diff --git a/src/utils.h b/src/utils.h
index 300bcbb..53f4a99 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -18,6 +18,8 @@
 #ifndef OPENOLT_UTILS_H_
 #define OPENOLT_UTILS_H_
 
+#include <string>
+
 extern "C"
 {
 #include <bcmos_system.h>
@@ -25,6 +27,7 @@
 #include <bal_api_end.h>
 }
 
-const char* serial_number_to_str(bcmbal_serial_number* serial_number);
+std::string serial_number_to_str(bcmbal_serial_number* serial_number);
+std::string vendor_specific_to_str(const char* const serial_number);
 
 #endif