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(libc.info)Advanced Signal Handling

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Advanced Signal Handling
------------------------

   The `sigaction' function has the same basic effect as `signal': to
specify how a signal should be handled by the process.  However,
`sigaction' offers more control, at the expense of more complexity.  In
particular, `sigaction' allows you to specify additional flags to
control when the signal is generated and how the handler is invoked.

   The `sigaction' function is declared in `signal.h'.

 - Data Type: struct sigaction
     Structures of type `struct sigaction' are used in the `sigaction'
     function to specify all the information about how to handle a
     particular signal.  This structure contains at least the following
     members:

    `sighandler_t sa_handler'
          This is used in the same way as the ACTION argument to the
          `signal' function.  The value can be `SIG_DFL', `SIG_IGN', or
          a function pointer.  Note: Basic Signal Handling.

    `sigset_t sa_mask'
          This specifies a set of signals to be blocked while the
          handler runs.  Blocking is explained in Note: Blocking for
          Handler.  Note that the signal that was delivered is
          automatically blocked by default before its handler is
          started; this is true regardless of the value in `sa_mask'.
          If you want that signal not to be blocked within its handler,
          you must write code in the handler to unblock it.

    `int sa_flags'
          This specifies various flags which can affect the behavior of
          the signal.  These are described in more detail in Note:
          Flags for Sigaction.

 - Function: int sigaction (int SIGNUM, const struct sigaction
          *restrict ACTION, struct sigaction *restrict OLD-ACTION)
     The ACTION argument is used to set up a new action for the signal
     SIGNUM, while the OLD-ACTION argument is used to return
     information about the action previously associated with this
     symbol.  (In other words, OLD-ACTION has the same purpose as the
     `signal' function's return value--you can check to see what the
     old action in effect for the signal was, and restore it later if
     you want.)

     Either ACTION or OLD-ACTION can be a null pointer.  If OLD-ACTION
     is a null pointer, this simply suppresses the return of
     information about the old action.  If ACTION is a null pointer,
     the action associated with the signal SIGNUM is unchanged; this
     allows you to inquire about how a signal is being handled without
     changing that handling.

     The return value from `sigaction' is zero if it succeeds, and `-1'
     on failure.  The following `errno' error conditions are defined
     for this function:

    `EINVAL'
          The SIGNUM argument is not valid, or you are trying to trap
          or ignore `SIGKILL' or `SIGSTOP'.