A thread can execute any part of the program's code, including a part executed by another thread. Threads are the basic entity to which the operating system allocates CPU time. Each thread maintains a set of structures for saving its context while waiting to be scheduled for processing time. The context includes the thread's set of machine registers, the kernel stack, a thread environment block, and a user stack in the address space of the thread's process. All threads of a process share the virtual address space and can access the global variables and system resources of the process.
A multitasking operating system divides the available CPU time among the threads that need it. In Windows, the Win32 API is designed for preemptive multitasking; this means that the system allocates small slices of CPU time among the competing threads. The currently executing thread is suspended when its time slice elapses, allowing another thread to run. When the system switches from one thread to another, it saves the context of the suspended thread and restores the saved context of the next thread in the queue.
Because each time slice is small (approximately 20 milliseconds), it appears that multiple threads are executing at the same time. This is actually the case on multiprocessor systems where the executable threads are distributed among the available processors. On a single processor system, however, using multiple threads does not result in more instructions being executed. In fact, the system can slow down if it is forced to keep track of too many threads.
How to Launch Win32 Applications from Visual BasicThere are two ways to launch another Win32 application from a Microsoft Visual Basic application:
- Use the Visual Basic shell command. This spawns a new process and returns its process ID. However, to be able to do anything useful, a process handle is required, which can be obtained by a subsequent call to the OpenProcess Win32 API function.
- Use the CreateProcess Win32 API function that creates both a process object and a main thread object. Both the process and the initial thread are assigned a 32-bit identifier that remains valid until the respective object (process or thread) terminates. The 32-bit identifier can be used to uniquely identify the object within the system. The new process and new thread handles are also created with full access rights. All these four values are returned in the PROCESS_INFORMATION structure that is passed by reference to CreateProcess.
It is important to understand that at creation time, the system gives each object an initial usage count of one. Then, just before CreateProcess returns, the function opens both the process and the thread object and places the process-relative handles for each in the hProcess and hThread members of the PROCESS_INFORMATION structure.
When CreateProcess opens these objects, the usage count for each increments to two. This means that before the Windows NT or Windows 2000 Executive can free the process object, the process must terminate (decrementing the usage count to one) and the parent process must call CloseHandle (decrementing the usage count to zero). To free the thread object, the thread must terminate and the parent process must close the handle to the thread object.
CAUTION: It is very important to close these handles. Failure to do so can result in a system memory leak because some Windows NT or Windows 2000 Executive objects are never destroyed.
Similar considerations are required when obtaining a process handle with OpenProcess. In this case too, the usage count is incremented by one, and unless the handle is closed, the process object will remain in memory even when the process itself has terminated.
- Start a new Standard EXE project in Visual Basic. Form1 is created by default.
- Copy the following code to the Code window of the Form1 form:
Private Type PROCESS_INFORMATION
hProcess As Long
hThread As Long
dwProcessId As Long
dwThreadId As Long
Private Type STARTUPINFO
cb As Long
lpReserved As String
lpDesktop As String
lpTitle As String
dwX As Long
dwY As Long
dwXSize As Long
dwYSize As Long
dwXCountChars As Long
dwYCountChars As Long
dwFillAttribute As Long
dwFlags As Long
wShowWindow As Integer
cbReserved2 As Integer
lpReserved2 As Long
hStdInput As Long
hStdOutput As Long
hStdError As Long
Private Declare Function CreateProcess Lib "kernel32" _
Alias "CreateProcessA" _
(ByVal lpApplicationName As String, _
ByVal lpCommandLine As String, _
lpProcessAttributes As Any, _
lpThreadAttributes As Any, _
ByVal bInheritHandles As Long, _
ByVal dwCreationFlags As Long, _
lpEnvironment As Any, _
ByVal lpCurrentDriectory As String, _
lpStartupInfo As STARTUPINFO, _
lpProcessInformation As PROCESS_INFORMATION) As Long
Private Declare Function OpenProcess Lib "kernel32.dll" _
(ByVal dwAccess As Long, _
ByVal fInherit As Integer, _
ByVal hObject As Long) As Long
Private Declare Function TerminateProcess Lib "kernel32" _
(ByVal hProcess As Long, _
ByVal uExitCode As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" _
(ByVal hObject As Long) As Long
Const SYNCHRONIZE = 1048576
Const NORMAL_PRIORITY_CLASS = &H20&
Private Sub Form_Click()
Dim pInfo As PROCESS_INFORMATION
Dim sInfo As STARTUPINFO
Dim sNull As String
Dim lSuccess As Long
Dim lRetValue As Long
sInfo.cb = Len(sInfo)
lSuccess = CreateProcess(sNull, _
ByVal 0&, _
ByVal 0&, _
ByVal 0&, _
MsgBox "Calculator has been launched!"
lRetValue = TerminateProcess(pInfo.hProcess, 0&)
lRetValue = CloseHandle(pInfo.hThread)
lRetValue = CloseHandle(pInfo.hProcess)
MsgBox "Calculator has terminated!"
- On the Run menu, select Start, or press the F5 key to start the program. Click on the Form1 form to launch the Calculator application. A message box appears indicating the application has launched successfully. Click OK to close the message box and the Calculator application. Another message box appears indicating the application has successfully terminated.
Article ID: 129797 - Last Review: Jul 15, 2004 - Revision: 1