diff --git a/tools/include/dwarfs/tool/safe_main.h b/tools/include/dwarfs/tool/safe_main.h
index 93b20067..6556a6a9 100644
--- a/tools/include/dwarfs/tool/safe_main.h
+++ b/tools/include/dwarfs/tool/safe_main.h
@@ -25,6 +25,6 @@
 
 namespace dwarfs::tool {
 
-int safe_main(std::function<int(void)> fn);
+int safe_main(std::function<int(void)> const& fn);
 
 } // namespace dwarfs::tool
diff --git a/tools/src/tool/safe_main.cpp b/tools/src/tool/safe_main.cpp
index 1c68e268..76300c1b 100644
--- a/tools/src/tool/safe_main.cpp
+++ b/tools/src/tool/safe_main.cpp
@@ -29,7 +29,7 @@
 
 namespace dwarfs::tool {
 
-int safe_main(std::function<int(void)> fn) {
+int safe_main(std::function<int(void)> const& fn) {
   try {
     install_signal_handlers();
     setup_default_locale();