diff --git a/libvncclient/tls_gnutls.c b/libvncclient/tls_gnutls.c
index 3daa416..91cea67 100644
--- a/libvncclient/tls_gnutls.c
+++ b/libvncclient/tls_gnutls.c
@@ -480,6 +480,14 @@ WriteToTLS(rfbClient* client, char *buf, unsigned int n)
   unsigned int offset = 0;
   ssize_t ret;
 
+  if (client->LockWriteToTLS)
+  {
+    if (!client->LockWriteToTLS(client))
+    {
+      rfbClientLog("Callback to get lock in WriteToTLS() failed\n");
+      return -1;
+    }
+  }
   while (offset < n)
   {
     ret = gnutls_record_send((gnutls_session_t)client->tlsSession, buf+offset, (size_t)(n-offset));
@@ -488,10 +496,23 @@ WriteToTLS(rfbClient* client, char *buf, unsigned int n)
     {
       if (ret == GNUTLS_E_AGAIN || ret == GNUTLS_E_INTERRUPTED) continue;
       rfbClientLog("Error writing to TLS: %s.\n", gnutls_strerror(ret));
+      if (client->UnlockWriteToTLS)
+      {
+        if (!client->UnlockWriteToTLS(client))
+          rfbClientLog("Callback to unlock WriteToTLS() failed\n");
+      }
       return -1;
     }
     offset += (unsigned int)ret;
   }
+  if (client->UnlockWriteToTLS)
+  {
+    if (!client->UnlockWriteToTLS(client))
+    {
+      rfbClientLog("Callback to unlock WriteToTLS() failed\n");
+      return -1;
+    }
+  }
   return offset;
 }
 
diff --git a/libvncclient/vncviewer.c b/libvncclient/vncviewer.c
index af0a50b..d81e298 100644
--- a/libvncclient/vncviewer.c
+++ b/libvncclient/vncviewer.c
@@ -220,6 +220,8 @@ rfbClient* rfbGetClient(int bitsPerSample,int samplesPerPixel,
   client->subAuthScheme = 0;
   client->GetCredential = NULL;
   client->tlsSession = NULL;
+  client->LockWriteToTLS = NULL;
+  client->UnlockWriteToTLS = NULL;
   client->sock = -1;
   client->listenSock = -1;
   client->listenAddress = NULL;
diff --git a/rfb/rfbclient.h b/rfb/rfbclient.h
index 157461e..c18eaf9 100644
--- a/rfb/rfbclient.h
+++ b/rfb/rfbclient.h
@@ -182,6 +182,8 @@ typedef void (*BellProc)(struct _rfbClient* client);
 */
 typedef void (*GotCursorShapeProc)(struct _rfbClient* client, int xhot, int yhot, int width, int height, int bytesPerPixel);
 typedef void (*GotCopyRectProc)(struct _rfbClient* client, int src_x, int src_y, int w, int h, int dest_x, int dest_y);
+typedef rfbBool (*LockWriteToTLSProc)(struct _rfbClient* client);
+typedef rfbBool (*UnlockWriteToTLSProc)(struct _rfbClient* client);
 
 typedef struct _rfbClient {
 	uint8_t* frameBuffer;
@@ -361,6 +363,10 @@ typedef struct _rfbClient {
         /* Output Window ID. When set, client application enables libvncclient to perform direct rendering in its window */
         unsigned long outputWindow;
 
+	/** Hooks for optional protection WriteToTLS() by mutex */
+	LockWriteToTLSProc LockWriteToTLS;
+	UnlockWriteToTLSProc UnlockWriteToTLS;
+
 } rfbClient;
 
 /* cursor.c */