LispWorks Foreign Language Interface User Guide and Reference Manual > 4 Advanced Uses of the FLI > 4.2 Modifying, passing and returning strings

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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"

LispWorks Foreign Language Interface User Guide and Reference Manual - 21 Dec 2009

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