Raise an error when attempting to modify the value of `%null-pointer'.

* libguile/foreign.c (sym_null_pointer_error): New variable.
  (null_pointer_error): New function.
  (scm_foreign_set_x): Raise an error if attempting to modify
  NULL_POINTER.
  (scm_foreign_to_bytevector): Use `null_pointer_error ()' instead of
  `scm_misc_error ()'.

* test-suite/tests/foreign.test: New file.

* test-suite/Makefile.am (SCM_TESTS): Add tests/foreign.test.

* test-suite/lib.scm (exception:null-pointer-error): New variable.

libguile/foreign.c

@@ -52,6 +52,7 @@ SCM_SYMBOL (sym_size_t, "size_t");
 SCM_SYMBOL (sym_asterisk, "*");
 
 SCM_SYMBOL (sym_null, "%null-pointer");
+SCM_SYMBOL (sym_null_pointer_error, "null-pointer-error");
 
 /* The cell representing the null pointer.  */
 static const scm_t_bits null_pointer[2] =
@@ -60,6 +61,15 @@ static const scm_t_bits null_pointer[2] =
     0
   };
 
+/* Raise a null pointer dereference error.  */
+static void
+null_pointer_error (const char *func_name)
+{
+  scm_error (sym_null_pointer_error, func_name,
+	     "null pointer dereference", SCM_EOL, SCM_EOL);
+}
+
+
 static SCM cif_to_procedure (SCM cif, SCM func_ptr);
 
 
@@ -161,6 +171,12 @@ SCM_DEFINE (scm_foreign_set_x, "foreign-set!", 2, 0, 0,
   scm_t_uint8 *ptr;
 
   SCM_VALIDATE_FOREIGN (1, foreign);
+
+  if (SCM_UNLIKELY (scm_is_eq (foreign, PTR2SCM (&null_pointer))))
+    /* Attempting to modify the pointer value of NULL_POINTER (which is
+       read-only anyway), so raise an error.  */
+    null_pointer_error (FUNC_NAME);
+
   ptr = SCM_FOREIGN_POINTER (foreign, scm_t_uint8);
   ftype = SCM_FOREIGN_TYPE (foreign);
 
@@ -237,7 +253,7 @@ SCM_DEFINE (scm_foreign_to_bytevector, "foreign->bytevector", 1, 3, 0,
   ptr = SCM_FOREIGN_POINTER (foreign, scm_t_int8);
 
   if (SCM_UNLIKELY (ptr == NULL))
-    scm_misc_error (FUNC_NAME, "null pointer dereference", SCM_EOL);
+    null_pointer_error (FUNC_NAME);
 
   if (SCM_UNBNDP (uvec_type))
     btype = SCM_ARRAY_ELEMENT_TYPE_VU8;

test-suite/Makefile.am

@@ -44,6 +44,7 @@ SCM_TESTS = tests/00-initial-env.test		\
 	    tests/exceptions.test		\
 	    tests/filesys.test			\
 	    tests/fluids.test			\
+	    tests/foreign.test			\
 	    tests/format.test			\
 	    tests/fractions.test		\
 	    tests/ftw.test			\

test-suite/lib.scm

@@ -36,6 +36,7 @@
  exception:miscellaneous-error
  exception:string-contains-nul
  exception:read-error
+ exception:null-pointer-error
 
  ;; Reporting passes and failures.
  run-test
@@ -278,6 +279,8 @@
   (cons 'misc-error "^.*"))
 (define exception:read-error
   (cons 'read-error "^.*$"))
+(define exception:null-pointer-error
+  (cons 'null-pointer-error "^.*$"))
 
 ;; as per throw in scm_to_locale_stringn()
 (define exception:string-contains-nul

test-suite/tests/foreign.test

@@ -0,0 +1,57 @@
+;;;; foreign.test --- FFI.           -*- mode: scheme; coding: utf-8; -*-
+;;;;
+;;;; 	Copyright (C) 2010 Free Software Foundation, Inc.
+;;;;
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+;;;
+;;; See also ../standalone/test-ffi for FFI tests.
+;;;
+
+(define-module (test-foreign)
+  #:use-module (system foreign)
+  #:use-module (rnrs bytevector)
+  #:use-module (test-suite lib))
+
+
+(with-test-prefix "null pointer"
+
+  (pass-if "zero"
+    (= 0 (foreign-ref %null-pointer)))
+
+  (pass-if-exception "foreign-set! %null-pointer"
+    exception:null-pointer-error
+    (foreign-set! %null-pointer 2))
+
+  (pass-if "foreign-set! other-null-pointer"
+    (let ((f (bytevector->foreign (make-bytevector 2))))
+      (and (not (= 0 (foreign-ref f)))
+           (begin
+             (foreign-set! f 0)
+             (= 0 (foreign-ref f)))
+           (begin
+             ;; Here changing the pointer value of F is perfectly valid.
+             (foreign-set! f 777)
+             (= 777 (foreign-ref f))))))
+
+  (pass-if-exception "foreign->bytevector %null-pointer"
+    exception:null-pointer-error
+    (foreign->bytevector %null-pointer))
+
+  (pass-if-exception "foreign->bytevector other-null-pointer"
+    exception:null-pointer-error
+    (let ((f (bytevector->foreign (make-bytevector 2))))
+      (foreign-set! f 0)
+      (foreign->bytevector f))))