fork
              Fork the current Tcl process.  Fork returns zero to
              the child process and the  process  number  of  the
              child  to the parent process.  If the fork fails, a
              Tcl error is generated.

              If an execl is not going to be performed before the
              child  process  does  output, or if a close and dup
              sequence is going to  be  performed  on  stdout  or
              stderr,  then a flush should be issued against std-
              out, stderr and any other open output  file  before
              doing  the fork. Otherwise characters from the par-
              ent process pending in the buffers will  be  output
              by both the parent and child processes.

              Note:  If you are forking in a Tk based apllication
              you must execl before doing any  window  operations
              in  the child or you will receive a BadWindow error
              from the X server.

              This command is provided by Extended Tcl.