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在头文件<stdatomic.h>中定义



_Bool atomic_compare_exchange_strong（volatile A * obj，C * expected，C desired）;

(1)

（自C11以来）

_Bool atomic_compare_exchange_weak（volatile A * obj，C * expected，C desired）;

(2)

（自C11以来）

_Bool atomic_compare_exchange_strong_explicit（volatile A * obj，C * expected，C desired，memory_order succ，memory_order fail）;

(3)

（自C11以来）

_Bool atomic_compare_exchange_weak_explicit（volatile A * obj，C * expected，C desired，memory_order succ，memory_order fail）;

(4)

（自C11以来）

将所指向的值obj与所指向的值进行原子比较expected，如果相等，则用前者替换前者desired（执行读取 - 修改 - 写入操作）。否则，加载实际值所指向的obj进入*expected（进行负载操作）。

读取 - 修改 - 写入和加载操作的内存模型分别为succ和fail。（1-2）版本memory_order_seq_cst默认使用。

函数的弱形式（（2）和（4））允许虚假地失败，即，*obj != *expected即使它们是相等的。当比较和交换处于循环中时，弱版本将在某些平台上产生更好的性能。当一个弱的比较和交换需要一个循环，而一个强大的循环不会，强壮的一个更可取。

这是为所有原子对象类型定义的通用函数A。该参数是指向易失性原子类型的指针，以接受非易失性和易失性（例如内存映射I/O）原子变量的地址。C是与之对应的非原子类型A。

纠错

参数

OBJ

-

指向要测试和修改的原子对象的指针

expected

-

指向预期在原子对象中找到的值的指针

disired

-

如果按预期方式存储在原子对象中的值

succ

-

如果比较成功，则读取 - 修改 - 写入操作的存储器同步排序。所有的值都是允许的。

fail

-

如果比较失败，则执行加载操作的内存同步排序。不能是memory_order_release或memory_order_acq_rel，并且不能指定比succ更强的排序

返回值

比较的结果：true如果*obj等于*exp，否则则返回false。

注意

行为atomic_compare_exchange_strong就好像下面的代码是以原子方式执行的：


if (memcmp(obj, expected, sizeof *obj) == 0)    memcpy(obj, &desired, sizeof *obj);else    memcpy(expected, obj, sizeof *obj);


（标准措辞中称比较是以“value”的形式被DR 431改为“bitwise equality”）。

参考

  • C11标准（ISO/IEC 9899：2011）：

    • 7.17.7.4 atomic_compare_exchange通用函数（p：283-284）

另请参阅

atomic_exchangeatomic_exchange_explicit（C11）

用原子对象（函数）的值交换一个值

| C ++文档：atomic_compare_exchange_weak，atomic_compare_exchange_strong，atomic_compare_exchange_weak_explicit，atomic_compare_exchange_strong_explicit |

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