#if ! defined (SYSTEM_MALLOC) && defined (HAVE_GTK_AND_PTHREAD)
+/* When GTK uses the file chooser dialog, different backends can be loaded
+ dynamically. One such a backend is the Gnome VFS backend that gets loaded
+ if you run Gnome. That backend creates several threads and also allocates
+ memory with malloc.
+
+ If Emacs sets malloc hooks (! SYSTEM_MALLOC) and the emacs_blocked_*
+ functions below are called from malloc, there is a chance that one
+ of these threads preempts the Emacs main thread and the hook variables
+ end up in a inconsistent state. So we have a mutex to prevent that (note
+ that the backend handles concurrent access to malloc within its own threads
+ but Emacs code running in the main thread is not included in that control).
+
+ When UNBLOCK_INPUT is called, revoke_input_signal may be called. If this
+ happens in one of the backend threads we will have two threads that tries
+ to run Emacs code at once, and the code is not prepared for that.
+ To prevent that, we only call BLOCK/UNBLOCK from the main thread. */
+
static pthread_mutex_t alloc_mutex;
pthread_t main_thread;