FFI

Foreign Function Interface (FFI) is a mechanism that allows code written in one programming language to call routines or use services written in another language. It provides a bridge between languages, handling symbol resolution, data type conversion (marshalling), and calling conventions. FFI can be provided by language runtimes, compilers, or external libraries, and can operate at compile time or at runtime.

Implementation approaches include static bindings generated at compile time or dynamic bindings resolved at runtime. A common tool is libffi, a portable library that enables calling into functions whose interface is not known until runtime. Languages expose FFI bindings to C libraries, such as Python's ctypes and cffi, Ruby's FFI, LuaJIT FFI, and Rust's extern "C" interfaces. Data types must be mapped between languages, including integers, floating point numbers, pointers, and structs; memory ownership and lifetimes must be managed to avoid leaks or undefined behavior.

FFI is widely used to access system APIs, to reuse performance-critical native libraries, or to interoperate with legacy code. It enables language ecosystems to leverage existing code without full reimplementation, but it can introduce portability challenges due to differences in ABIs, alignment, and platform conventions. Proper use requires attention to error translation, threading models, and security implications.