Signals

Description

A signal is a notification sent to a process or thread to indicate that a particular type of event has occurred. Processes can specify the action to be taken when a given signal is raised. This could be calling a given handler function, or ignoring the signal, or performing a default action (usually ignoring the signal or terminating the process). Common uses of signals include:

• handling SIGINT to prevent a program from being terminated by control-C, or alternatively, to provide for an orderly shutdown. Similarly for SIGTERM and SIGQUIT;
• handling SIGPIPE to prevent termination when writing to a broken pipe or disconnected socket;
• handling SIGCHLD to detect when there are zombie processes in need of reaping; and
• handling SIGHUP as a signal to a daemon that it should reload its configuration.

Some care is needed when writing handler functions because they may be invoked at any point during the execution of a program. They cannot safely access global data unless it is of type sig_atomic_t, or call system functions outside of a limited subset. If a signal handler interrupts a system function that was executing in the foreground then the latter may return early, typically with the error code EINTR.

microHOWTOs

• Ignore SIGPIPE without affecting other threads in a process
• Prevent a process from terminating when writing to a broken pipe
• Reap zombie processes using a SIGCHLD handler

Further reading

• Signal Concepts, Base Specifications, Issue 7, The Open Group, 2008
• Signal Handling, GNU C Library Reference Manual, The GNU Project

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