TIP 49: I/O Subsystem: Add API Tcl_OutputBuffered(chan)

Author:       Rolf Schroedter <[email protected]>
State:        Final
Type:         Project
Vote:         Done
Created:      25-Jul-2001
Tcl-Version:  8.4


This document proposes the new public function Tcl_OutputBuffered(), analogous to the existing public function Tcl_InputBuffered().


Tcl has a Tcl_InputBuffered() function but no analog function for the output buffer. A Tcl_OutputBuffered() function would be useful for non-blocking channel drivers which need to know the number of bytes pending in Tcl's output queue.

The implementation of [35] allows one to query the number of bytes in the channels input and output queues with a [fconfigure -queue] option. This is a useful feature especially for serial ports because the input/output may be really slow or even stall.

On the driver level only the number of bytes in the system queue can be queried. For a non-blocking channel there may also be some pending output in Tcl buffers. Obviously there is not much sense to know only the byte counter at driver level without knowing Tcl_OutputBuffered().

Related Ideas

It could also be useful to add general [fconfigure -inputbuffer -outputbuffer] options for all channels returning the values from Tcl_InputBuffered(chan) and Tcl_OutputBuffered(chan) respectively.

At this opportunity the code of Tcl_Seek() and Tcl_Tell() may be shortened, because it repeats the code of Tcl_InputBuffered() and Tcl_OutputBuffered().


This function would be added to generic/tclIO.c and be stubs enabled. This new API should not have any impact on existing applications.

The implementation is analog to what is done in Tcl_Tell():

 * Tcl_OutputBuffered --
 *	Returns the number of bytes of output currently buffered in the
 *	common internal buffer of a channel.
 * Results:
 *	The number of output bytes buffered, or zero if the channel is not
 *	open for writing.
 * Side effects:
 *	None.

    Tcl_Channel chan;			/* The channel to query. */
    ChannelState *statePtr = ((Channel *) chan)->state;
					/* State of real channel structure. */
    ChannelBuffer *bufPtr;
    int bytesBuffered;

    for (bytesBuffered = 0, bufPtr = statePtr->outQueueHead;
	bufPtr != (ChannelBuffer *) NULL;
	bufPtr = bufPtr->nextPtr) {
	bytesBuffered += (bufPtr->nextAdded - bufPtr->nextRemoved);
    if ((statePtr->curOutPtr != (ChannelBuffer *) NULL) &&
	    (statePtr->curOutPtr->nextAdded > statePtr->curOutPtr->nextRemoved)) {
        statePtr->flags |= BUFFER_READY;
        bytesBuffered +=
            (statePtr->curOutPtr->nextAdded - statePtr->curOutPtr->nextRemoved);
    return bytesBuffered;


This document has been placed in the public domain.