Why there is an error like "Xlib:unexpected async reply"
 

I wrote a gtk+ program, and used pthread to create new threads.
But when I want to call functions of gtk+, such as add text to GtkText widget, there will be the error "Xlib:unexpected async reply".
Now I have to use pthread_join() to synchronize threads.I want to use mutex, but it doesn't work, there is still such an error.
Now I don't know what to do.