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Tcl_CreateChannelHandler(3)                             Tcl Library Procedures

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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 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(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).

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

Tcl                                   7.5          Tcl_CreateChannelHandler(3)