Steady Timer Messages in a Dynamic-Link Library (DLL)

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

In the Microsoft Windows graphical environment, a dynamic-link library (DLL) can receive steady timer messages, even as various applications that link into the DLL are started and terminated. However, it cannot do so by calling the SetTimer() API directly as an application typically does. To receive steady timer messages, a DLL can launch a hidden application and use that application's message queue to receive the timer messages.

MORE INFORMATION

The SetTimer() API is used to create a Windows timer. When the timer expires, Windows needs a message queue into which it can place a WM_TIMER message. A message queue is required even if a timer notification function is specified as the lpTimerFunc parameter to SetTimer().

Because a DLL is not a task, it does not have a message queue. When a DLL calls SetTimer(), Windows places the resulting WM_TIMER messages into the message queue of the currently active task, which is the application instance that called into the DLL.

If this application instance should terminate, its message queue is destroyed, and the DLL does not receive any more timer messages. If the DLL specified a timer notification function, the notification function is not called again.

For a DLL to receive a steady flow of timer messages, the timer messages must go into a message queue that will remain present while the various applications that link into the DLL are started and terminated. One way to achieve this is to launch a hidden application from the DLL. The DLL can use that application's message queue to receive steady timer messages.

This technique could be implemented as follows:

1. Export a routine from the DLL with syntax similar to the following:

         void FAR PASCAL StartDLLTimer(HWND hWndHiddenApp)
   

2. To start a steady timer, the DLL launches a hidden application with a call similar to the following:

         WinExec("MYTIMER.EXE", SW_HIDE);
   

   MYTIMER is a minimal Windows-based application. The only special processing MYTIMER requires is to link to the DLL and call the StartDLLTimer function in the processing of its WM_CREATE case, supplying the handle for its main window as the value for the hWndHiddenApp parameter.

3. In the StartDLLTimer function, the DLL calls SetTimer, specifying hWndHiddenApp as the hWnd parameter.

Timers are a scarce system resource. If the SetTimer() call fails, no timers are available for use.

Before the DLL terminates, it should free the timer. It can do so by calling KillTimer(). It then sends a WM_CLOSE message to the MYTIMER application, which terminates MYTIMER.