在C11 / C ++ 11中，是否可以在同一内存上混合原子/非原子操作？(In C11/C++11, possible to mix atomic/non-atomic ops on the same memory?)

 是否有可能在同一个内存位置上执行原子操作和非原子操作？ 
 
 我问的不是因为我真的想这样做，而是因为我想了解C11 / C ++ 11内存模型。 他们定义了如下的“数据竞赛”： 
 
 
 
 如果一个程序的执行包含一个数据争用，如果它包含两个不同线程中的冲突动作，其中至少有一个不是原子的，而且两个都不会发生在另一个线程之前。 任何这样的数据竞争都会导致未定义的行为。 - C11§5.1.2.4p25， C ++ 11§1.10 p21 
 
 
 它的“至少其中之一不是原子的”部分令我感到困扰。 如果不可能混合原子操作和非原子操作，它只会说“在一个不是原子的对象上”。 
 
 我看不到在原子变量上执行非原子操作的直接方式。 C ++中的std::atomic<T>没有定义任何使用非原子语义的操作。 在C中，原子变量的所有直接读取/写入似乎都被转换为原子操作。 
 
 我想memcpy()和其他直接内存操作可能是一种对原子变量执行非原子读/写的方式？ 即。 memcpy(&atomicvar, othermem, sizeof(atomicvar)) ？ 但是，这是甚至定义的行为？ 在C ++中， std::atomic是不可复制的，所以它会被定义为memcpy()在C或C ++中的行为吗？ 
 
 初始化一个原子变量（无论是通过构造函数还是atomic_init() ）都被定义为不是原子的。 但是这是一次性的操作：你不允许第二次初始化一个原子变量。 放置新的或显式的析构函数调用也可能不是原子的。 但是，在所有这些情况下，它似乎并不像定义的行为那样具有可能在未初始化值上运行的并发原子操作。 
 
 对非原子变量执行原子操作看起来完全不可能：C和C ++都没有定义任何可以对非原子变量操作的原子函数。 
 
 那么这里的故事是什么？ 它真的是关于memcpy() ，或初始化/销毁，或其他？ 

Is it possible to perform atomic and non-atomic ops on the same memory location?
 
I ask not because I actually want to do this, but because I'm trying to understand the C11/C++11 memory model. They define a "data race" like so:
 
 
 
The execution of a program contains a data race if it contains two conflicting actions in different threads, at least one of which is not atomic, and neither happens before the other. Any such data race results in undefined behavior. -- C11 §5.1.2.4 p25, C++11 § 1.10 p21
 
 
Its the "at least one of which is not atomic" part that is troubling me. If it weren't possible to mix atomic and non-atomic ops, it would just say "on an object which is not atomic."
 
I can't see any straightforward way of performing non-atomic operations on atomic variables. std::atomic<T> in C++ doesn't define any operations with non-atomic semantics. In C, all direct reads/writes of an atomic variable appear to be translated into atomic operations.
 
I suppose memcpy() and other direct memory operations might be a way of performing a non-atomic read/write on an atomic variable? ie. memcpy(&atomicvar, othermem, sizeof(atomicvar))? But is this even defined behavior? In C++, std::atomic is not copyable, so would it be defined behavior to memcpy() it in C or C++?
 
Initialization of an atomic variable (whether through a constructor or atomic_init()) is defined to not be atomic. But this is a one-time operation: you're not allowed to initialize an atomic variable a second time. Placement new or an explicit destructor call could would also not be atomic. But in all of these cases, it doesn't seem like it would be defined behavior anyway to have a concurrent atomic operation that might be operating on an uninitialized value.
 
Performing atomic operations on non-atomic variables seems totally impossible: neither C nor C++ define any atomic functions that can operate on non-atomic variables.
 
So what is the story here? Is it really about memcpy(), or initialization/destruction, or something else?

原文：https://stackoverflow.com/questions/35109419