A window timer causes the system to post WM_TIMER messages to a message queue at specified time intervals called timeout values. A timeout value is expressed in milliseconds.

An application starts the timer for a given window, specifying the timeout value. The system counts down approximately that number of milliseconds and posts a WM_TIMER message to the message queue for the corresponding window. The system repeats the countdown-post cycle continuously until the application stops the timer.

The timeout value can be any value in the range from 0 through 4,294,967,295 (full magnitude of ULONG) for OS/2 Version 3; for previous versions, the maximum value is 65535. However, the operating system cannot guarantee that all values are accurate. The actual timeout depends on how often the application retrieves messages from the queue and the system clock rate. In many computers, the operating system clock ticks about every 50 milliseconds, but this can vary widely from computer to computer. In general, a timer message cannot be posted more frequently than every system clock tick. To make the system post a timer message as often as possible, an application can set the timeout value to 0.

An application starts a timer by using the WinStartTimer function. If a window handle is given, the timer is created for that window. In such case, the WinDispatchMsg function dispatches the WM_TIMER message to the given window when the message is retrieved from the message queue. If a NULL window handle is given, it is up to the application to check the message queue for WM_TIMER messages and dispatch them to the appropriate window.

A new timer starts counting down as soon as it is created. An application can reset or change a timer's timeout value in subsequent calls to the WinStartTimer function. To stop a timer, an application can use the WinStopTimer function.

The system contains a limited number of timers that must be shared among all PM applications; each application should use as few timers as possible. An application can determine how many timers currently are available by checking the SV_CTIMERS system value.

Every timer has a unique timer identifier. An application can request that a timer be created with a particular identifier or have the system choose a unique value. When a WM_TIMER message is received, the timer identifier is contained in the first message parameter. Timer identifiers enable an application to determine the source of the WM_TIMER message. Three timer identifiers are reserved by and for the system and cannot be used by applications; these system timer identifiers and their symbolic constants are shown in the following table:

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³Value          ³Meaning                                      ³
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³TID_CURSOR     ³Identifies the timer that controls cursor    ³
³               ³blinking. Its timeout value is stored in the ³
³               ³os2.ini file under the CursorBlinkRate       ³
³               ³keyname in the PM_ControlPanel section.      ³
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³TID_FLASHWINDOW³Identifies the window-flashing timer.        ³
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³TID_SCROLL     ³Identifies the scroll-bar repetition timer   ³
³               ³that controls scroll-bar response when the   ³
³               ³mouse button or a key is held down.  Its     ³
³               ³timeout value is specified by the system     ³
³               ³value SV_SCROLLRATE.                         ³
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WM_TIMER messages, like WM_PAINT and semaphore messages, are not actually posted to a message queue. Instead, when the time elapses, the system sets a record in the queue indicating which timer message was posted. The system builds the WM_TIMER message when the application retrieves the message from the queue.

Although a timer message may be in the queue, if there are any messages with higher priority in the queue, the application retrieves those messages first. If the time elapses again before the message is retrieved, the system does not create a separate record for this timer, meaning that the application should not depend on the timer messages being processed at precise intervals. To check the accuracy of the message, an application can retrieve the actual system time by using the WinGetCurrentTime function. Comparing the actual time with the time of the previous timer message is useful in determining what action to take for the timer.