diff --git a/mio/lib/fmt.c b/mio/lib/fmt.c
index 2ca4e82..c608fb6 100644
--- a/mio/lib/fmt.c
+++ b/mio/lib/fmt.c
@@ -1553,12 +1553,25 @@ mio_ooi_t mio_logbfmtv (mio_t* mio, mio_bitmask_t mask, const mio_bch_t* fmt, va
 	fo.putbchars = log_bcs;
 	fo.putuchars = log_ucs;
 
-	x = fmt_outv(&fo, ap);
-
-	if (mio->log.len > 0 && mio->log.ptr[mio->log.len - 1] == MIO_EOL)
+	if (mio->option.log_mask & MIO_LOG_GUARDED) 
 	{
-		MIO_SYS_WRITE_LOG (mio, mio->log.last_mask, mio->log.ptr, mio->log.len);
-		mio->log.len = 0;
+		mio_sys_locklog (mio);
+		x = fmt_outv(&fo, ap);
+		if (mio->log.len > 0 && mio->log.ptr[mio->log.len - 1] == MIO_EOL)
+		{
+			MIO_SYS_WRITE_LOG (mio, mio->log.last_mask, mio->log.ptr, mio->log.len);
+			mio->log.len = 0;
+		}
+		mio_sys_unlocklog (mio);
+	}
+	else
+	{
+		x = fmt_outv(&fo, ap);
+		if (mio->log.len > 0 && mio->log.ptr[mio->log.len - 1] == MIO_EOL)
+		{
+			MIO_SYS_WRITE_LOG (mio, mio->log.last_mask, mio->log.ptr, mio->log.len);
+			mio->log.len = 0;
+		}
 	}
 
 	return (x <= -1)? -1: fo.count;
@@ -1607,12 +1620,25 @@ mio_ooi_t mio_logufmtv (mio_t* mio, mio_bitmask_t mask, const mio_uch_t* fmt, va
 	fo.putbchars = log_bcs;
 	fo.putuchars = log_ucs;
 
-	x = fmt_outv(&fo, ap);
-
-	if (mio->log.len > 0 && mio->log.ptr[mio->log.len - 1] == MIO_EOL)
+	if (mio->option.log_mask & MIO_LOG_GUARDED) 
 	{
-		MIO_SYS_WRITE_LOG (mio, mio->log.last_mask, mio->log.ptr, mio->log.len);
-		mio->log.len = 0;
+		mio_sys_locklog (mio);
+		x = fmt_outv(&fo, ap);
+		if (mio->log.len > 0 && mio->log.ptr[mio->log.len - 1] == MIO_EOL)
+		{
+			MIO_SYS_WRITE_LOG (mio, mio->log.last_mask, mio->log.ptr, mio->log.len);
+			mio->log.len = 0;
+		}
+		mio_sys_unlocklog (mio);
+	}
+	else
+	{
+		x = fmt_outv(&fo, ap);
+		if (mio->log.len > 0 && mio->log.ptr[mio->log.len - 1] == MIO_EOL)
+		{
+			MIO_SYS_WRITE_LOG (mio, mio->log.last_mask, mio->log.ptr, mio->log.len);
+			mio->log.len = 0;
+		}
 	}
 
 	return (x <= -1)? -1: fo.count;
diff --git a/mio/lib/mio.h b/mio/lib/mio.h
index ee742df..3eb32d5 100644
--- a/mio/lib/mio.h
+++ b/mio/lib/mio.h
@@ -542,6 +542,7 @@ enum mio_log_mask_t
 	MIO_LOG_ALL_LEVELS = (MIO_LOG_DEBUG  | MIO_LOG_INFO | MIO_LOG_WARN | MIO_LOG_ERROR | MIO_LOG_FATAL),
 	MIO_LOG_ALL_TYPES  = (MIO_LOG_UNTYPED | MIO_LOG_CORE | MIO_LOG_DEV | MIO_LOG_TIMER),
 
+	MIO_LOG_GUARDED    = (1u << 13), /* make logging thread-safe */
 	MIO_LOG_STDOUT     = (1u << 14), /* write log messages to stdout without timestamp. MIO_LOG_STDOUT wins over MIO_LOG_STDERR. */
 	MIO_LOG_STDERR     = (1u << 15)  /* write log messages to stderr without timestamp. */
 };
diff --git a/mio/lib/sys-log.c b/mio/lib/sys-log.c
index 8f7012d..b162f26 100644
--- a/mio/lib/sys-log.c
+++ b/mio/lib/sys-log.c
@@ -319,7 +319,6 @@ void mio_sys_writelog (mio_t* mio, mio_bitmask_t mask, const mio_ooch_t* msg, mi
 int mio_sys_initlog (mio_t* mio)
 {
 	mio_sys_log_t* log = &mio->sysdep->log;
-	mio_bitmask_t logmask;
 	/*mio_oow_t pathlen;*/
 
 /* TODO: */
@@ -340,9 +339,13 @@ int mio_sys_initlog (mio_t* mio)
 	if (isatty(log->fd)) log->fd_flag |= LOGFD_TTY;
 #endif
 
+/*
+	mio_bitmask_t logmask;
 	logmask = MIO_LOG_ALL_TYPES | MIO_LOG_ALL_LEVELS;
 	mio_setoption (mio, MIO_LOG_MASK, &logmask);
+*/
 
+	pthread_mutex_init (&log->mtx, MIO_NULL);
 	return 0;
 }
 
@@ -350,6 +353,8 @@ void mio_sys_finilog (mio_t* mio)
 {
 	mio_sys_log_t* log = &mio->sysdep->log;
 
+	pthread_mutex_destroy (&log->mtx);
+
 	if ((log->fd_flag & LOGFD_OPENED_HERE) && log->fd >= 0) 
 	{
 		close (log->fd);
@@ -357,3 +362,13 @@ void mio_sys_finilog (mio_t* mio)
 		log->fd_flag = 0;
 	}
 }
+
+void mio_sys_locklog (mio_t* mio)
+{
+	pthread_mutex_lock (&mio->sysdep->log.mtx);
+}
+
+void mio_sys_unlocklog (mio_t* mio)
+{
+	pthread_mutex_unlock (&mio->sysdep->log.mtx);
+}
diff --git a/mio/lib/sys-prv.h b/mio/lib/sys-prv.h
index d9c2ca0..46505b8 100644
--- a/mio/lib/sys-prv.h
+++ b/mio/lib/sys-prv.h
@@ -41,6 +41,8 @@
 #	error NO SUPPORTED MULTIPLEXER
 #endif
 
+#include <pthread.h>
+
 /* -------------------------------------------------------------------------- */
 
 #if defined(USE_POLL)
@@ -87,6 +89,8 @@ struct mio_sys_log_t
 		mio_bch_t buf[4096];
 		mio_oow_t len;
 	} out;
+
+	pthread_mutex_t mtx;
 };
 typedef struct mio_sys_log_t mio_sys_log_t;
 
@@ -133,6 +137,15 @@ void mio_sys_finilog (
 	mio_t* mio
 );
 
+void mio_sys_locklog (
+	mio_t* mio
+);
+
+void mio_sys_locklog (
+	mio_t* mio
+);
+
+
 int mio_sys_initmux (
 	mio_t* mio
 );