diff --git a/internal/ceres/test_util.h b/internal/ceres/test_util.h
index 93dc343..4c530de 100644
--- a/internal/ceres/test_util.h
+++ b/internal/ceres/test_util.h
@@ -28,6 +28,9 @@
 //
 // Author: keir@google.com (Keir Mierle)
 
+#include <string>
+#include "ceres/internal/port.h"
+
 #ifndef CERES_INTERNAL_TEST_UTIL_H_
 #define CERES_INTERNAL_TEST_UTIL_H_
 
@@ -58,6 +61,10 @@
                                 const double* q,
                                 double tolerance);
 
+// Construct a fully qualified path for the test file depending on the
+// local build/testing environment.
+string TestFileAbsolutePath(const string& filename);
+
 }  // namespace internal
 }  // namespace ceres