[cffi-devel] C to Lisp

[Greg Bennett]

Good morning,
I (think I) might be close to getting a result in having a C function
return its output to Lisp. I shall be very grateful for comments and
corrections to what I have done, and thank readers for their patience.

Here is the C-function from which I wish to get the result  Hello Lisp
from C  on the lisp side:
/* Hello.c  program */


#include<stdio.h>

void Hello(void)
{
    printf("Hello Lisp from C\n");
    return 0 ;

}

At the end of the process, I hope to be able to do this on the Lisp
side:
(defcfun ("Hello" HiFromC) (:void))
and then
(HiFromC) -> Hello Lisp from C

Both C and Lisp sessions are below in case details are relevant.
Somewhere I have missed a critical element.

Thanks to a message from Luis Oliveira pointing me to the necessity of
using a shared library as the intermediary between C and Lisp. There
seem to be as many recipes for creating such libraries as there are
posters of the topic, and as many variations on the code too. I have
put together the C side following very much this page:
 www.cprogramming.com/tutorial/shared-libraries-linux-gcc.html

In file /home/gwbennett/Hello.h
#ifndef Hello_h__
#define  Hello_h__

extern void Hello(void);

#endif  // Hello_h__

In file /home/gwbennett/Hello.c
/* Hello  program */


#include<stdio.h>

void Hello(void)
{
    printf("Hello Lisp from C\n");
    return 0 ;

}

In file main.c
/* This is main.c to go with Hello.c,h */
#include <stdio.h>
#include "Hello.h"

int main (void)
{
	Hello();
	return 0;
}

Then
gcc -c -Wall -Werror -fpic Hello.c -> OK
gcc -shared -o libfHello.so Hello.o -> OK
gcc -L/home/gwbennett -Wall -o test main.c -lHello -> OK

Step 4 of the above page is about making the library visible.
I have tried all 3 routes with the same results; at the moment I have
taken the path of installing libHello.so in /usr/lib (and
in /usr/loca/lib too)as described in the Section  Using ldconfig to
modify ld.so

gcc -Wall -o test main.c -lfoo -> OK
ldd test | grep foo -> /usr/locl/lib/libHello.so (0x00007f946c0c4000)
./test -> Hello Lisp from C

so things seem to work down the C-side

On the Lisp side I (hope I) am following the cffi manual page 10. I
assume that cffi looks for shared libraries in the same sort of 'system'
places as C, but just in case, see page 106 on
*foreign-library-directories*.

Start ccl version 1.9-r15972M (LinuxX8664)
(require :cffi) -> OK
(defpackage :cffi-user
  (:use :common-lisp :cffi)) -> OK
(in-package :cffi-user) -> OK
;P106
(pushnew #P/"usr/lib" *foreign-library-directories* :test #'equal) -> OK
(pushnew #P"/usr/local/lib" *foreign-library-directories* :test #'equal)
-> OK
(load-foreign-library '(:default "LibHello")) -> OK
;; I think I should be able to define the C -> Lisp name function
(defcfun ("Hello" HiFromC) (:void)) -> HIFROMC
;; and execute it to get the message
(HiFromC) -> NIL

;; .. so evidently I have not completely understood what is necessary to
achieve the desired result.

Again, thanks for any and all assistance.
Cheers /Greg Bennett