diff --git a/aos/testing/tmpdir.h b/aos/testing/tmpdir.h
new file mode 100644
index 0000000..7e64342
--- /dev/null
+++ b/aos/testing/tmpdir.h
@@ -0,0 +1,15 @@
+#ifndef AOS_TESTING_TMPDIR_H_
+#define AOS_TESTING_TMPDIR_H_
+
+#include <string>
+
+namespace aos {
+namespace testing {
+
+// Returns a usable temporary directory.
+std::string TestTmpDir();
+
+}  // namespace testing
+}  // namespace aos
+
+#endif  // AOS_TESTING_TMPDIR_H_