Tcl_CreateChannelHandler(3)				      Tcl Library Procedures				       Tcl_CreateChannelHandler(3)

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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 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.
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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 speci-
       fied  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_CreateChannelHan-
       dler 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(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).

KEYWORDS

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

Tcl									7.5					       Tcl_CreateChannelHandler(3)