(libc.info.gz) Delivery of Signal

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(libc.info.gz) Signal Generation

(libc.info.gz) Concepts of Signals

 
 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.
  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' ( 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 ( 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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(libc.info.gz) Signal Generation

(libc.info.gz) Concepts of Signals