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'\" Copyright (c) 1996 Sun Microsystems, Inc.
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.so man.macros
.TH Tcl_CreateChannelHandler 3 7.5 Tcl "Tcl Library Procedures"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_CreateChannelHandler, Tcl_DeleteChannelHandler \- call a procedure when a channel becomes readable or writable
.SH SYNOPSIS
.nf
.nf
\fB#include <tcl.h>\fR
.sp
void
\fBTcl_CreateChannelHandler\fR(\fIchannel, mask, proc, clientData\fR)
.sp
void
\fBTcl_DeleteChannelHandler\fR(\fIchannel, proc, clientData\fR)
.sp
.SH ARGUMENTS
.AS Tcl_ChannelProc clientData
.AP Tcl_Channel channel in
Tcl channel such as returned by \fBTcl_CreateChannel\fR.
.AP int mask in
Conditions under which \fIproc\fR should be called: OR-ed combination of
\fBTCL_READABLE\fR, \fBTCL_WRITABLE\fR and \fBTCL_EXCEPTION\fR. Specify
a zero value to temporarily disable an existing handler.
.AP Tcl_FileProc *proc in
Procedure to invoke whenever the channel indicated by \fIchannel\fR meets
the conditions specified by \fImask\fR.
.AP ClientData clientData in
Arbitrary one-word value to pass to \fIproc\fR.
.BE

.SH DESCRIPTION
.PP
\fBTcl_CreateChannelHandler\fR arranges for \fIproc\fR to be called in the
future whenever input or output becomes possible on the channel identified
by \fIchannel\fR, or whenever an exceptional condition exists for
\fIchannel\fR. The conditions of interest under which \fIproc\fR will be
invoked are specified by the \fImask\fR argument.
See the manual entry for \fBfileevent\fR for a precise description of
what it means for a channel to be readable or writable.
\fIProc\fR must conform to the following prototype:
.CS
typedef void Tcl_ChannelProc(
        ClientData \fIclientData\fR,
        int \fImask\fR);
.CE
.PP
The \fIclientData\fR argument is the same as the value passed to
\fBTcl_CreateChannelHandler\fR when the handler was created. Typically,
\fIclientData\fR points to a data structure containing application-specific
information about the channel. \fIMask\fR 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 \fImask\fR argument to
\fBTcl_CreateChannelHandler\fR when the handler was created.
.PP
Each channel handler is identified by a unique combination of \fIchannel\fR,
\fIproc\fR and \fIclientData\fR.
There may be many handlers for a given channel as long as they don't
have the same \fIchannel\fR, \fIproc\fR, and \fIclientData\fR.
If \fBTcl_CreateChannelHandler\fR is invoked when there is already a handler
for \fIchannel\fR, \fIproc\fR, and \fIclientData\fR, then no new
handler is created;  instead, the \fImask\fR is changed for the
existing handler.
.PP
\fBTcl_DeleteChannelHandler\fR deletes a channel handler identified by
\fIchannel\fR, \fIproc\fR and \fIclientData\fR; if no such handler exists,
the call has no effect.
.PP
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 \fImask\fR argument
to \fIproc\fR may no longer exist when \fIproc\fR is invoked:  for
example, if there are two handlers for \fBTCL_READABLE\fR 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.

.SH "SEE ALSO"
Notifier(3), Tcl_CreateChannel(3), Tcl_OpenFileChannel(3), vwait(n).

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