How do arrange to have a (Cancel) button to stop a calacuation? When you are doing cpu-intensive calculations, your program probably isn't calling the XView notifier, so that button presses aren't noticed until the computation is over. There are several possible solutions: * use the implicit or explicit notify displatch mechanism, described in the Notifier chapter of the XView Programming Manual (O'Reilly); * use multiple processes, and send signals; * split up the work into small chunks and use notify_stop to return to the main loop for each chunk.

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