在c ++ 11中，可以使用std :: atomic在两个线程之间传输非原子数据

Question

In c++11, can std::atomic be used to transmit non-atomic data between two thread? In detail, are the following 4 semantics all established by atomic?

1. all statements(when talking about execution, including all machine instructions generated by those c++ statements) before an atomic-write statement are executed before the atomic-write.

2. all statements(when talking about execution, including all machine instructions generated by those c++ statements) after an atomic-read are executed after the atomic-read.

3. all other memory-write before writing an atomic are committed to main memory.

4. all other memory-read after reading an atomic will read from main memory again(that means discard thread cache).

I have seen an example here: http://bartoszmilewski.com/2008/12/01/c-atomics-and-memory-ordering/

However, in the example, the data is an atomic, so my question is, what if the data is non-atomic?

Here is some code, showing what i want:

common data:

std::atomic_bool ready;
char* data; // or data of any other non atomic

write thread:

data = new char[100];
data[0] = 1;
ready.store(true); // use default memory_order(memory_order_seq_cst), witch i think is the most restrict one

read thread:

if(ready.load()) { // use default memory_order(memory_order_seq_cst)
    assert(data[0] == 1); // both data(of type char*) and data[0~99](each of type char) are loaded
}

Answer 1

I think you must use memory orders:

data = new char[100];
data[0] = 1;
ready.store_explicit(true, std::memory_order_release);

if(ready.load_explicit(std::memory_order_aqcuire)) {
    assert(data[0] == 1); // both data(of type char*) and data[0~99](each of type char) are loaded
}

(i am not sure about this syntax _explicit )

Actually your code is correct, but in this case there is no need to sec/cst memory order and acquire/release is correct. with release no write can be reordered after atomic write and with acquire no load can be reordered before atomic load, so all non-atomic store before atomic store will be visible to all loads after atomic load.