Signal Handling

• A signal is used in UNIX systems to notify a process that a particular event has occurred.
• A signal may be received either synchronously or asynchronously, depending on the source of and the reason for the event being signaled.
  1. A signal is generated by the occurrence of a particular event.
  2. A generated signal is delivered to a process.
  3. Once delivered, the signal must be handled.
• Examples of synchronous signals include illegal memory access and division by O. If a running program performs either of these actions, a signal is generated.
• Synchronous signals are delivered to the same process that performed the operation that caused the signal (that is the reason they are considered synchronous).
• When a signal is generated by an event external to a running process, that process receives the signal asynchronously.
• Examples of such signals include terminating a process with specific keystrokes (such as $<control> <C>$ and having a timer expire.
• Typically, an asynchronous signal is sent to another process.
• Every signal may be handled by one of two possible handlers:
  • A default signal handler.
  • A user-defined signal handler
• Every signal has a default signal handler that is run by the kernel when handling that signal. This default action can be overridden by a user-defined signal handler that is called to handle the signal.