Function unix_signal_add 

pub fn unix_signal_add<F>(signum: i32, func: F) -> SourceId
where
    F: FnMut() -> ControlFlow + Send + 'static,

Available on Unix only.

Adds a closure to be called by the default main loop whenever a UNIX signal is raised.

func will be called repeatedly every time signum is raised until it returns ControlFlow::Break.

The default main loop almost always is the main loop of the main thread. Thus, the closure is called on the main thread.