
Foreign Function Interface for RPython
=======================================

Purpose
-------

This document describes an FFI for the RPython language, concentrating
on low-level backends like C. It describes
how to declare and call low-level (C) functions from RPython level.

Declaring low-level external function
-------------------------------------

Declaring external C function in RPython is easy, but one needs to
remember that low level functions eat `low level types`_ (like
lltype.Signed or lltype.Array) and memory management must be done
by hand. To declare a function, we write::

  from rpython.rtyper.lltypesystem import rffi

  external_function = rffi.llexternal(name, args, result)

where:

* name - a C-level name of a function (how it would be rendered)
* args - low level types of args
* result - low level type of a result

You can pass in additional information about C-level includes,
libraries and sources by passing in the optional ``compilation_info``
parameter::

  from rpython.rtyper.lltypesystem import rffi
  from rpython.translator.tool.cbuild import ExternalCompilationInfo

  info = ExternalCompilationInfo(includes=[], libraries=[])

  external_function = rffi.llexternal(
    name, args, result, compilation_info=info
    )

See cbuild_ for more info on ExternalCompilationInfo.

.. _`low level types`: rtyper.html#low-level-type
.. _cbuild: https://bitbucket.org/pypy/pypy/src/tip/rpython/translator/tool/cbuild.py


Types
------

In rffi_ there are various declared types for C-structures, like CCHARP
(char*), SIZE_T (size_t) and others. Refer to file for details. 
Instances of non-primitive types must be alloced by hand, with call 
to lltype.malloc, and freed by lltype.free both with keyword argument 
flavor='raw'. There are several helpers like string -> char*
converter, refer to the source for details.

.. _rffi: https://bitbucket.org/pypy/pypy/src/tip/pypy/rpython/lltypesystem/rffi.py

Registering function as external
---------------------------------

Once we provided low-level implementation of an external function,
would be nice to wrap call to some library function (like os.open)
with such a call. For this, there is a `register_external` routine,
located in `extfunc.py`_, which provides nice API for declaring such a
functions, passing llimpl as an argument and eventually llfakeimpl
as a fake low-level implementation for tests performed by an llinterp.

.. _`extfunc.py`: https://bitbucket.org/pypy/pypy/src/tip/pypy/rpython/extfunc.py
