cffi doesn't allow creating pointers to existing C functions

Created originally on Bitbucket by pitrou (Antoine Pitrou)

Browsing through the docs, it isn't possible to create a function pointer to a plain existing C function. cffi has ffi.callback which allows to create a C function pointer to a Python function; but you can't just create a C function pointer from either the function's name (symbol) or its numeric address. ctypes lets you do it from the function's address.

(for the record, in my use case, the function doesn't exist at the C level, as it's generated by LLVM, but the basic point stands)