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Signals Overview

  • A signal is a notification (usually from the kernel) to a process that an event has occurred. Signals are sometimes described as software interrupts.

  • Among the types of events that cause the kernel to generate a signal for a process are the following:

    • A hardware exception occurred, meaning that the hardware detected a fault condition that was notified to the kernel, which in turn sent a corresponding signal to the process concerned. Example: dividing by 0, referencing a part of memory that is inaccessible
    • The user typed one of the terminal special characters that generate signals. These characters include the interrupt character (usually Control-C) and the suspend character (usually Control-Z).
    • A software event occurred. Example: input became available on a file descriptor, the terminal window was resized, a timer went off, the process’s CPU time limit was exceeded, or a child of this process terminated.

  • Signals fall into two broad categories:

    • standard signals: used by the kernel to notify process of events (numbered from 1 to 31 on Linux)
    • realtime signals

  • Signal flow: -> Generated (from the kernel or process itself) -> Pending -> Delivered to process -> Blocked (if the signal is added to the process’s signal mask) -> Arrived at the process

  • Upon delivery of a signal, a process carries out one of the following default actions, depending on the signal:

    • The signal is ignored: discarded by the kernel, has no effect on the process (The process never even knows that it occurred.)
    • The process is terminated (killed).
    • A core dump file is generated, the process is terminated. A core dump file contains an image of the virtual memory of the process which is used for debugging.
    • The process is stopped - execution of the process is suspended.
    • Execution of the process is resumed after previously being stopped.

  • Instead of accepting the default for a particular signal, a program can change the action that occurs when the signal is delivered (aka setting the disposition of the signal). A program can set one of the following dispositions for a signal:

    • The default action should occur. This is useful to undo an earlier change of the disposition of the signal to something other than its default
    • The signal is ignored. This is useful for a signal whose default action would be to terminate the process.
    • A signal handler is executed.