CFFI documentation

C Foreign Function Interface for Python. The goal is to provide a convenient and reliable way to call compiled C code from Python using interface declarations written in C.


The interface is based on LuaJIT’s FFI, and follows a few principles:

  • The goal is to call C code from Python without learning a 3rd language: existing alternatives require users to learn domain specific language (Cython, SWIG) or API (ctypes). The CFFI design requires users to know only C and Python, minimizing the extra bits of API that need to be learned.
  • Keep all the Python-related logic in Python so that you don’t need to write much C code (unlike CPython native C extensions).
  • The preferred way is to work at the level of the API (Application Programming Interface): the C compiler is called from the declarations you write to validate and link to the C language constructs. Alternatively, it is also possible to work at the ABI level (Application Binary Interface), the way ctypes work. However, on non-Windows platforms, C libraries typically have a specified C API but not an ABI (e.g. they may document a “struct” as having at least these fields, but maybe more).
  • Try to be complete. For now some C99 constructs are not supported, but all C89 should be, including macros (and including macro “abuses”, which you can manually wrap in saner-looking C functions).
  • Attempt to support both PyPy and CPython, with a reasonable path for other Python implementations like IronPython and Jython.
  • Note that this project is not about embedding executable C code in Python, unlike Weave. This is about calling existing C libraries from Python.

Get started by reading the overview.

Comments and bugs

The best way to contact us is on the IRC #pypy channel of Feel free to discuss matters either there or in the mailing list. Please report to the issue tracker any bugs.

As a general rule, when there is a design issue to resolve, we pick the solution that is the “most C-like”. We hope that this module has got everything you need to access C code and nothing more.

— the authors, Armin Rigo and Maciej Fijalkowski