Index: file.c
===================================================================
--- file.c	(revision 23091)
+++ file.c	(working copy)
@@ -2594,7 +2594,7 @@
 #endif
 
 #define BUFCHECK(cond) do {\
-    long bdiff = p - buf;\
+    size_t bdiff = p - buf;\
     if (cond) {\
 	do {buflen *= 2;} while (cond);\
 	rb_str_resize(result, buflen);\