[Python-Dev] PEP needed? Introducing Tcl objects

Martin v. Loewis martin@v.loewis.de
19 Feb 2002 21:22:16 +0100

"Jeffrey Hobbs" <jeffh@ActiveState.com> writes:

> IIRC, Tcl_SetMaxBlockTime is a one-short call - it sets the next
> block time, not all block times.  I'm sure there was a reason for
> this, but that was implemented before I was a core guy.  Anyway,
> I think you just need to try:
> -		result = Tcl_DoOneEvent(TCL_DONT_WAIT);
> +		Tcl_SetMaxBlockTime(&blocktime);
> +		result = Tcl_DoOneEvent(0);
> and see if that satisfies the need for responsiveness as well as
> not blocking.

Thanks, but that won't help. Tcl still performs a blocking select.
Studying the Tcl source, it seems that the SetMaxBlockTime feature is
broken in Tcl 8.3. DoOneEvent has

	 * If TCL_DONT_WAIT is set, be sure to poll rather than
	 * blocking, otherwise reset the block time to infinity.

	if (flags & TCL_DONT_WAIT) {
	    tsdPtr->blockTime.sec = 0;
	    tsdPtr->blockTime.usec = 0;
	    tsdPtr->blockTimeSet = 1;
	} else {
	    tsdPtr->blockTimeSet = 0;

So if TCL_DONT_WAIT is set, the blocktime is 0, otherwise, it is
considered not set. It then goes on doing

	if ((flags & TCL_DONT_WAIT) || tsdPtr->blockTimeSet) {
	    timePtr = &tsdPtr->blockTime;
	} else {
	    timePtr = NULL;
	result = Tcl_WaitForEvent(timePtr);

So if TCL_DONT_WAIT isn't set, it will block; if it is, it will
busy-wait. Looks like we lose either way.

In-between, it invokes the setupProcs of each input source, so that
they can set a maxblocktime, but I don't think _tkinter should hack
itself into that process.

So I don't see a solution on the path of changing how Tcl invokes

About thread-safety: Is Tcl 8.3 thread-safe in its standard
installation, so that we can just use it from multiple threads? If
not, what is the compile-time check to determine whether it is
thread-safe? If there is none, I really don't see a solution, and the
Sleep must stay.