diff --git a/selfdrive/common/util.h b/selfdrive/common/util.h
index 2797cbd44a..4edc15cd4c 100644
--- a/selfdrive/common/util.h
+++ b/selfdrive/common/util.h
@@ -34,7 +34,7 @@ int set_core_affinity(int core);
 
 namespace util {
 
-inline bool starts_with(std::string s, std::string prefix) {
+inline bool starts_with(const std::string &s, const std::string &prefix) {
   return s.compare(0, prefix.size(), prefix) == 0;
 }
 
@@ -74,7 +74,7 @@ inline std::string dir_name(std::string const & path) {
   return path.substr(0, pos);
 }
 
-inline std::string readlink(std::string path) {
+inline std::string readlink(const std::string &path) {
   char buff[4096];
   ssize_t len = ::readlink(path.c_str(), buff, sizeof(buff)-1);
   if (len != -1) {