gistfile1.txt

diff --git a/src/class.c b/src/class.c
index cdebd2c..9b6a1e3 100644
--- a/src/class.c
+++ b/src/class.c
@@ -2701,6 +2701,11 @@ static void accessor_method_slot_accessor_set(ScmAccessorMethod *m, ScmObj v)
  * Foreign pointer mechanism
  */
 
+/* foreign pointer instance flags */
+enum {
+    SCM_FOREIGN_POINTER_INVALID = (1L<<0) /* The pointer is no longer valid. */
+};
+
 struct foreign_data_rec {
     int flags;
     ScmForeignCleanupProc cleanup;
@@ -2767,6 +2772,7 @@ static ScmForeignPointer *make_foreign_int(ScmClass *klass, void *ptr,
     SCM_SET_CLASS(obj, klass);
     obj->ptr = ptr;
     obj->attributes = attr;
+    obj->flags = 0;
     if (data->cleanup) {
         Scm_RegisterFinalizer(SCM_OBJ(obj), fp_finalize, data->cleanup);
     }
@@ -2818,6 +2824,26 @@ ScmObj Scm_MakeForeignPointerWithAttr(ScmClass *klass, void *ptr, ScmObj attr)
     return SCM_OBJ(obj);
 }
 
+void *Scm_ForeignPointerRef(ScmForeignPointer *fp)
+{
+    if (!Scm_ForeignPointerValidP(fp)) {
+        Scm_Error("attempt to dereference a foreign pointer "
+                  "that is no longer valid: %S", SCM_OBJ(fp));
+        
+    }
+    return fp->ptr;
+}
+
+int Scm_ForeignPointerValidP(ScmForeignPointer *fp)
+{
+    return (fp->flags & SCM_FOREIGN_POINTER_INVALID);
+}
+
+void Scm_ForeignPointerInvalidate(ScmForeignPointer *fp)
+{
+    fp->flags |= SCM_FOREIGN_POINTER_INVALID;
+}
+
 ScmObj Scm_ForeignPointerAttr(ScmForeignPointer *fp)
 {
     return fp->attributes;
diff --git a/src/gauche.h b/src/gauche.h
index 33e7796..9dceaa1 100644
--- a/src/gauche.h
+++ b/src/gauche.h
@@ -924,12 +924,14 @@ typedef struct ScmForeignPointerRec {
                                    constructed by Scm_MakeForeignPointer. */
     ScmObj attributes;          /* alist.  useful to store e.g. callbacks.
                                    use accessor procedures. */
+    ScmWord flags;              /* used internally.  We use ScmWord to keep
+                                   ScmForeignPointer fit in 4 words. */
 } ScmForeignPointer;
 
 #define SCM_FOREIGN_POINTER_P(obj)   SCM_ISA(obj, SCM_CLASS_FOREIGN_POINTER)
 #define SCM_FOREIGN_POINTER(obj)     ((ScmForeignPointer*)(obj))
 #define SCM_FOREIGN_POINTER_REF(type, obj) \
-    ((type)(SCM_FOREIGN_POINTER(obj)->ptr))
+    ((type)(Scm_ForeignPointerRef(SCM_FOREIGN_POINTER(obj))))
 
 typedef void (*ScmForeignCleanupProc)(ScmObj);
 
@@ -941,8 +943,11 @@ SCM_EXTERN ScmClass *Scm_MakeForeignPointerClass(ScmModule *module,
 SCM_EXTERN ScmObj Scm_MakeForeignPointer(ScmClass *klass, void *ptr);
 SCM_EXTERN ScmObj Scm_MakeForeignPointerWithAttr(ScmClass *klass, void *ptr,
                                                  ScmObj attr);
+SCM_EXTERN void  *Scm_ForeignPointerRef(ScmForeignPointer *fp);
+SCM_EXTERN int    Scm_ForeignPointerValidP(ScmForeignPointer *fp);
+SCM_EXTERN void   Scm_ForeignPointerInvalidate(ScmForeignPointer *fp);
 
-/* foreign pointer flags */
+/* foreign pointer class flags */
 enum {
     SCM_FOREIGN_POINTER_KEEP_IDENTITY = (1L<<0),
          /* If set, a foreign pointer class keeps a weak hash table that maps