Foreign Function Interface (FFI) in Racket | Schema Programming Part 20

Sometimes you need raw speed, or access to a specific hardware library written in C. Racket's FFI allows you to load dynamic libraries (.dll, .so, .dylib) and call their functions without writing any C glue code.

A Simple Example: Calling printf from C

#lang racket
(require ffi/unsafe)

; Load the C library (libc is usually available by default, or passing #f)
(define ffi-printf
  (get-ffi-obj "printf" #f (_fun _string -> _int)))

(ffi-printf "Hello from C! Number: %d\n" 42)

Defining Types

The FFI library defines mappings for C types:

• _int -> C int
• _float -> C float
• _string -> C char*
• _void -> C void

Designing a Wrapper

Usually, you wrap the unsafe FFI calls in a safe Racket module.

(provide my-fast-add)

(define c-add
  (get-ffi-obj "add_numbers" "mylib.so" 
    (_fun _int _int -> _int)))

(define (my-fast-add x y)
  (c-add x y))

Summary

Racket acts as a fantastic "glue language". With FFI, you can script low-level system operations while keeping the high-level logic in safe, functional Racket code.