diff --git a/builtin/string.h b/builtin/string.h
index d30dda8324..0d87c7fa0d 100644
--- a/builtin/string.h
+++ b/builtin/string.h
@@ -10,6 +10,7 @@
 
 #include <stddef.h>
 
+int memcmp(const void *s1, const void *s2, size_t len);
 void *memcpy(void *dest, const void *src, size_t len);
 void *memset(void *dest, int c, size_t len);