ref: a28d43658e3347d55d70655e6ee3d87d0d3fba8a
dir: /vpx_ports/vpx_once.h/
/*
* 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_VPX_PORTS_VPX_ONCE_H_
#define VPX_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_VPX_PORTS_VPX_ONCE_H_