diff --git a/src/cluster/np_cluster.cpp b/src/cluster/np_cluster.cpp
index 66fb783bc2e9d51e32b5dbc6fb0c033793348936..de9c30fea71cf4a06a5e53e5b6989f1a4f2303c9 100644
--- a/src/cluster/np_cluster.cpp
+++ b/src/cluster/np_cluster.cpp
@@ -31,7 +31,7 @@
 
 using namespace std;
 
-extern void cluster(string config_file, string data_file, string output_path);
+extern void cluster(const string& config_file, const string& data_file, const string& output_path);
 
 /*! \brief Main program entry point.
  *
diff --git a/src/sphere/np_sphere.cpp b/src/sphere/np_sphere.cpp
index 7f381e41ee28b645b53376828ddf33c4c68b7a88..afb3fdbd2a0d4d2256e5ef9c9b33f6bedca1d9fa 100644
--- a/src/sphere/np_sphere.cpp
+++ b/src/sphere/np_sphere.cpp
@@ -30,7 +30,7 @@
 
 using namespace std;
 
-extern void sphere(string config_file, string data_file, string output_path);
+extern void sphere(const string& config_file, const string& data_file, const string& output_path);
 
 /*! \brief Main program entry point.
  *