Tcl_CreateChannelHandler(3) Tcl (7.5) 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
Page 1 (printed 2/19/99)
Tcl_CreateChannelHandler(3) Tcl (7.5) Tcl_CreateChannelHandler(3)
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 don't 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.
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