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Gets the non-blocking mode of the native descriptor implementation.
bool native_non_blocking() const;
This function is used to retrieve the non-blocking mode of the underlying native descriptor. This mode has no effect on the behaviour of the descriptor object's synchronous operations.
            true if the underlying descriptor is in non-blocking mode
            and direct system calls may fail with asio::error::would_block
            (or the equivalent system error).
          
The current non-blocking mode is cached by the descriptor object. Consequently, the return value may be incorrect if the non-blocking mode was set directly on the native descriptor.