calling racket code from C lib

callback.rkt

#lang racket

(require
  ffi/unsafe
  ffi/unsafe/define)

;; helper to define racket symbols to refer to C symbols
(define-ffi-definer my-define (ffi-lib "libmyprint"))

;; typedef. this corresponds to the argument type of my_print
(define _my-callback (_cprocedure (list _string) _string))

(my-define my_print (_fun _my-callback -> _string))

;; racket function which will be passed to c code as a callback
;; returns a string of the first character of its input concatenated
;; four times
(define (four-times-the-first str)
  (make-string 4 (string-ref str 0)))

;; call the C function with our callback
;; output should be "CCCC"
(my_print four-times-the-first)

Makefile

CFLAGS=-fPIC
CC=cc

all: libmyprint.so

clean:
	rm -f *.o *.so

libmyprint.so: myprint.o
	${CC} ${CFLAGS} -shared -o libmyprint.so myprint.o

myprint.o: myprint.c
	${CC} ${CFLAGS} -c myprint.c

myprint.c

#include <stdio.h>

typedef char*(*callback)(char*);

char *
my_print(callback cb)
{
  static char text[] = "Cat";

  return cb(text);
}