diff --git a/aos/testing/tmpdir.cc b/aos/testing/tmpdir.cc
index 15e3c13..7287603 100644
--- a/aos/testing/tmpdir.cc
+++ b/aos/testing/tmpdir.cc
@@ -3,16 +3,24 @@
 #include <cstdlib>
 #include <string>
 
+#include "aos/ipc_lib/shm_base.h"
+
 namespace aos {
 namespace testing {
 
-std::string TestTmpDir() {
+namespace {
+std::string TestTmpDirOr(std::string fallback) {
   const char *tmp_dir = std::getenv("TEST_TMPDIR");
   if (tmp_dir != nullptr) {
     return tmp_dir;
   }
-  return "/tmp";
+  return fallback;
 }
+}  // namespace
+
+std::string TestTmpDir() { return TestTmpDirOr("/tmp"); }
+
+void SetTestShmBase() { SetShmBase(TestTmpDirOr(FLAGS_shm_base)); }
 
 }  // namespace testing
 }  // namespace aos