How can the Task Manager in Windows or Force Quit in macOS close frozen, suspended, unresponding programs instantly. What do they do different than clicking the X button in the corner?
In: Technology
The Task Manager in Windows (or Finder in macOS) communicate with programs by sending them *signals*. When a program receives a signal, it briefly pauses (gets *interrupted*) to respond to the signal.
There are a couple common signals an OS might send to a program:
* **SIGTERM**. (Terminate signal). This signal is a request for the program to exit gracefully. For example, the program could save its data to a file before exiting. It is functionally similar to a user issuing a File > Exit or Quit command. Note: since SIGTERM is only a request, a program could ignore this signal and continue processing. An unresponsive program will not be able to handle the signal, and therefore may fail to exit.
* **SIGKILL**. (Kill signal). This signal is sent to forcefully terminate a program. Unlike SIGTERM, SIGKILL is not a request and cannot be ignored by the program. SIGKILL always terminates a program unless there’s an operating system kernel bug or an extremely anomalous condition.
So if we take macOS’s Finder as an example, selecting Quit from the dock menu will cause a SIGTERM to be sent to the application’s main process, while selecting Force Quit sends a SIGKILL instead.
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