Tcl_CreateChannelHandler(3) Tcl Library Procedures Tcl_CreateChannelHandler(3)
Tcl_CreateChannelHandler, Tcl_DeleteChannelHandler - call a procedure when a channel
becomes readable or writable
Tcl_CreateChannelHandler(channel, mask, proc, clientData)
Tcl_DeleteChannelHandler(channel, proc, clientData)
Tcl_Channel channel (in) Tcl channel such as returned by Tcl_CreateChan-
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 temporar-
ily disable an existing handler.
Tcl_FileProc *proc (in) Procedure to invoke whenever the channel indi-
cated by channel meets the conditions specified
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(
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 sub-
set of the bits from the mask argument to Tcl_CreateChannelHandler when the handler was
Each channel handler is identified by a unique combination of channel, proc and client-
Data. 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 client-
Data; 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.
Notifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).
blocking, callback, channel, events, handler, nonblocking.
Tcl 7.5 Tcl_CreateChannelHandler(3)