4.2.9 Mapping Nil to a Null Pointer

If you wish a string argument to accept nil and pass it as a null pointer, or to return a null pointer as Lisp value nil , use the :allow-null argument to the :reference types.

The C function strcap in the following example modifies a string, but also accepts and returns a null pointer if passed.

Windows version:

#include <string.h>

#include <ctype.h>

__declspec(dllexport) void __cdecl strcap(char *string)

{

  int len;

  int ii;

  if (string) {

     len = strlen(string);

     if (len > 0) {

        for (ii = len - 1; ii > 0; ii--)

            if (isupper(string[ii]))

               string[ii] = tolower(string[ii]); 

        if (islower(string[0]))

           string[0] = toupper(string[0]);

     }

  }

  return string;

}

Linux/Unix/Macintosh version:

#include <string.h>

#include <ctype.h>

void strcap(char *string)

{

  int len;

  int ii;

  if (string) {

     len = strlen(string);

     if (len > 0) {

        for (ii = len - 1; ii > 0; ii--)

            if (isupper(string[ii]))

               string[ii] = tolower(string[ii]); 

        if (islower(string[0]))

           string[0] = toupper(string[0]);

     }

  }

  return string;

}

With this following foreign function definition:

(fli:define-foreign-function (strcap "strcap" :source)

    ((string (:reference :ef-mb-string)))

  :language

  :c

  :calling-convention

  :cdecl)

(strcap "abC")

=>

"Abc"

However (strcap nil) signals error because the :ef-mb-string type expects a string.

Using :allow-null allows nil to be passed:

(fli:define-foreign-function (strcap "strcap" :source)

    ((string (:reference :ef-mb-string :allow-null t)))

  :language

  :c

  :calling-convention

  :cdecl)

(strcap nil) 

=>

nil

Note that with-foreign-string, convert-to-foreign-string and convert-from-foreign-string also accept an :allow-null argument. So another way to call strcap and allow the null pointer is:

(fli:define-foreign-function (strcap "strcap" :source)

    ((string :pointer))

  :language

  :c

  :calling-convention

  :cdecl)

(defun c-string-capitalize (string)

  (fli:with-foreign-string (ptr elts bytes :allow-null t)

      string

    (declare (ignore  elts bytes))

    (strcap ptr)

    (fli:convert-from-foreign-string ptr :allow-null t)))

(c-string-capitalize "abC")

=>

"Abc"

(c-string-capitalize nil)

=>

nil

LispWorks Foreign Language Interface User Guide and Reference Manual - 7 Dec 2011