TIP: | 49 |
Title: | I/O Subsystem: Add API Tcl_OutputBuffered(chan) |
Version: | $Revision: 1.4 $ |
Author: | Rolf Schroedter <rolf dot schroedter at dlr dot de> |
State: | Final |
Type: | Project |
Tcl-Version: | 8.4 |
Vote: | Done |
Created: | Wednesday, 25 July 2001 |
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 TIP #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().
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. * *---------------------------------------------------------------------- */ int Tcl_OutputBuffered(chan) 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.
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