       select readfileIds ?writefileIds? ?exceptfileIds? ?time-
              out?
              This command allows an Extended Tcl program to wait
              on  zero or more files being ready for for reading,
              writing, have an exceptional condition pending,  or
              for  a  timeout  period  to  expire.   readFileIds,
              writeFileIds,  exceptFileIds  are  each  lists   of
              fileIds, as returned from open, to query.  An empty
              list ({}) may be specified if  a  category  is  not
              used.

              The  files  specified  by  the readFileIds list are
              checked to see if data is  available  for  reading.
              The writeFileIds are checked if the specified files
              are  clear  for  writing.   The  exceptFileIds  are
              checked  to  see  if  an  exceptional condition has
              occurred (typically,  an  error).   The  write  and
              exception  checking is most useful on devices, how-
              ever, the read checking is very useful when  commu-
              nicating  with  multiple  processes  through pipes.
              Select considers data pending in  the  stdio  input
              buffer  for  read files as being ready for reading,
              the files do.  not have to be unbuffered.

              Timeout is a floating point timeout value, in  sec-
              onds.  If an empty list is supplied (or the parame-
              ter is omitted), then no timeout is  set.   If  the
              value is zero, then the select command functions as
              a poll of the files, returning immediately even  if
              none are ready.

              If  the  timeout  period  expires  with none of the
              files becoming ready, then the command  returns  an
              empty  list.   Otherwise the command returns a list
              of three elements, each of those elements is a list
              of  the  fileIds  that are ready in the read, write
              and exception classes.  If  none  are  ready  in  a
              class,  then  that  element  will be the null list.
              For example:

                      select {file3 file4 file5} {file6 file7} {} 10.5

              could return

                      {file3 file4} {file6} {}

              or perhaps

                      file3 {} {}

              On Windows 95/NT, only sockets can be used with the
              select  command.   Pipes,  as  returned by the open
              command, are not supported.
              This command is provided by Extended Tcl.
