Tcl_CreateChannelHandler,  Tcl_DeleteChannelHandler - call
       a procedure when a channel becomes readable or writable


       #include <tcl.h>

       Tcl_CreateChannelHandler(channel, mask, proc, clientData)

       Tcl_DeleteChannelHandler(channel, proc, clientData)


       Tcl_Channel       channel      (in)      Tcl channel  such
                                                as   returned  by

       int               mask         (in)      Conditions  under
                                                which proc should
                                                be  called: OR-ed
                                                combination    of
                                                TCL_WRITABLE  and
                                                Specify  a   zero
                                                value   to   tem­
                                                porarily  disable
                                                an  existing han­

       Tcl_FileProc      *proc        (in)      Procedure      to
                                                invoke   whenever
                                                the channel indi­
                                                cated  by channel
                                                meets the  condi­
                                                tions   specified
                                                by mask.

       ClientData        clientData   (in)      Arbitrary    one-
                                                word   value   to
                                                pass to proc.


       Tcl_CreateChannelHandler arranges for proc to be called in
       the  future  whenever  input or output becomes possible on
       the channel identified by channel, or whenever  an  excep­
       the mask argument.  See the manual entry for fileevent for
       a precise description of what it means for a channel to be
       readable  or writable.  Proc must conform to the following
              typedef void Tcl_ChannelProc(
                ClientData clientData,
                int mask);

       The clientData argument is the same as the value passed to
       Tcl_CreateChannelHandler  when  the  handler  was created.
       Typically, clientData points to a data structure  contain­
       ing  application-specific  information  about the channel.
       Mask is an integer mask indicating which of the  requested
       conditions  actually  exists for the channel; it will con­
       tain a subset of  the  bits  from  the  mask  argument  to
       Tcl_CreateChannelHandler when the handler was created.

       Each channel handler is identified by a unique combination
       of channel, proc and clientData.  There may be  many  han­
       dlers  for  a given channel as long as they don't have the
       same channel, proc, and clientData.  If Tcl_CreateChannel­
       Handler  is  invoked  when  there is already a handler for
       channel, proc, and clientData, then no new handler is cre­
       ated;   instead, the mask is changed for the existing han­

       Tcl_DeleteChannelHandler deletes a channel handler identi­
       fied  by  channel, proc and clientData; if no such handler
       exists, the call has no effect.

       Channel handlers are invoked via the Tcl event  mechanism,
       so  they  are  only useful in applications that are event-
       driven.  Note also that the conditions  specified  in  the
       mask  argument  to  proc  may no longer exist when proc is
       invoked:  for example,  if  there  are  two  handlers  for
       TCL_READABLE  on the same channel, the first handler could
       consume all of the available input so that the channel  is
       no  longer  readable  when  the second handler is invoked.
       For this reason it may be useful to use nonblocking I/O on
       channels for which there are event handlers.


       Notifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3),


       blocking, callback, channel, events, handler, nonblocking.