4.2.4 Returning a string via a buffer

In this example no Lisp string is needed when calling. The :reference-return type converts a foreign string of lowercase ASCII characters to a Lisp string on return. Here is the C code for the example.

Windows version:

#include <string.h>

#include <stdlib.h>

__declspec(dllexport) void __cdecl random_string(int length, char *string)

{

  int ii;

  for (ii = 0; ii < length ; ii++)

      string[ii] = 97 + rand() % 26;

  string[length] = 0;

}

Linux/Unix/Macintosh version:

#include <string.h>

#include <stdlib.h>

void random_string(int length, char *string)

{

  int ii;

  for (ii = 0; ii < length ; ii++)

      string[ii] = 97 + rand() % 26;

  string[length] = 0;

}

In this foreign function definition the :reference-return type must specify a size, since memory is allocated for it before calling the C function. Note also the use of :lambda-list so that the caller doesn't have to pass a dummy argument for the return-string, and :result-type nil correponding to the void declaration of the C function.

(fli:define-foreign-function (random-string 

                              "random_string" 

                              :source)

   ((length :int)

    (return-string (:reference-return 

                    (:ef-mb-string 

                     :limit 256))))

 :result-type nil

 :lambda-list (length &aux return-string)

 :calling-convention :cdecl)

(random-string 3)

=>

"uxw"

(random-string 6)

=>

"fnfozv"