使用 Boost Asio 异步等待文件描述符

问题描述

我正在尝试将 D-Bus 与我的 boost::asio 应用程序集成.

I'm trying to integrate D-Bus with my boost::asio application.

D-Bus 有一个 API，可以枚举一组要观察的 Unix 文件描述符(主要是套接字，但也可以是 FIFO).当这些描述符需要读取时，我应该通知 D-Bus API，以便它可以读取它们并执行操作.

D-Bus has an API that enumerates a set of Unix file descriptors (mainly sockets but could also be FIFOs) to be watched. When those descriptors have something to be read I should inform the D-Bus API so it can read them and do it's thing.

目前我正在这样做:

using boost::asio::posix::stream_descriptor;
void read_handle(stream_descriptor* desc, const boost::system::error_code& ec,
                 std::size_t bytes_read)
{
    if (!ec) {
        stream_descriptor::bytes_readable command(true);
        descriptor->io_control(command);
        std::size_t bytes_readable = command.get();
        std::cout << "It thinks I should read" << bytes_readable
            << " bytes" << std::endl;
    } else {
        std::cout << "There was an error" << std::endl;
    }
}

void watch_descriptor(boost::asio::io_service& ios, int file_descriptor)
{
    // Create the asio representation of the descriptor
    stream_descriptor* desc = new stream_descriptor(ios);
    desc->assign(file_descriptor);

    // Try to read 0 bytes just to be informed that there is something to be read
    std::vector<char> buffer(0);
    desc->async_read_some(boost::asio::buffer(buffer, 0),
        boost::bind(read_handle, desc, _1, _2));
}

但是立即调用处理程序说它有 0 个字节要读取.我希望只有在有东西要读取时才调用它，但是 boost::asio 不能 读取它.它应该像一个美化的 select().有没有简单的方法可以做到这一点?

But the handler is called right away saying that it has 0 bytes to be read. I would like it to be called only when there is something to be read, but boost::asio CAN NOT read it. It should act just as a glorified select(). Is there a simple way to do that?

PS:我在我的软件中广泛使用了 boost::asio，这只是其中的一小部分，所以我不想依赖 glib或其他主循环.

PS: I'm extensively using boost::asio in my software, this is just a small part of it, so I would like not to depend on glib or other mainloops.

推荐答案

这正是问题所在 null_buffers 是设计的 为.

This is precisely the problem null_buffers was designed for.

有时必须集成一个程序与想要的第三方库执行 I/O 操作本身.为了促进这一点，Boost.Asio包括一个 null_buffers 类型，可以与读和写一起使用操作.null_buffers 操作直到 I/O 对象出现时才返回准备好"执行操作.

Sometimes a program must be integrated with a third-party library that wants to perform the I/O operations itself. To facilitate this, Boost.Asio includes a null_buffers type that can be used with both read and write operations. A null_buffers operation doesn't return until the I/O object is "ready" to perform the operation.

例如，执行一个非阻塞读取类似可以使用以下内容:

As an example, to perform a non-blocking read something like the following may be used:

ip::tcp::socket socket(my_io_service);
...
ip::tcp::socket::non_blocking nb(true);
socket.io_control(nb);
...
socket.async_read_some(null_buffers(), read_handler);
...
void read_handler(boost::system::error_code ec)
{
  if (!ec)
  {
    std::vector<char> buf(socket.available());
    socket.read_some(buffer(buf));
  }
}

还有一个优秀示例文档.

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