diff -r 591473851c95 -r 3d80d425543b src/ucx/ucx.h
--- a/src/ucx/ucx.h	Tue Jan 02 17:00:21 2018 +0100
+++ b/src/ucx/ucx.h	Sun Jan 21 10:13:21 2018 +0100
@@ -131,6 +131,15 @@
  */
 typedef size_t(*read_func)(void*, size_t, size_t, void*);
 
+
+
+#if defined(__GNUC__) || defined(__clang__)
+#define UCX_MUL_BUILTIN
+#define ucx_szmul(a, b, result) __builtin_umull_overflow(a, b, result)
+#else
+int ucx_szmul(size_t a, size_t b, size_t *result);
+#endif
+
 #ifdef	__cplusplus
 }
 #endif