16.1. Signals

Perl uses a simple signal-handling model: the %SIG hash contains references (either symbolic or hard) to user-defined signal handlers. Certain events cause the operating system to deliver a signal to the affected process. The handler corresponding to that event is called with one argument containing the name of the signal that triggered it. To send a signal to another process, you use the kill function. Think of it as sending a one-bit piece of information to the other process.^[2] If that process has installed a signal handler for that signal, it can execute code when it receives the signal. But there's no way for the sending process to get any sort of return value, other than knowing that the signal was legally sent. The sender receives no feedback saying what, if anything, the receiving process did with the signal.

^[2] Actually, it's more like five or six bits, depending on how many signals your OS defines and on whether the other process makes use of the fact that you didn't send a different signal.

We've classified this facility as a form of IPC, but in fact, signals can come from various sources, not just other processes. A signal might also come from your own process, or it might be generated when the user at the keyboard types a particular sequence like Control-C or Control-Z, or it might be manufactured by the kernel when a special event transpires, such as when a child process exits, or when your process runs out of stack space or hits a file size or memory limit. But your own process can't easily distinguish among these cases. A signal is like a package that arrives mysteriously on your doorstep with no return address. You'd best open it carefully.