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
                                                temporarily disable an existing handler.

       Tcl_FileProc *proc (in)                  Procedure  to   invoke   whenever   the   channel
                                                indicated   by   channel   meets  the  conditions
                                                specified by mask.

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

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. Typically, 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 do not 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  identified  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.

SEE ALSO

       Notifier(3tcl), Tcl_CreateChannel(3tcl), Tcl_OpenFileChannel(3tcl), vwait(3tcl).

KEYWORDS

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