### 25.4 Defining Signal Handlers ¶

This section describes how to write a signal handler function that can be established with the `signal` or `sigaction` functions.

A signal handler is just a function that you compile together with the rest of the program. Instead of directly invoking the function, you use `signal` or `sigaction` to tell the operating system to call it when a signal arrives. This is known as establishing the handler. See Specifying Signal Actions.

There are two basic strategies you can use in signal handler functions:

- You can have the handler function note that the signal arrived by tweaking some global data structures, and then return normally.
- You can have the handler function terminate the program or transfer control to a point where it can recover from the situation that caused the signal.

You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid.