diff --git a/include/spdlog/sinks/ostream_sink.h b/include/spdlog/sinks/ostream_sink.h
index 68ecec87..91580301 100644
--- a/include/spdlog/sinks/ostream_sink.h
+++ b/include/spdlog/sinks/ostream_sink.h
@@ -39,7 +39,7 @@ template<class Mutex>
 class ostream_sink: public base_sink<Mutex>
 {
 public:
-    explicit ostream_sink(std::ostream& os) :_ostream(os) {}
+    explicit ostream_sink(std::ostream& os, bool force_flush=false) :_ostream(os), _force_flush(force_flush) {}
     ostream_sink(const ostream_sink&) = delete;
     ostream_sink& operator=(const ostream_sink&) = delete;
     virtual ~ostream_sink() = default;
@@ -48,8 +48,11 @@ protected:
     virtual void _sink_it(const details::log_msg& msg) override
     {
         _ostream.write(msg.formatted.data(), msg.formatted.size());
+        if (_force_flush)
+            _ostream.flush();
     }
     std::ostream& _ostream;
+    bool _force_flush;
 };
 
 typedef ostream_sink<std::mutex> ostream_sink_mt;
diff --git a/include/spdlog/sinks/stdout_sinks.h b/include/spdlog/sinks/stdout_sinks.h
index b633b2dc..e944e273 100644
--- a/include/spdlog/sinks/stdout_sinks.h
+++ b/include/spdlog/sinks/stdout_sinks.h
@@ -38,7 +38,7 @@ template <class Mutex>
 class stdout_sink : public ostream_sink<Mutex>
 {
 public:
-    stdout_sink() : ostream_sink<Mutex>(std::cout) {}
+    stdout_sink() : ostream_sink<Mutex>(std::cout, true) {}
 
 };
 
@@ -50,7 +50,7 @@ template <class Mutex>
 class stderr_sink : public ostream_sink<Mutex>
 {
 public:
-    stderr_sink() : ostream_sink<Mutex>(std::cerr) {}
+    stderr_sink() : ostream_sink<Mutex>(std::cerr, true) {}
 };
 
 typedef stderr_sink<std::mutex> stderr_sink_mt;