What are the asynchronous I/O models in C++? What are their advantages and disadvantages?

Asynchronous I/O model in C

The asynchronous I/O model allows I/O operations to be performed concurrently without blocking the main thread . This is critical for high-performance applications, as computation and other activities can occur simultaneously.

There are two main types of asynchronous I/O models in C:

1. Event-driven I/O (I/O Completion Port)

• The event-driven model uses notification handles at the operating system level. When an I/O operation completes, the system sends a notification to the handle, which can then be processed by the application.
• Advantages: high performance, low overhead, and strong scalability.
• Disadvantages: Requires more code and configuration.

Sample code:

    // 创建 I/O 完成端口
    HANDLE ioPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);

    // 为每个套接字创建通知句柄
    SOCKET sock = ...;
    CreateIoCompletionPort((HANDLE)sock, ioPort, 0, 0);

    // 在单独的线程中处理通知
    DWORD bytesTransferred;
    LPOVERLAPPED overlapped;
    while (GetQueuedCompletionStatus(ioPort, &bytesTransferred, &overlapped, NULL, INFINITE)) {
        // 处理完成的 I/O 操作
    }

2. Asynchronous file I/O

• Asynchronous file I /O uses the ReadFileEx() and WriteFileEx() functions. Unlike the event-driven model, it does not use notification handles, but returns directly on completion.
• Advantages: No additional configuration is required, and the code is simpler.
• Disadvantages: Performance may be lower and scalability is limited.

Sample code:

    HANDLE fileHandle = ...;
    DWORD bytesTransferred;
    OVERLAPPED overlapped;

    ReadFileEx(fileHandle, buffer, sizeof(buffer), &overlapped, NULL);

    while (GetOverlappedResult(fileHandle, &overlapped, &bytesTransferred, FALSE)) {
        // 处理已完成的 I/O 操作
    }

Choose the right model

Choose the best asynchronous I/O model Depends on the specific requirements of the application:

• For applications with high performance and scalability requirements, event-driven I/O (I/O Completion Port) is better choose.
• For applications that require simplicity and ease of use, asynchronous file I/O may be a better choice.

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