diff --git a/include/log.h b/include/log.h
index ba2e912..9197a08 100644
--- a/include/log.h
+++ b/include/log.h
@@ -33,6 +33,7 @@
 #define LOG_H
 
 #include <string>
+#include <boost/thread.hpp>
 
 #include "singleton.h"
 #include "shared.h"
@@ -70,8 +71,20 @@ namespace swd {
 			void send(swd::log_level level, std::string message);
 
 		private:
+			/**
+			 * @brief Get the current date and time as string.
+			 */
 			std::string get_current_time();
+
+			/**
+			 * @brief The log file. If empty stderr is used instead.
+			 */
 			std::string file_;
+
+			/**
+			 * @brief Mutex for output.
+			 */
+			boost::mutex mutex_;
 	};
 }
 
diff --git a/include/storage.h b/include/storage.h
index 45e29c1..9c71fb0 100644
--- a/include/storage.h
+++ b/include/storage.h
@@ -101,7 +101,9 @@ namespace swd {
 			 */
 			bool stop_;
 
-			/* @brief Notify consumer threads on new requests in the queue. */
+			/**
+			 * @brief Notify consumer threads on new requests in the queue.
+			 */
 			boost::condition_variable cond_;
 	};
 }
diff --git a/src/log.cpp b/src/log.cpp
index 62c9976..f93426e 100644
--- a/src/log.cpp
+++ b/src/log.cpp
@@ -37,6 +37,10 @@
 #include "config.h"
 
 void swd::log::open_file(std::string file) {
+	/* Use mutex to avoid race conditions. */
+	boost::unique_lock<boost::mutex> scoped_lock(mutex_);
+
+	/* Set the file. */
 	file_ = file;
 }
 
@@ -70,6 +74,9 @@ void swd::log::send(swd::log_level level, std::string message) {
 
 	line << "\t" << message << "\n";
 
+	/* Use mutex to avoid race conditions. */
+	boost::unique_lock<boost::mutex> scoped_lock(mutex_);
+
 	/* Write the final line into a log file or stderr. */
 	if (!file_.empty()) {
 		std::ofstream out_file(file_.c_str(), std::ios_base::app);