diff --git a/example/example.cpp b/example/example.cpp
index 07d6be29..ce4bab88 100644
--- a/example/example.cpp
+++ b/example/example.cpp
@@ -18,7 +18,10 @@ using namespace utils;
 
 int main(int argc, char* argv[])
 {
-    if(argc || argv) {};
+
+	if(argc || argv) {};
+	std::string s;
+	cout << "STRING cap: " << format(s.capacity()) << endl;
 
     auto fsink = std::make_shared<sinks::rotating_file_sink>("log", "txt", 1024*1024*50 , 5, 0);
 	//auto fsink = std::make_shared<sinks::simple_file_sink>("simplelog", "txt");
diff --git a/include/c11log/details/fast_oss.h b/include/c11log/details/fast_oss.h
index 9cfb7371..b02ff4ac 100644
--- a/include/c11log/details/fast_oss.h
+++ b/include/c11log/details/fast_oss.h
@@ -40,6 +40,12 @@ protected:
 
     virtual std::streamsize xsputn(const char_type* s, std::streamsize count) override
     {
+
+		auto ssize = _str.size();
+		auto cap_left = _str.capacity() - ssize;
+		if(cap_left < static_cast<std::size_t>(count))
+			_str.reserve(ssize + count + 128);
+
         _str.append(s, static_cast<unsigned int>(count));
         return count;
     }