diff --git i/ext/fiddle/conversions.c w/ext/fiddle/conversions.c
index 14b444f..b299ad6 100644
--- i/ext/fiddle/conversions.c
+++ w/ext/fiddle/conversions.c
@@ -27,7 +27,7 @@ int_to_ffi_type(int type)
 	return rb_ffi_type_of(long);
 #if HAVE_LONG_LONG
       case TYPE_LONG_LONG:
-	return rb_ffi_type_of(int64);
+	return rb_ffi_type_of(long_long);
 #endif
       case TYPE_FLOAT:
 	return &ffi_type_float;
@@ -56,23 +56,35 @@ value_to_generic(int type, VALUE src, fiddle_generic * dst)
 	dst->pointer = NUM2PTR(rb_Integer(src));
 	break;
       case TYPE_CHAR:
-	dst->schar = NUM2INT(src);
+	if (signed_p)
+	    dst->schar = NUM2INT(src);
+	else
+	    dst->uchar = NUM2UINT(src);
 	break;
       case TYPE_SHORT:
-	dst->sshort = NUM2INT(src);
+	if (signed_p)
+	    dst->sshort = NUM2INT(src);
+	else
+	    dst->ushort = NUM2UINT(src);
 	break;
       case TYPE_INT:
-	dst->sint = NUM2INT(src);
+	if (signed_p)
+	    dst->sint = NUM2INT(src);
+	else
+	    dst->uint = NUM2UINT(src);
 	break;
       case TYPE_LONG:
 	if (signed_p)
 	    dst->slong = NUM2LONG(src);
 	else
-	    dst->ulong = NUM2LONG(src);
+	    dst->ulong = NUM2ULONG(src);
 	break;
 #if HAVE_LONG_LONG
       case TYPE_LONG_LONG:
-	dst->long_long = NUM2ULL(src);
+	if (signed_p)
+	    dst->slong_long = NUM2LL(src);
+	else
+	    dst->ulong_long = NUM2ULL(src);
 	break;
 #endif
       case TYPE_FLOAT:
@@ -120,7 +132,8 @@ generic_to_value(VALUE rettype, fiddle_generic retval)
 	return ULONG2NUM(retval.ulong);
 #if HAVE_LONG_LONG
       case TYPE_LONG_LONG:
-	return rb_ll2inum(retval.long_long);
+	if (signed_p) return LL2NUM(retval.slong_long);
+	return ULL2NUM(retval.ulong_long);
 	break;
 #endif
       case TYPE_FLOAT:
diff --git i/ext/fiddle/conversions.h w/ext/fiddle/conversions.h
index 5a605b3..d0a08d6 100644
--- i/ext/fiddle/conversions.h
+++ w/ext/fiddle/conversions.h
@@ -18,7 +18,8 @@ typedef union
     float ffloat;          /* ffi_type_float */
     double ddouble;        /* ffi_type_double */
 #if HAVE_LONG_LONG
-    unsigned LONG_LONG long_long; /* ffi_type_uint64 */
+    unsigned LONG_LONG ulong_long; /* ffi_type_ulong_long */
+    signed LONG_LONG slong_long; /* ffi_type_ulong_long */
 #endif
     void * pointer;        /* ffi_type_pointer */
 } fiddle_generic;
diff --git i/ext/fiddle/fiddle.h w/ext/fiddle/fiddle.h
index 7165969..3a829fe 100644
--- i/ext/fiddle/fiddle.h
+++ w/ext/fiddle/fiddle.h
@@ -67,6 +67,15 @@
 #  error "long size not supported"
 # endif
 
+#if HAVE_LONG_LONG
+# if SIZEOF_LONG_LONG == 8
+#   define ffi_type_slong_long ffi_type_sint64
+#   define ffi_type_ulong_long ffi_type_uint64
+# else
+#  error "long long size not supported"
+# endif
+#endif
+
 #include <closure.h>
 #include <conversions.h>
 #include <function.h>