ANN: Stackless Python 0.2

Tim Peters tim_one at
Tue Jun 29 10:12:07 CEST 1999

[G. David Kuhlman]
> I'll bite. How *does* a thread that calls out to C release the global
> lock?

This is covered in detail in the Python/C API manual (look in your Python
doc distribution), chapter 8.  You typically just use a pair of
Python-supplied bracket macros:

... here you own the global lock


... now you do not:  other Python threads can run, and you can do
... C stuff here as long as you like, in parallel with them


... now you own the global lock again, and can return to Python

Many examples of these macros can be found in the distribution's C source
code too.

> Would it have to do this thingy that is at the beginning of the
> interpreter loop in python/ceval.c?
>     if (PyThreadState_Swap(NULL) != tstate)
>         Py_FatalError("ceval: tstate mix-up");
>     PyThread_release_lock(interpreter_lock);
> Would this work? or would it screw up the interpreter?

It won't even compile.  interpreter_lock is not an extern symbol:  you can't
get at it directly.  That's a feature, of course, to discourage chuckleheads
from trying to subvert the published API <wink>.

> Is there a way to do this in a Python script *before* it calls out
> to C.

No:  Python-level code must never run unless the thread running it holds the
global lock.  Therefore no Python-level code can ever release the global
lock (the instant it did so, it would be illegal code -- so no possibility
for screwing up this way is provided).

This is no real restriction, though.  To run external C code at all you have
to put it in a module known to Python (so that Python can resolve the name
*from* Python), and in the typical case where you're calling some
pre-existing thread-safe C function ThreadedSpam, you're not going to stick
ThreadedSpam in a Python module anyway.  Instead you'd write your Python C
module with a trivial wrapper function and tell Python about *that*; its
guts will consist of


it's-only-confusing-if-you-think-about-it-too-much<wink>-ly y'rs  - tim

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