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# 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.

## 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.

## 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.

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