1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
|
/*
* Copyright (c) 2016, Alliance for Open Media. All rights reserved
*
* This source code is subject to the terms of the BSD 2 Clause License and
* the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
* was not distributed with this source code in the LICENSE file, you can
* obtain it at www.aomedia.org/license/software. If the Alliance for Open
* Media Patent License 1.0 was not distributed with this source code in the
* PATENTS file, you can obtain it at www.aomedia.org/license/patent.
*/
#ifndef AOM_PORTS_AOM_ONCE_H_
#define AOM_PORTS_AOM_ONCE_H_
#include "config/aom_config.h"
/* Implement a function wrapper to guarantee initialization
* thread-safety for library singletons.
*
* NOTE: This function uses static locks, and can only be
* used with one common argument per compilation unit. So
*
* file1.c:
* aom_once(foo);
* ...
* aom_once(foo);
*
* file2.c:
* aom_once(bar);
*
* will ensure foo() and bar() are each called only once, but in
*
* file1.c:
* aom_once(foo);
* aom_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, aom_once_state will be zero-initialized as program start.
*/
static LONG aom_once_state;
static void aom_once(void (*func)(void)) {
/* Try to advance aom_once_state from its initial value of 0 to 1.
* Only one thread can succeed in doing so.
*/
if (InterlockedCompareExchange(&aom_once_state, 1, 0) == 0) {
/* We're the winning thread, having set aom_once_state to 1.
* Call our function. */
func();
/* Now advance aom_once_state to 2, unblocking any other threads. */
InterlockedIncrement(&aom_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 aom_once_state from 2 to 2, which is only possible
* after the winning thead advances it from 1 to 2.
*/
while (InterlockedCompareExchange(&aom_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 aom_once_state advance to 2, so we know func()
* has been called. And we've left aom_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 aom_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 aom_once(void (*func)(void)) {
static pthread_once_t lock = PTHREAD_ONCE_INIT;
pthread_once(&lock, func);
}
#else
/* Default version that performs no synchronization. */
static void aom_once(void (*func)(void)) {
static int done;
if (!done) {
func();
done = 1;
}
}
#endif
#endif // AOM_PORTS_AOM_ONCE_H_
|
