differential-datalog | programming language for incremental computation | Database library

There are two steps to using CFFI. The first one is to decide if you're using the ABI or the API mode---ffi.dlopen() versus ffi.set_source(); see https://cffi.readthedocs.io/en/latest/overview.html#abi-versus-api for details. Then you need to write the call to ffi.cdef() accordingly. I generally recommend to use the API mode, which is much more flexible at the cost of requiring a C compiler at install-time (just like if you wrote a standard CPython C extension module). In both modes, you should copy manually parts of the C header inside the call to ffi.cdef(); in the API mode you can leave many more details out and replace them with ... (dot-dot-dot).

In neither of the two modes can you just paste a random C header making use of all the standard C features. Calling gcc -E just makes the problem even harder (but has been done in some cases with very large libraries, with lots of custom post-processing). The point of ffi.cdef() is that normally, you paste a simplified version of just the features you need.

To answer your precise question:

• macros that are just constants, like an integer, are directly supported. In API mode you can also write #define MY_CONSTANT ... with dot-dot-dot.

• macros that work like functions are supported in the API mode (only) by using the following trick: write them in ffi.cdef() as if they were regular functions. That's enough to be able to call them.

• for more complex cases, you might have to write a real C function that wraps the usage of the macro or macros as needed. Just invent a new function name, declare the function in ffi.cdef(), and finally implement the function directly in ffi.set_source().

In the ABI mode, the last two cases cannot be supported at all---macros don't exist in the ABI any more.