+// A class used to do something once. This is an abstract parent
+// class; any actual use will involve a child of this.
+
+class Once
+{
+ public:
+ Once();
+
+ virtual
+ ~Once()
+ { }
+
+ // Call this function to do whatever it is. We pass an argument
+ // even though you have to use a child class because in some uses
+ // setting the argument would itself require a Once class.
+ void
+ run_once(void* arg);
+
+ // This is an internal function, which must be public because it is
+ // run by an extern "C" function called via pthread_once.
+ void
+ internal_run(void* arg);
+
+ protected:
+ // This must be implemented by the child class.
+ virtual void
+ do_run_once(void* arg) = 0;
+
+ private:
+ // True if we have already run the function.
+ bool was_run_;
+#if defined(ENABLE_THREADS) && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
+ // Internal compare-and-swap lock on was_run_;
+ uint32_t was_run_lock_;
+#endif
+ // The lock to run the function only once.
+ Once_initialize* once_;
+};
+