24.4 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 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.

• Handler Returns: Handlers that return normally, and what this means.
• Termination in Handler: How handler functions terminate a program.
• Longjmp in Handler: Nonlocal transfer of control out of a signal handler.
• Signals in Handler: What happens when signals arrive while the handler is already occupied.
• Merged Signals: When a second signal arrives before the first is handled.
• Nonreentrancy: Do not call any functions unless you know they are reentrant with respect to signals.
• Atomic Data Access: A single handler can run in the middle of reading or writing a single object.