Tcl_CreateChannelHandler(3) Tcl Library Procedures

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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 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 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 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

Tcl 7.5 Tcl_CreateChannelHandler(3)