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Tcl_CreateChannelHandler(3) Tcl	Library	Procedures Tcl_CreateChannelHandler(3)


       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 Tcl_CreateChannel.

       int mask	(in)				Conditions  under  which  proc
						should be called: OR-ed	combi-
						nation	   of	 TCL_READABLE,
						TION.  Specify a zero value to
						temporarily disable an	exist-
						ing handler.

       Tcl_FileProc *proc (in)			Procedure  to  invoke whenever
						the channel indicated by chan-
						nel meets the conditions spec-
						ified 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 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_Create-
       ChannelHandler 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  re-
       quested	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;  in-
       stead, the mask is changed for the existing handler.

       Tcl_DeleteChannelHandler	deletes	a channel handler identified by	 chan-
       nel,  proc  and	clientData; if no such handler exists, the call	has no

       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), vwait(n).

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

Tcl				      7.5	   Tcl_CreateChannelHandler(3)


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