Android系统中提供的原子操作
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代码的实现位于文件system/core/include/cutils中
http://androidxref.com/4.4.3_r1.1/xref/system/core/include/cutils/atomic.h
16
17#ifndef ANDROID_CUTILS_ATOMIC_H
18#define ANDROID_CUTILS_ATOMIC_H
19
20#include <stdint.h>
21#include <sys/types.h>
22
23#ifdef __cplusplus
24extern "C" {
25#endif
26
27/*
28 * A handful of basic atomic operations. The appropriate pthread
29 * functions should be used instead of these whenever possible.
30 *
31 * The "acquire" and "release" terms can be defined intuitively in terms
32 * of the placement of memory barriers in a simple lock implementation:
33 * - wait until compare-and-swap(lock-is-free --> lock-is-held) succeeds
34 * - barrier
35 * - [do work]
36 * - barrier
37 * - store(lock-is-free)
38 * In very crude terms, the initial (acquire) barrier prevents any of the
39 * "work" from happening before the lock is held, and the later (release)
40 * barrier ensures that all of the work happens before the lock is released.
41 * (Think of cached writes, cache read-ahead, and instruction reordering
42 * around the CAS and store instructions.)
43 *
44 * The barriers must apply to both the compiler and the CPU. Note it is
45 * legal for instructions that occur before an "acquire" barrier to be
46 * moved down below it, and for instructions that occur after a "release"
47 * barrier to be moved up above it.
48 *
49 * The ARM-driven implementation we use here is short on subtlety,
50 * and actually requests a full barrier from the compiler and the CPU.
51 * The only difference between acquire and release is in whether they
52 * are issued before or after the atomic operation with which they
53 * are associated. To ease the transition to C/C++ atomic intrinsics,
54 * you should not rely on this, and instead assume that only the minimal
55 * acquire/release protection is provided.
56 *
57 * NOTE: all int32_t* values are expected to be aligned on 32-bit boundaries.
58 * If they are not, atomicity is not guaranteed.
59 */
60
61/*
62 * Basic arithmetic and bitwise operations. These all provide a
63 * barrier with "release" ordering, and return the previous value.
64 *
65 * These have the same characteristics (e.g. what happens on overflow)
66 * as the equivalent non-atomic C operations.
67 */
68int32_t android_atomic_inc(volatile int32_t* addr);
69int32_t android_atomic_dec(volatile int32_t* addr);
70int32_t android_atomic_add(int32_t value, volatile int32_t* addr);
71int32_t android_atomic_and(int32_t value, volatile int32_t* addr);
72int32_t android_atomic_or(int32_t value, volatile int32_t* addr);
73
74/*
75 * Perform an atomic load with "acquire" or "release" ordering.
76 *
77 * This is only necessary if you need the memory barrier. A 32-bit read
78 * from a 32-bit aligned address is atomic on all supported platforms.
79 */
80int32_t android_atomic_acquire_load(volatile const int32_t* addr);
81int32_t android_atomic_release_load(volatile const int32_t* addr);
82
83/*
84 * Perform an atomic store with "acquire" or "release" ordering.
85 *
86 * This is only necessary if you need the memory barrier. A 32-bit write
87 * to a 32-bit aligned address is atomic on all supported platforms.
88 */
89void android_atomic_acquire_store(int32_t value, volatile int32_t* addr);
90void android_atomic_release_store(int32_t value, volatile int32_t* addr);
91
92/*
93 * Compare-and-set operation with "acquire" or "release" ordering.
94 *
95 * This returns zero if the new value was successfully stored, which will
96 * only happen when *addr == oldvalue.
97 *
98 * (The return value is inverted from implementations on other platforms,
99 * but matches the ARM ldrex/strex result.)
100 *
101 * Implementations that use the release CAS in a loop may be less efficient
102 * than possible, because we re-issue the memory barrier on each iteration.
103 */
104int android_atomic_acquire_cas(int32_t oldvalue, int32_t newvalue,
105 volatile int32_t* addr);
106int android_atomic_release_cas(int32_t oldvalue, int32_t newvalue,
107 volatile int32_t* addr);
这是个跟处理器相关的一个函数，它执行原子性的加1操作可能更有效率。它的用法是这样，如果oldvalue等于* addr则赋值给* addr，返回0，否则返回1
108
109/*
110 * Aliases for code using an older version of this header. These are now
111 * deprecated and should not be used. The definitions will be removed
112 * in a future release.
113 */
114#define android_atomic_write android_atomic_release_store
115#define android_atomic_cmpxchg android_atomic_release_cas
116
117#ifdef __cplusplus
118} // extern "C"
119#endif
120
121#endif // ANDROID_CUTILS_ATOMIC_H
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