Threading problems with SWIG generated code

[Yves Perrenoud]

I've ran into some problems when trying to use multiple threads which
make use of modules generated with SWIG. Here's the shortest example I
could come up with to illustrate the problem:

Given the following SWIG interface file:

----
%module blip

unsigned int sleep(unsigned int seconds);
----

I generate the C code using "swig -python", compile it and generate the
shared library.

Here's some sample code using the newly generated "blip" module:

----
import threading, blip

def foo():
    blip.sleep(5)

threading.Thread(target=foo).start()
threading.Thread(target=foo).start()
----

You would expect this function to run for a total of approximately 5
seconds, but instead, it runs for 10 seconds! Clearly, the sleep
function calls are running one after the other, when they should be
running in parallel. It's almost as if I was acquiring a lock before the
sleep and releasing it afterwards.

Any idea what's causing this?

 ..Yves