boost asio中的并发读取和async_read_some

Question

Assume that an async_read_some service has been enabled on a socket in boost::asio, what will happen if a blocking read on the same socket is called?

A piece of pseudo code looks like:

using boost::asio::local::stream_protocol;

boost::asio::io_service io;

stream_protocol::socket s(io);
s.connect(stream_protocol::endpoint(address));

s.async_read_some(aBuffer, aCallback);  // start async_read

boost::thread thread(boost::bind(&boost::asio::io_service::run, &io));

usleep(1000000); // do some stuff    

boost::asio::read(bBuffer);  // request a blocking read

My naive test shows that the blocking read always get priority : data will first fill the bBuffer before the async callback being called. It's a desired behaviour on my side.

Question: It is a guaranteed behaviour? On all socket types?

Answer 1

Boost.Asio does not make this guarantee on any I/O object. When a synchronous operation is initiated on an I/O object that has an outstanding asynchronous operation of the same type, the order in which the underlying system calls occur is unspecified.