       signal action siglist ?command?
              Warning:    The  behavior  of  signals  under  Unix
              doesn't make signals  a  general  event  mechanism.
              Interrupted  system  calls,  caused by signals, can
              cause errors in code where they are  not  expected.
              This  can  cause  a program to behave in unexpected
              ways.  In general, the only safe usage for  signals
              is to generate errors to terminate a blocked opera-
              tion.

              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  per-
              formed  on  receipt  of the signal.  To specify all
              modifiable signals, use `*' (this will not  include
              SIGKILL  and SIGSTOP, as they can not be modified).

              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:
                  POSIX 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 occurs, execute command  and
              continue  execution  if an error is not returned by
              command.  The  command  will  be  executed  in  the
              global  context.  The command will be edited before
              execution, replacing occurrences of "%S"  with  the
              signal  name.  Occurrences of "%%" result in a sin-
              gle "%".  This editing occurs just before the  trap
              command  is  evaluated.   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 speci-
              fied signals.  A keyed list will be  returned  were
              the  keys  are one of the specified signals and the
              values are a list consisting of the action  associ-
              ated  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.  The action
              `unknown' is returned if a non-Tcl  signal  handler
              has been associated with the signal.
              set  -  Set signals from a keyed list in the format
              returned by the get.  For this action,  siglist  is
              the  keyed  list  of signal state.  Signals with an
              action of `unknown' are not modified.

              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  comple-
              tion  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).

              This command is provided by Extended Tcl.
