use C shared lib with cffi

demo-cffi.lisp

;;;building a C shared library,
;;;and calling it in Common Lisp with CFFI

(require :asdf)
(asdf:load-system :cffi)
 
(defpackage :cffi-user
  (:use :common-lisp :cffi))
 
(in-package :cffi-user)
 
(define-foreign-library libmystuff
  (t (:default "libmystuff")))
 
(use-foreign-library libmystuff)
 
(defcfun "hello" :void (name :pointer))
 
(defcfun "double_it" :int (n :int))
 
;;use it
(let ((n 4))
  (format t "~a double it! ~a~%" n (double-it 4)))
 
(with-foreign-string (name "Billy")
  (hello name))

gistfile2.sh

#compile with 'position independent code' flag
gcc -fpic -c greeting.c mymath.c

#build the shared object library
gcc -shared -o libmystuff.so greeting.o mymath.o
 
#copy library somewhere we can see it, see /etc/ld.so.conf
cp libmystuff.so ~/local/lib

#configure linker to see it
sudo ldconfig

greeting.c

#include <stdio.h>
#include "greeting.h"
 
void hello (char* name) {
    printf("Hello, %s!\n", name);
}

greeting.h

void hello (char*);

mymath.c

#include "mymath.h"
 
int double_it (int n) {
    return n * 2;
}

mymath.h

int double_it (int);