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_CreateChan
nel.
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 tem
porarily disable
an existing han
dler.
Tcl_FileProc *proc (in) Procedure to
invoke whenever
the channel indi
cated by channel
meets the condi
tions specified
by mask.
ClientData clientData (in) Arbitrary one-
word value to
pass to proc.
_________________________________________________________________
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 excep
tional condition exists for channel. The conditions of
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 contain
ing application-specific information about the channel.
Mask is an integer mask indicating which of the requested
conditions actually exists for the channel; it will con
tain 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 han
dlers for a given channel as long as they don't have the
same channel, proc, and clientData. If Tcl_CreateChannel
Handler is invoked when there is already a handler for
channel, proc, and clientData, then no new handler is cre
ated; instead, the mask is changed for the existing han
dler.
Tcl_DeleteChannelHandler deletes a channel handler identi
fied 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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