Index: tools/c-arcmt-test/c-arcmt-test.c
===================================================================
--- tools/c-arcmt-test/c-arcmt-test.c
+++ tools/c-arcmt-test/c-arcmt-test.c
@@ -97,14 +97,20 @@
 void thread_runner(void *client_data_v) {
   thread_info *client_data = client_data_v;
   client_data->result = carcmttest_main(client_data->argc, client_data->argv);
-#ifdef __CYGWIN__
-  fflush(stdout);  /* stdout is not flushed on Cygwin. */
-#endif
+}
+
+static void flush_atexit(void) {
+  // stdout, and surprisingly even stderr, are not always flushed on process
+  // and thread exit, particularly when the system is under heavy load.
+  fflush(stdout);
+  fflush(stderr);
 }
 
 int main(int argc, const char **argv) {
   thread_info client_data;
 
+  atexit(flush_atexit);
+
 #if defined(_WIN32)
   if (getenv("LIBCLANG_LOGGING") == NULL)
     putenv("LIBCLANG_LOGGING=1");
Index: tools/c-index-test/c-index-test.c
===================================================================
--- tools/c-index-test/c-index-test.c
+++ tools/c-index-test/c-index-test.c
@@ -4116,14 +4116,20 @@
 void thread_runner(void *client_data_v) {
   thread_info *client_data = client_data_v;
   client_data->result = cindextest_main(client_data->argc, client_data->argv);
-#ifdef __CYGWIN__
-  fflush(stdout);  /* stdout is not flushed on Cygwin. */
-#endif
+}
+
+static void flush_atexit(void) {
+  // stdout, and surprisingly even stderr, are not always flushed on process
+  // and thread exit, particularly when the system is under heavy load.
+  fflush(stdout);
+  fflush(stderr);
 }
 
 int main(int argc, const char **argv) {
   thread_info client_data;
 
+  atexit(flush_atexit);
+
 #ifdef CLANG_HAVE_LIBXML
   LIBXML_TEST_VERSION
 #endif