[mod_python] Output from C functions

I am running a long calculation using a web interface, and am having the
problem that the web client times out before the results come back.  I fixed
this by first printing a valid HTTP header and then having the Python script
print a counter, which the web browser picks up and prints - and then
doesn't time out.  Works very well.

BUT - I have now recoded the central calculation routine in C (all the
control/UI stuff is still Python), and placed a printf routine in there to
output the iterations like I did in the Python, but it doesn't get to the
webbrowser.  Any ideas why?  It prints to stdout if I just run the script
from the console, but not if I call it from apache.  Is there some buffering
when calling a C routine?

I am using mod_python 2.6.2 and I wrapped the C function using SWIG.

Grateful for any pointers.

Thanks
Alexis