Скачать или смотреть Understanding Success Responses in asio::ip::tcp::socket::write_some: Why Does It Happen?

Explore why Boost Asio's `write_some` method might return `Success` even after a connection is aborted, and learn about TCP protocols in networking.
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Understanding Success Responses in asio::ip::tcp::socket::write_some: Why Does It Happen?

When working with Boost Asio and TCP sockets in C+ + , you might come across a perplexing situation: your asio::ip::tcp::socket::write_some call returns Success even after a connection has been abruptly terminated. If you've ever faced such a scenario, you're not alone! This guide aims to shed light on this issue and explain the underlying principles of how TCP communication works, particularly in relation to Boost Asio’s implementation.

The Problem: Unexpected Success After Connection Abort

Consider the following code snippet that sets up a TCP server using Boost Asio (Boost version 1.76 and Asio version 1.18.2):

[[See Video to Reveal this Text or Code Snippet]]

When running this code, and you actively connect to it (for instance, using curl), you might observe that on sending the message, it outputs Success. However, if you abort the connection prematurely (like sending a SIGINT to curl), it still prints Success. This can be troubling because it raises the question: Does TCP not guarantee delivery as intended?

Understanding TCP and the Behavior of write_some

How TCP Handles Write Operations

Immediate Feedback: The function write_some signals success when the data has been queued in the operating system's TCP stack. At this moment, the system is not concerned with actual delivery to the recipient.

Error Detection: TCP is designed to manage robustness. It will eventually detect if there was an error in the connection, such as a connection reset or an acknowledgment timeout. However, this feedback is not sent back immediately.

Delayed Error Report: Since the sockets API does not allow for instantaneous error reporting during the write_some execution, any issues are deferred until the next call to the socket API, leading to subsequent calls generating a Broken pipe error.

Why You Encounter Success in a Failed Scenario

The fact that write_some returns Success doesn't mean your data was successfully delivered to the recipient. Instead, it merely confirms that the operating system accepted your request and enqueued it for transmission. If the recipient disconnects, the operating system will recognize this error later, but you will only see it with subsequent function calls.

Ensuring Message Delivery

To ensure that your messages have been successfully delivered over TCP, consider these techniques:

Graceful Shutdown: The most reliable way to check delivery involves performing a graceful shutdown of the socket connection which includes communicating with the recipient before closing the connection.

Using Acknowledgments: Build a protocol for your application that requires the recipient to acknowledge receipt of critical messages.

Error Handling: Regularly check the error codes after any socket operation to gracefully handle potential issues.

Conclusion

In conclusion, the perplexing behavior of asio::ip::tcp::socket::write_some returning Success after an abrupt connection abort can be attributed to how TCP manages the status of transmission. Understanding this behavior is vital for robust network programming. Implementing strategies for ensuring message delivery and using proper error handling can lead to more resilient applications that effectively deal with the realities of network communication.

In scenarios where quick responses are essential, be proactive in managing your socket connections and monitoring the return statuses for a worry-free experience in networked environments.