diff --git a/aos/testing/tmpdir.h b/aos/testing/tmpdir.h
index 7e64342..8eabf86 100644
--- a/aos/testing/tmpdir.h
+++ b/aos/testing/tmpdir.h
@@ -9,6 +9,10 @@
 // Returns a usable temporary directory.
 std::string TestTmpDir();
 
+// Sets shm_base to a folder inside of TEST_TMPDIR if set, or --shm_base
+// otherwise.
+void SetTestShmBase();
+
 }  // namespace testing
 }  // namespace aos