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+/*
+ * Copyright (c) 2015 The WebM project authors. All Rights Reserved.
+ *
+ * Use of this source code is governed by a BSD-style license
+ * that can be found in the LICENSE file in the root of the source
+ * tree. An additional intellectual property rights grant can be found
+ * in the file PATENTS. All contributing project authors may
+ * be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef VPX_PORTS_VPX_ONCE_H_
+#define VPX_PORTS_VPX_ONCE_H_
+
+#include "vpx_config.h"
+
+/* Implement a function wrapper to guarantee initialization
+ * thread-safety for library singletons.
+ *
+ * NOTE: These functions use static locks, and can only be
+ * used with one common argument per compilation unit. So
+ *
+ * file1.c:
+ * vpx_once(foo);
+ * ...
+ * vpx_once(foo);
+ *
+ * file2.c:
+ * vpx_once(bar);
+ *
+ * will ensure foo() and bar() are each called only once, but in
+ *
+ * file1.c:
+ * vpx_once(foo);
+ * vpx_once(bar):
+ *
+ * bar() will never be called because the lock is used up
+ * by the call to foo().
+ */
+
+#if CONFIG_MULTITHREAD && defined(_WIN32)
+#include <windows.h>
+#include <stdlib.h>
+/* Declare a per-compilation-unit state variable to track the progress
+ * of calling func() only once. This must be at global scope because
+ * local initializers are not thread-safe in MSVC prior to Visual
+ * Studio 2015.
+ *
+ * As a static, once_state will be zero-initialized as program start.
+ */
+static LONG once_state;
+static void once(void (*func)(void))
+{
+ /* Try to advance once_state from its initial value of 0 to 1.
+ * Only one thread can succeed in doing so.
+ */
+ if (InterlockedCompareExchange(&once_state, 1, 0) == 0) {
+ /* We're the winning thread, having set once_state to 1.
+ * Call our function. */
+ func();
+ /* Now advance once_state to 2, unblocking any other threads. */
+ InterlockedIncrement(&once_state);
+ return;
+ }
+
+ /* We weren't the winning thread, but we want to block on
+ * the state variable so we don't return before func()
+ * has finished executing elsewhere.
+ *
+ * Try to advance once_state from 2 to 2, which is only possible
+ * after the winning thead advances it from 1 to 2.
+ */
+ while (InterlockedCompareExchange(&once_state, 2, 2) != 2) {
+ /* State isn't yet 2. Try again.
+ *
+ * We are used for singleton initialization functions,
+ * which should complete quickly. Contention will likewise
+ * be rare, so it's worthwhile to use a simple but cpu-
+ * intensive busy-wait instead of successive backoff,
+ * waiting on a kernel object, or another heavier-weight scheme.
+ *
+ * We can at least yield our timeslice.
+ */
+ Sleep(0);
+ }
+
+ /* We've seen once_state advance to 2, so we know func()
+ * has been called. And we've left once_state as we found it,
+ * so other threads will have the same experience.
+ *
+ * It's safe to return now.
+ */
+ return;
+}
+
+
+#elif CONFIG_MULTITHREAD && defined(__OS2__)
+#define INCL_DOS
+#include <os2.h>
+static void once(void (*func)(void))
+{
+ static int done;
+
+ /* If the initialization is complete, return early. */
+ if(done)
+ return;
+
+ /* Causes all other threads in the process to block themselves
+ * and give up their time slice.
+ */
+ DosEnterCritSec();
+
+ if (!done)
+ {
+ func();
+ done = 1;
+ }
+
+ /* Restores normal thread dispatching for the current process. */
+ DosExitCritSec();
+}
+
+
+#elif CONFIG_MULTITHREAD && HAVE_PTHREAD_H
+#include <pthread.h>
+static void once(void (*func)(void))
+{
+ static pthread_once_t lock = PTHREAD_ONCE_INIT;
+ pthread_once(&lock, func);
+}
+
+
+#else
+/* No-op version that performs no synchronization. *_rtcd() is idempotent,
+ * so as long as your platform provides atomic loads/stores of pointers
+ * no synchronization is strictly necessary.
+ */
+
+static void once(void (*func)(void))
+{
+ static int done;
+
+ if(!done)
+ {
+ func();
+ done = 1;
+ }
+}
+#endif
+
+#endif // VPX_PORTS_VPX_ONCE_H_