Handles the user clicking on the close button of the window.
On platforms that have a close button, this routine installs a callback
function to handle the close event. In other words, when the user clicks
the close button on your program's window or any equivalent device, the
function you specify here will be called.
This function should not generally attempt to exit the program or save
any data itself. The function could be called at any time, and there is
usually a risk of conflict with the main thread of the program. Instead,
you should set a flag during this function, and test it on a regular
basis in the main loop of the program.
Pass NULL as the `proc' argument to this function to disable the close
button functionality, which is the default state.
Note that Allegro cannot intercept the close button of a DOS box in
Also note that the supplied callback is also called under MacOS X when
the user hits Command-Q or selects "Quit" from the application menu.
volatile int close_button_pressed = FALSE;
close_button_pressed = TRUE;
Returns zero on success and non-zero on failure (e.g. the feature is not
supported by the platform).
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