memory leak in extension

I am using python-3.8.5 with reportlab to produce charts.

I have an ongoing memory issue with a program that uses the reportlab _renderPM extension along with pillow.

I don't see the issue in linux (at least I don't see the same linear growth in memory usage

I used both objgraph & pympler to see if I could spot dangling objects, but failed to see anything obvious.

I have access to the vs 14.2 tools and can probably remember enough C to modify the _renderPM code, but I seek advice on 
what a suitable approach to detecting leaks in C extensions as I don't think the allocated python objects show growth in 
the Gb range.
-- 
Robin Becker