

          signal action siglist [command]
               Specify the action to take when a Unix signal is
               received by Extended Tcl, or a program that embeds it.
               Siglist is a list of either the symbolic or numeric
               Unix signal (the SIG prefix is optional).  Action is
               one of the following actions to be performed on receipt
               of the signal.

               default - Perform system default action when signal is
               received (see signal system call documentation).

               ignore - Ignore the signal.

               error - Generate a catchable Tcl error.  It will be as
               if the command that was running returned an error.  The
               error code will be in the form:
                   UNIX SIG signame
               For the death of child signal, signame will always be
               SIGCHLD, rather than SIGCLD, to allow writing portable
               code.

               trap - When the signal occures, execute command and
               continue execution if an error is not returned by
               command.  If will be executed in the global context and
               the symbolic signal name (e.g. SIGINT) will be supplied
               in a global variable signalReceived.  If an error is
               returned, then follow the standard Tcl error mechanism.
               Often command will just do an exit.

               get - Retrieve the current settings of the specified
               signals.  A keyed list will be returned were the keys
               are one of the specified signals and the values are a
               list cosisting of the action associated with the
               signal, a 0 if the signal may be delivered (not block)
               and a 1 if it is blocked. The actions maybe one of
               `default',`ignore', `error' or `trap.  If the action is
               trap, the third element is the command associated with
               the action.

               block - Block the specified signals from being
               received. (Posix systems only).

               unblock - Allow the specified signal to be received.
               Pending signals will not occur. (Posix systems only).

               The signal action will remain enabled after the
               specified signal has occurred.  The exception to this
               is SIGCHLD on systems without Posix signals.  For these
               systems, SIGCHLD is not be automatically reenabled.
               After a SIGCHLD signal is received, a call to wait must
               be performed to retrieve the exit status of the child
               process before issuing another signal SIGCHLD ...
               command.  For code that is to be portable between both
               types of systems, use this approach.

               Signals are not processed until after the completion of
               the Tcl command that is executing when the signal is
               received.  If an interactive Tcl shell is running, then
               the SIGINT will be set to error, non-interactive Tcl
               sessions leave SIGINT unchanged from when the process
               started (normally default for foreground processes and
               ignore for processes in the background).
