VMS Help  —  CRTL  sigaction  Description
    When a process requests the sigaction function, the process
    can both examine and specify what action to perform when the
    specified signal is delivered. The arguments determine the
    behavior of the sigaction function as follows:

    o  Specifying the sig argument identifies the affected signal.
       Use any one of the signal values defined in the <signal.h>
       header file, except SIGKILL.

       If sig is SIGCHLD and the SA_NOCLDSTOP flag is not set in
       sa_flags, then a SIGCHLD signal is generated for the calling
       process whenever any of its child processes stop. If sig is
       SIGCHLD and the SA_NOCLDSTOP flag is set in sa_flags, then
       SIGCHLD signal is not generated in this way.

    o  Specifying the action argument, if not null, points to a
       sigaction structure that defines what action to perform when
       the signal is received. If the action argument is null, signal
       handling remains unchanged, so you can use the call to inquire
       about the current handling of the signal.

    o  Specifying the o_action argument, if not null, points to
       a sigaction structure that contains the action previously
       attached to the specified signal.

    The sigaction structure consists of the following members:

      void        (*sa_handler)(int);
      sigset_t    sa_mask;
      int         sa_flags;

    The sigaction structure members are defined as follows:

    sa_        This member can contain the following values:
    handler
               o  SIG_DFL - Specifies the default action taken when
                  the signal is delivered.

               o  SIG_IGN - Specifies that the signal has no effect on
                  the receiving process.

               o  Function pointer - Requests to catch the signal. The
                  signal causes the function call.

    sa_mask    This member can request that individual signals, in
               addition to those in the process signal mask, are
               blocked from delivery while the signal handler function
               specified by the sa_handler member is executing.
    sa_flags   This member can set the flags to enable further control
               over the actions taken when a signal is delivered.

    The sa_flags member of the sigaction structure has the following
    values:

    SA_ONSTACK     Setting this bit causes the system to run the
                   signal catching function on the signal stack
                   specified by the sigstack function. If this bit
                   is not set, the function runs on the stack of the
                   process where the signal is delivered.
    SA_RESETHAND   Setting this bit resets the signal to SIG_DFL. Be
                   aware that you cannot automatically reset SIGILL
                   and SIGTRAP.
    SA_NODEFER     Setting this bit does not automatically block the
                   signal as it is intercepted.
    SA_NOCLDSTOP   If this bit is set and the sig argument is equal
                   to SIGCHLD and a child process of the calling
                   process stops, then a SIGCHLD signal is sent to
                   the calling process only if SA_NOCLDSTOP is not
                   set for SIGCHLD.

    When a signal is intercepted by a signal-catching function
    installed by sigaction, a new signal mask is calculated and
    installed for the duration of the signal-catching function (or
    until a call to either sigprocmask or sigsuspend is made. This
    mask is formed by taking the union of the current signal mask and
    the value of the sa_mask for the signal being delivered unless
    SA_NODEFER or SA_RESETHAND is set, and then including the signal
    being delivered. If and when the user's signal handler returns
    normally, the original signal mask is restored.

    Once an action is installed for a specific signal, it remains
    installed until another action is explicitly requested (by
    another call to sigaction), until the SA_RESETHAND flag causes
    resetting of the handler, or until one of the exec functions is
    called.

    If the previous action for a specified signal had been
    established by signal, the values of the fields returned in
    the structure pointed to by the o_action argument of sigaction
    are unspecified, and in particular o_action->sa_handler is
    not necessarily the same value passed to signal. However, if a
    pointer to the same structure or a copy thereof is passed to a
    subsequent call to sigaction by means of the action argument
    of sigaction), the signal is handled as if the original call to
    signal were repeated.

    If sigaction fails, no new signal handler is installed.

    It is unspecified whether an attempt to set the action for a
    signal that cannot be intercepted or ignored to SIG_DFL is
    ignored or causes an error to be returned with errno set to
    EINVAL.

    See the "Error and Signal Handling" chapter of the VSI C RTL
    Reference Manual for more information on signal handling.

                                   NOTE

       The sigvec and signal functions are provided for
       compatibility to old UNIX systems; their function is a
       subset of that available with the sigaction function.

    See also sigvec, signal, wait, read, and write.
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