How are asynchronous signals handled in Linux?

Question

This seems like a silly question, but I can't find the answer to it anywhere I look. I know that in UNIX, signals are handled asynchronously. If I write a function that handles a signal, where is that function run? Is a new thread spawned? Is an existing thread interrupted somehow? Or is this handled in a system thread like asynchronous I/O is?

Answer 1

A signal function is executed as if a thread in the process has been interrupted. That is, the signal handler is called using the signaled thread and the stack is rearranged so that when the signal handler returns the thread continues execution. No new threads are introduced.

Answer 2

An existing process thread is interrupted until the function returns. There are serious restrictions on what it can safely do to ensure it doesn't corrupt state of function calls the thread was in the middle of - specifically, any functions it calls that the thread may have already been calling must be async reentrant. See the man pages e.g. signal, sigaction for further details or ask more specific questions as you like.