Secondary threads without object windows are typically created to perform a lengthy processing operation within the scope of a single event. A window procedure under the control of the primary thread creates the secondary thread to process a particular subroutine, and the secondary thread terminates automatically when this subroutine reaches an exit point. However, the DosExit() function should be called as the last action in the secondary thread, in order to ensure that the memory allocated for the thread's stack is correctly released by the operating system.
However, some checks may be necessary to ensure orderly termination of a secondary thread, particularly where access to data objects is involved. Upon termination of an application's primary thread, all secondary threads that have not already been terminated by the application are forcibly terminated by OS/2. Where the secondary thread's processing involves a critical data operation such as the update of a database, the primary thread should ensure that the secondary thread has completed its processing before allowing itself to terminate.
It is recommended that before creating a secondary thread without an object window, a window procedure should set an event semaphore, and pass the handle of this semaphore to the secondary thread. The event semaphore is then cleared by the secondary thread as the last action before it terminates.
Upon receiving a WM_DESTROY message, the window procedure in the primary thread should test the state of the event semaphore and wait for the semaphore to clear before completing the WM_DESTROY message processing (which should include releasing the semaphore) and returning control to Presentation Manager. This will ensure that the secondary thread terminates in an orderly manner before the primary thread is terminated.
The use of event semaphores is described in more detail in Maintaining Synchronization.