1 files changed, 38 insertions, 0 deletions
diff --git a/src/thread/pthread_once.c b/src/thread/pthread_once.c
new file mode 100644
index 00000000..72230054
--- /dev/null
+++ b/src/thread/pthread_once.c
@@ -0,0 +1,38 @@
+#include "pthread_impl.h"
+
+static void undo(void *control)
+{
+	a_store(control, 0);
+	__wake(control, 1, 0);
+}
+
+int pthread_once(pthread_once_t *control, void (*init)(void))
+{
+	static int waiters;
+
+	/* Return immediately if init finished before */
+	if (*control == 2) return 0;
+
+	/* Try to enter initializing state. Three possibilities:
+	 *  0 - we're the first or the other cancelled; run init
+	 *  1 - another thread is running init; wait
+	 *  2 - another thread finished running init; just return */
+
+	for (;;) switch (a_swap(control, 1)) {
+	case 0:
+		break;
+	case 1:
+		__wait(control, &waiters, 1, 0);
+		continue;
+	case 2:
+		a_store(control, 2);
+		return 0;
+	}
+
+	pthread_cleanup_push(undo, control);
+	init();
+	pthread_cleanup_pop(0);
+
+	if (waiters) __wake(control, -1, 0);
+	return 0;
+}