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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 \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.