Close-Aware Applications

A close-aware application is any MS-DOS – based application that periodically checks the state of an internal close flag and terminates if the flag is set. Windows 95/98 sets this flag when the user chooses the Close command from the system menu of the window in which the MS-DOS – based application runs. Close-aware applications enable the Close command, which gives the user an alternate way to exit the application and close the window.

An application enables or disables the Close command by using the Enable or Disable Close Command function. The function takes a flag indicating whether to enable or disable the command. Once the command is enabled, the application must periodically check the close flag by using the Query Close function. The function returns zero in the AX register if the user has chosen the command.

If the Query Close function returns zero in AX, the application should call the Acknowledge Close function to acknowledge the close state of the internal close flag. After the application acknowledges the close state, subsequent calls to the Query Close function will return 1 in AX, indicating that the user has chosen the Close command and the close state has been acknowledged. After acknowledging the close state, an application should take all necessary steps to shut down and eventually exit, or it should cancel the close operation by calling the Cancel Close function.

An application should acknowledge the close state if it needs to perform additional keyboard input before exiting. When the close state has been signaled but has not yet been acknowledged by the application, all keyboard reads will return NULL and buffered line input will return an empty string.

After an application acknowledges the close state, the state reverts to unacknowledged if the application either exits or cancels the close operation. If the application acknowledges the close state and then exits, the parent process will be in an unacknowledged close state. The application must then acknowledge the close state to perform additional keyboard input before exiting or canceling the close operation.

For example, if a text editor receives a positive response from the Query Close function and has some buffers that have not been saved, it should call Acknowledge Close and ask the user if the buffers should be saved with these possible responses: "Yes," "No," or "Cancel."

If the user responds "Yes" or "No," the text editor should save (or not save if the response was "No") the buffers and then exit. The close state remains active, and the parent process (probably the command interpreter) will also receive a positive response from Query Close and will also terminate.

If the response is "Cancel," the application should call the Cancel Close function and not exit. Canceling the close operation informs the system that any attempted shutdown should be abandoned.

This sequence of operations is analogous to the way that Windows-based applications handle the WM_QUERYENDSESSION message.

Depending on the tasking option chosen for the application, there may be some time between when the user chooses the Close command and the application checks the internal close flag. During this time, the system changes the window title of the application, appending the word "Closing" to it, and gives the user the opportunity to cancel the command by changing the command name to Cancel Close. If the user chooses the Cancel Close command, the close flag is reset, preventing the application from closing. If a close-aware application fails to check the close flag within a system-defined amount of time, the system automatically abandons the operation and resets the close flag.

The system tracks the close-awareness and close state for each process. For the virtual machine to close, all applications in the virtual machine must close. When the user chooses the Close command, the operating system detects applications that are not close-aware and displays a dialog box with a warning message, but gives the user the option of forcing the application to exit anyway. If you make an application close-aware, it can shut down cleanly.