原子增加和比较线程安全吗

Question

I'm writing multithreaded socket class. BelowBounds() function can be invoked from multiple threads simultaneously, i need to prevent using mutexes. Does this code is thread-safe ?

class UDPSocketHT
{
public:
    std::atomic<size_t> m_nSimultaneousRecvCalls;
    std::atomic<size_t> m_nPendingOperations;
    //...
    bool UDPSocketHT::BelowBounds ( )
    {
         return ( !m_nSimultaneousRecvCalls || ( m_nPendingOperations + 1 <= m_nSimultaneousRecvCalls ) ) ? true : false;
    }
}

Or i must write in this way?

bool UDPSocketHT::BelowBounds ( )
{
     size_t x = m_nSimultaneousRecvCalls;
     size_t y = m_nPendingOperations;
     return ( !x || ( y + 1 <= x) ) ? true : false;
}

Answer 1

Both of your alternatives are unsafe. Each atomic variable by itself is atomic, but using two of them in a single statement is not.

You can wrap your check in a mutex or come up with a way to use a single atomic.

What operations on std::atomic are atomic?

• operator= stores a new value atomically

• load() or operator T (using in an expression) reads the value atomically

• operator++ increments a value atomically

• compare_exchange_weak/strong check and set the value atomically

• more details

Using two atomics in an expression is not atomic: a + b will read a atomically, then read b atomically, but anything can happen in between reading a and b ; by the time you read b , a can already have another value.