green,meaning that it tries to connect
to it's main server and when the connection completes, the flower get's green.

How do they do it? I mean. do they have an IsConnected() function on a timer with some interval?

Definetly no!

What they do is take advantage of WaitForMultipleObjects with another function located in rasapi32.dll; RasConnectionNotification

The RasConnectionNotification function specifies an event object that the system sets to the signaled state when

a RAS connection is created or terminated. The function accepts three values :

hrasconnas the handle to a RAS connection

hEvent as the handle to an event object

dwFlagsas the type of event to receive notifications for (RASCN_Connection or RASCN_Disconnection)

Nowwe are going to use WaitForMultipleObjects to monitor both events

Public Const RASCN_Connection = &H1 'Our two flags
Public Const RASCN_Disconnection = &H2

Public Const WAIT_OBJECT_0 = &H0&
Public Const WAIT_ABANDONED = &H80&
Public Const WAIT_TIMEOUT = &H102&

nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type

Public Declare Function CreateEvent Lib "kernel32" Alias"CreateEventA" (lpEventAttributes AsSECURITY_ATTRIBUTES, ByVal bManualReset AsLong, ByVal bInitialState AsLong, ByVal lpName AsString) As Long
Public Declare Function RasConnectionNotification Lib "rasapi32.dll" Alias "RasConnectionNotificationA" (hRasConn AsLong, ByVal hEvent AsLong, ByVal dwFlags As Long) As Long
Public Declare Function WaitForMultipleObjects Lib "kernel32" (ByVal nCount As Long, lpHandles As Long, ByVal bWaitAll As Long, ByVal dwMilliseconds AsLong) As Long
Public Declare Function ResetEvent Lib "kernel32"(ByVal hEvent As Long) As Long
Public Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long

Public Sub MonitorRASStatusAsync()

Dim hEvents(1) As Long 'Array of event handles. Since there are two events we'd like to monitor, i have already
dimention it.

Dim RasNotif As Long
Dim WaitRet As Long
Dim hRasConn As Long

hRasConn = 0

'We are going to create and register two event objects with CreateEvent API function

'There aren't any special treated events that need any kind of security attributes sowe just initialize the structure

With sd
.nLength = Len(sd) 'we pass the length of sd
.lpSecurityDescriptor = 0
.bInheritHandle = 0
End With

'We create the event by passing in CreateEvent any security attributes,

'we want to manually reset the event after it gets signaled,

'we also want it's initial state not signaled assuming that we don't have yet any connection to the internet,

'last but not least we give the event a name (RASStatusNotificationObject1)

hEvents(0) = CreateEvent(sd, True, False, "RASStatusNotificationObject1")
'If the returned value was zero, something went wrong so exit the sub

If hEvents(0) = 0 Then MsgBox "Couldn't assign an event handle": Exit Sub

'If we succesfully created the first event object we pass it toRasConnectionNotification

'with the flag RASCN_Connection so that this event will monitor for internet connection

RasNotif = RasConnectionNotification(ByVal hRasConn, hEvents(0), RASCN_Connection)

If RasNotif <> 0 Then MsgBox "Ras Notification failure": GoTo ras_TerminateEvent
'We create the second event object exactly like the first one

'but we name it RASStatusNotificationObject2

hEvents(1) = CreateEvent(sd,True, False, "RASStatusNotificationObject2")

If hEvents(1) = 0 Then MsgBox "Couldn't assignan event handle": Exit Sub

'If we succesfully created the second event object too, we pass it toRasConnectionNotification

'with the flag RASCN_Disconnection. This event will monitor for disconnection

RasNotif = RasConnectionNotification(ByVal hRasConn, hEvents(1), RASCN_Disconnection)

If RasNotif <> 0 Then MsgBox "Ras Notification failure": GoTo ras_TerminateEvent

'We then issue the loop

'Notice that we have put hEvents array to it's first array item.

'and we used False cause we want to get notifications

'whenany of the two events occur.

WaitRet = WaitForMultipleObjects(2, hEvents(0),False, 20)
Select Case WaitRet

GoTo ras_TerminateEvent

MsgBox "Connected"

ResetEvent hEvents(0) 'Reset the event to avoid a second message box

DoEvents 'Free any pending messages

Case WAIT_OBJECT_0 + 1
MsgBox "Disconnected"
ResetEvent hEvents(1) 'Reset the event to place it in nosignal state (Manual reset, remember?)

End Select


'Close all event handles

'For more than two events you could apply a For.. Next

Call CloseHandle(hEvents(0))

'Free any pending messages from the application message queue

End Sub

Now imagine that you could monitor events from different objects like
a file or folder change, along with connection status, shelled applications, multiple printer objects, different processes and threads etc etc etc.

(64 maximum event objects i think)

I twill appear that you program is multithreading but the truth behind that, is that you will be taking advantage of WaitForMultipleObjects internal multithreading mechanism.

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posted by Admin @ 8:24 AM  
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