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

SYNOPSIS
     #include <tcl.h>

     void
     Tcl_CreateChannelHandler(channel, mask, proc, clientData)

     void
     Tcl_DeleteChannelHandler(channel, proc, clientData)


ARGUMENTS
     Tcl_Channel       channel      (in)      Tcl channel such as
                                              returned         by
                                              Tcl_CreateChannel.

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

     Tcl_FileProc      *proc        (in)      Procedure to invoke
                                              whenever  the chan-
                                              nel  indicated   by
                                              channel  meets  the
                                              conditions   speci-
                                              fied by mask.

     ClientData        clientData   (in)      Arbitrary  one-word
                                              value  to  pass  to
                                              proc.
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DESCRIPTION
     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  exceptional
     condition  exists  for  channel.  The conditions of interest
     under which proc will be invoked are specified by  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 prototype:
          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. Typi-
     cally, clientData points  to  a  data  structure  containing
     application-specific  information about the channel. Mask is
     an integer mask indicating which of the requested conditions
     actually exists for the channel; it will contain 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 handlers
     for a given channel as long as  they  don't  have  the  same
     channel,  proc, and clientData.  If Tcl_CreateChannelHandler
     is invoked when there is  already  a  handler  for  channel,
     proc,  and  clientData,  then  no  new  handler  is created;
     instead, the mask is changed for the existing handler.

     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 exam-
     ple, if there are two handlers for TCL_READABLE on the  same
     channel,  the  first handler could consume all of the avail-
     able 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.


SEE ALSO
     Notifier(3),  Tcl_CreateChannel(3),  Tcl_OpenFileChannel(3),
     vwait(n).


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

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