diff --git a/src/rime/setup.cc b/src/rime/setup.cc
index 738f0d80eb..3091838783 100644
--- a/src/rime/setup.cc
+++ b/src/rime/setup.cc
@@ -80,7 +80,11 @@ RIME_API void SetupLogging(const char* app_name,
   FLAGS_minloglevel = min_log_level;
   FLAGS_alsologtostderr = true;
   if (log_dir) {
-    FLAGS_log_dir = log_dir;
+    if (log_dir[0] == '\0') {
+      FLAGS_logtostderr = true;
+    } else {
+      FLAGS_log_dir = log_dir;
+    }
   }
   // Do not allow other users to read/write log files created by current
   // process.
diff --git a/src/rime_api.h b/src/rime_api.h
index f36c3a745c..ae4467abc8 100644
--- a/src/rime_api.h
+++ b/src/rime_api.h
@@ -94,6 +94,7 @@ typedef struct rime_traits_t {
   int min_log_level;
   /*! Directory of log files.
    *  Value is passed to Glog library using FLAGS_log_dir variable.
+   *  NULL means temporary directory, and "" means only writing to stderr.
    */
   const char* log_dir;
   //! prebuilt data directory. defaults to ${shared_data_dir}/build