diff --git a/webrtc/test/testsupport/fileutils.cc b/webrtc/test/testsupport/fileutils.cc
index c8054ba826..2d3ea00925 100644
--- a/webrtc/test/testsupport/fileutils.cc
+++ b/webrtc/test/testsupport/fileutils.cc
@@ -101,7 +101,7 @@ void SetExecutablePath(const std::string& path) {
   relative_dir_path_set = true;
 }
 
-bool FileExists(std::string& file_name) {
+bool FileExists(const std::string& file_name) {
   struct stat file_info = {0};
   return stat(file_name.c_str(), &file_info) == 0;
 }
@@ -206,7 +206,7 @@ std::string TempFilename(const std::string &dir, const std::string &prefix) {
 #endif
 }
 
-bool CreateDir(std::string directory_name) {
+bool CreateDir(const std::string& directory_name) {
   struct stat path_info = {0};
   // Check if the path exists already:
   if (stat(directory_name.c_str(), &path_info) == 0) {
@@ -226,7 +226,8 @@ bool CreateDir(std::string directory_name) {
   return true;
 }
 
-std::string ResourcePath(std::string name, std::string extension) {
+std::string ResourcePath(const std::string& name,
+                         const std::string& extension) {
 #if defined(WEBRTC_IOS)
   return IOSResourcePath(name, extension);
 #else
@@ -270,7 +271,7 @@ std::string ResourcePath(std::string name, std::string extension) {
 #endif  // defined (WEBRTC_IOS)
 }
 
-size_t GetFileSize(std::string filename) {
+size_t GetFileSize(const std::string& filename) {
   FILE* f = fopen(filename.c_str(), "rb");
   size_t size = 0;
   if (f != NULL) {
diff --git a/webrtc/test/testsupport/fileutils.h b/webrtc/test/testsupport/fileutils.h
index 2467f0c8e5..1a5f9bbb56 100644
--- a/webrtc/test/testsupport/fileutils.h
+++ b/webrtc/test/testsupport/fileutils.h
@@ -58,7 +58,8 @@ std::string TempFilename(const std::string &dir, const std::string &prefix);
 //           If a directory path is prepended to the filename, a subdirectory
 //           hierarchy reflecting that path is assumed to be present.
 //    extension - File extension, without the dot, i.e. "bmp" or "yuv".
-std::string ResourcePath(std::string name, std::string extension);
+std::string ResourcePath(const std::string& name,
+                         const std::string& extension);
 
 // Gets the current working directory for the executing program.
 // Returns "./" if for some reason it is not possible to find the working
@@ -68,14 +69,14 @@ std::string WorkingDir();
 // Creates a directory if it not already exists.
 // Returns true if successful. Will print an error message to stderr and return
 // false if a file with the same name already exists.
-bool CreateDir(std::string directory_name);
+bool CreateDir(const std::string& directory_name);
 
 // Checks if a file exists.
-bool FileExists(std::string& file_name);
+bool FileExists(const std::string& file_name);
 
 // File size of the supplied file in bytes. Will return 0 if the file is
 // empty or if the file does not exist/is readable.
-size_t GetFileSize(std::string filename);
+size_t GetFileSize(const std::string& filename);
 
 // Sets the executable path, i.e. the path to the executable that is being used
 // when launching it. This is usually the path relative to the working directory