Monthly Archives: June 2012

A C API for C++ and Python ones — or making of libh2o

Lately I spent a lot of time working on a small project of mine called libh2o. Its goal is to provide a library of routines implementing IAPWS IF97 equations for water and steam properties. With the core written in C, and providing a nice-to-use API for C++ and Python.

At first, I thought about not providing a «high level» C API at all. It was like: if you want to use plain C, you’ve gotta glue all the low-level equations yourself. However, after some thinking I decided to provide one, and built the two remaining APIs (C++ and Python) on top of it.

The main reason for doing this was that Python (well, CPython) is written in C. Although I’ve seen people writing Python extensions in C++, and even using some of C++ features to make them a little nicer, that’s still a bunch of ugly C hacks and pointer casts. I don’t see a really good reason to write a Python extension in C++, nor to make it depend on a C++ compiler when it’s all limited to C-based CPython API anyway.

And that means that I have either to duplicate all the high-level logic in the Python extension, or just create a C API first and reuse that. Since the whole logic was simple enough to be covered completely and clearly in C, I have chosen this way.

As it happens when people choose C, I had to implement some kind of poor man’s objectivity. Not something as wide (and ugly) as GObject (someone, please kill it!) but a few bits necessary to keep the state. In other words, a structure keeping the «object» and a bunch of nicely named functions taking it as their first argument.

Before I learnt C++, I would assume that the object structure should be a private (and obscured) blob, and the object type should be an incomplete pointer to it. User should just grab that pointer from a «constructor», pass it around and finally free it through a «destructor». Advantage: the exact struct contents are not the part of ABI.

But now I’ve decided to go the other way; way similar to how C++ classes work. I’ve created a structure with explicitly listed private fields (and a very simple /*private:*/ comment), and used that as the public type. It doesn’t need to keep any memory allocated, and is simple enough to be allocated on stack. Advantages: no need for a destructor, and an ability to pack that struct in the C++ class which will wrap it.

Then the usual stuff: a bunch of functions with common prefixes. One prefix for the «namespace», another one for the function (new, get…). All in nice and clear fashion, either to be used directly or wrapped in the C++ or Python APIs.