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24.1.3 How Signals Are Delivered

When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely—until signals of that kind are unblocked. Once unblocked, it will be delivered immediately. See Blocking Signals.

When the signal is delivered, whether right away or after a long delay, the specified action for that signal is taken. For certain signals, such as SIGKILL and SIGSTOP, the action is fixed, but for most signals, the program has a choice: ignore the signal, specify a handler function, or accept the default action for that kind of signal. The program specifies its choice using functions such as signal or sigaction (see Signal Actions). We sometimes say that a handler catches the signal. While the handler is running, that particular signal is normally blocked.

If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.

If a signal arrives which the program has neither handled nor ignored, its default action takes place. Each kind of signal has its own default action, documented below (see Standard Signals). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent “harmless” events, the default action is to do nothing.

When a signal terminates a process, its parent process can determine the cause of termination by examining the termination status code reported by the wait or waitpid functions. (This is discussed in more detail in Process Completion.) The information it can get includes the fact that termination was due to a signal and the kind of signal involved. If a program you run from a shell is terminated by a signal, the shell typically prints some kind of error message.

The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a core dump file which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error.

If you raise a “program error” signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.

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