原子比较（不等于）和swap [英] atomic compare(not equal) and swap

问题描述

我想使用原子比较和交换，但不是等于，我想只交换如果存储器位置不等于旧值。是否有可能用C？

I want to use atomic compare and swap, but instead of equal to, I want to swap only if the memory location is not equal to the old value. Is it possible in C?

推荐答案

这个怎么样：

void compare_and_swap_if_not_equal(word_t const required_non_value, word_t const new_value, word_t* ptr_to_shared_variable) { 
    for (;;) {
        word_t const snapshot_value = *ptr_to_shared_variable;
        if (required_non_value == snapshot_value) {
            break;
            // or (sleep and) 'continue;', if you want to wait for the stored value to be different 
            // -- but you might of course miss a transient change to a different state and back.
        } else { 
            if (compare_and_swap_if_equal(ptr_to_shared_variable, snapshot_value, new_value)) {
                // we know the stored value was different; if this 'theory' still matches reality: swap! done!
                break;
            }
        }
    }
}

未经检验。未编译。使用，因为我喜欢这样:)的常量。 word_t是一种占位符，我不知道真正的类型应该是什么。我不知道怎么样'compare_and_swap_if_equal被称为stdatomic.h。

Untested. Uncompiled. The 'const' used because I like it that way :). 'word_t' is a type placeholder, I don't know what the real type should be. And I don't know how what 'compare_and_swap_if_equal' is called in stdatomic.h.

（添加）atomic_compare_exchange_weak（）是门票。出于某种原因，它需要一个指向预期的说法，所以你必须修改上述code到

(added) atomic_compare_exchange_weak() is the ticket. For some reason, it takes a pointer to the 'expected' argument, so you'll have to modify above code to

如果（atomic_compare_exchange_weak（ptr_to_shared_variable，＆安培; snapshot_value，NEW_VALUE））...

if (atomic_compare_exchange_weak(ptr_to_shared_variable, &snapshot_value, new_value)) ...

弱版本应该在上面code工作;返回假不合逻辑只会增加另一次环路之旅。仍然未编译的，未经检验的;不靠家里此code。

The 'weak' version should work in the above code; returning 'false' spuriously will only add another trip around the loop. Still uncompiled, untested; don't rely on this code at home.

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