diff --git a/lib/diagnostics.cpp b/lib/diagnostics.cpp
index ec3704abce..63ea1d7a07 100644
--- a/lib/diagnostics.cpp
+++ b/lib/diagnostics.cpp
@@ -666,8 +666,8 @@ int diagnostics_cycle_logs() {
             boinc_copy(stderr_log, stderr_archive);
             stderr_file_size = 0;
             stderr_file = freopen(stderr_log, "w", stderr);
-            setbuf(stderr_file, 0);
             if (NULL == stderr_file) return ERR_FOPEN;
+            setbuf(stderr_file, 0);
         }
     }
 
@@ -679,6 +679,7 @@ int diagnostics_cycle_logs() {
             boinc_copy(stdout_log, stdout_archive);
             stdout_file = freopen(stdout_log, "w", stdout);
             if (NULL == stdout_file) return ERR_FOPEN;
+            setvbuf(stdout_file, NULL, _IOLBF, BUFSIZ);
         }
     }
     return BOINC_SUCCESS;