Boost asio non-blocking IO without callbacks

Question

Is it possible to use Boost's asio to do non-blocking IO without using async callbacks? Ie equivalent to the O_NONBLOCK socket option.

I basically want this function:

template<typename SyncWriteStream,
         typename ConstBufferSequence>
std::size_t write_nonblock(
    SyncWriteStream & s,
    const ConstBufferSequence & buffers);

This function will write as many bytes as it can and return immediately. It may write 0 bytes.

Is it possible?

Answer 1

Yes, using the non_blocking() method to put the socket into Asio non-blocking mode:

template<typename SyncWriteStream,
         typename ConstBufferSequence>
std::size_t write_nonblock(
    SyncWriteStream & s,
    const ConstBufferSequence & buffers)
{
    s.non_blocking(true);
    boost::system::error_code ec;
    auto bytes = s.send(buffers, 0, ec);
    if (bytes == 0 && !(ec == boost::asio::error::would_block))
        throw boost::system::system_error(ec, "write_nonblock send");
    return bytes;
}

Answer 2

The way with s.non_blocking(true) would not work. If you check send implementation, it uses socket_ops::sync_send , which does poll_write if send failed.

So it is still blocking on top level.