FFI概念
https://segmentfault.com/t/ffi/info
FFI即Foreign Function Interface，外部函数调用接口，是一个语言提供的使用其他语言现有库（以及其他组件）的接口。典型的例子包括C++的extern “C”、Java的JNI、.Net的P/Invoke、Python的Python/C API等。对于新的语言，提供FFI机制是很有现实意义的。提供这样的接口，意味着可以复用已有的无数现有的库。
https://en.wikipedia.org/wiki/Foreign_function_interface
A foreign function interface (FFI) is a mechanism by which a program written in one programming language can call routines or make use of services written in another.
The term comes from the specification for Common Lisp, which explicitly refers to the language features for inter-language calls as such;[1] the term is also used officially by the Haskell[2] and Python programming languages.[3] Other languages use other terminology: the Ada programming language talks about "language bindings", while Java refers to its FFI as the JNI (Java Native Interface) or JNA (Java Native Access). Foreign function interface has become generic terminology for mechanisms which provide such services.
Lua FFI
http://luajit.org/ext_ffi.html
The FFI library allows calling external C functions and using C data structures from pure Lua code.
The FFI library largely obviates the need to write tedious manual Lua/C bindings in C. No need to learn a separate binding language — it parses plain C declarations! These can be cut-n-pasted from C header files or reference manuals. It's up to the task of binding large libraries without the need for dealing with fragile binding generators.
demo
local ffi = require("ffi")
ffi.cdef[[
int printf(const char *fmt, ...);
]]
ffi.C.printf("Hello %s!", "world")
参考：
https://moonbingbing.gitbooks.io/openresty-best-practices/content/lua/FFI.html
Python FFI
https://cffi.readthedocs.io/en/latest/
CFFI documentation
C Foreign Function Interface for Python. Interact with almost any C code from Python, based on C-like declarations that you can often copy-paste from header files or documentation.
libFFI
http://sourceware.org/libffi/
What is libffi?
Compilers for high level languages generate code that follows certain conventions. These conventions are necessary, in part, for separate compilation to work. One such convention is the "calling convention". The "calling convention" is a set of assumptions made by the compiler about where function arguments will be found on entry to a function. A "calling convention" also specifies where the return value for a function is found.
Some programs may not know at the time of compilation what arguments are to be passed to a function. For instance, an interpreter may be told at run-time about the number and types of arguments used to call a given function. Libffi can be used in such programs to provide a bridge from the interpreter program to compiled code.
The libffi library provides a portable, high level programming interface to various calling conventions. This allows a programmer to call any function specified by a call interface description at run-time.
FFI stands for Foreign Function Interface. A foreign function interface is the popular name for the interface that allows code written in one language to call code written in another language. The libffi library really only provides the lowest, machine dependent layer of a fully featured foreign function interface. A layer must exist above libffi that handles type conversions for values passed between the two languages.