# Application control

The compositor is able to control the behaviour of running applications using [IPC socket](https://en.wikipedia.org/wiki/Unix_domain_socket) communication with the Linux Kernel. It is also possible to start new instances of applications.

The following signals are implemented:

| Signal  | #  | Description                                                                             |
| --------|----| ----------------------------------------------------------------------------------------|
| SIGCONT | 18 | Continue execution after being paused by the *SIGSTOP* signal.                          |
| SIGSTOP | 19 | Pause execution by not receiving any more CPU cycles.                                   |

The signalling is implemented using the C++ POSIX standard ``int kill(pid_t pid, int sig)`` function.

Application control is used by the compositor to control the minimized mode from parameter `X-Minimized Mode` of the [desktop file](application_registration.html) configuration. If minimized mode is set to `Paused`, the application will instantly be paused while it is being registered in the compositor.

## Controller Class

In the implementation of the compositor, each *Application* object has access to the *Controller* class which has a few methods to control the execution of the application.

### pauseApplication

The *pauseApplication* method pauses a running application by sending the *SIGSTOP* signal. This is useful when an application is started but is minimized or otherwise not visible for the user, to save CPU load.

>![Note](/content/images/note.png) If the user interface of the application is visible while it is being paused, the UI will still be shown but is completely non-responsive.

### resumeApplication

The *resumeApplication* method resumes execution of a paused application. It is normally used to invoke application after the *pauseApplication* call.

## Read more

- List of [Linux/Unix signals](https://www-uxsup.csx.cam.ac.uk/courses/moved.Building/signals.pdf)
- The [kill](https://man7.org/linux/man-pages/man2/kill.2.html) shell command
- The [kill](https://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html) programmatically using C