How to use the operating system API to detect Socket connection disconnection?

Explore the API for Socket connection disconnect detection

Many developers encounter this problem when programming the network: How to reliably determine whether a Socket connection has been disconnected? Especially after the client actively closes the connection, how does the server sense the termination of the connection? This article will dive into the APIs provided by the operating system and how to use these APIs to detect disconnection of Socket connections.

The title of the article is "What API does the operating system provide to let us know if a socket connection is disconnected?" This is the core issue of this article. The questioner pointed out that after TCP four waves completed, the client (A) that actively closed the connection knew that the connection had been closed, but the server (B) that passively closed the connection could be sensed, and whether the operating system provided the corresponding API provisioning layer query. The questioner believes that the server application layer cannot directly perceive the four waves of the underlying operating system, and the operating system should provide corresponding APIs to solve this problem.

The questioner's doubts about not finding relevant APIs in Python are actually not a problem unique to Python. The key is to understand the mechanism of TCP connection shutdown and how the operating system exposes this information to the application layer.

The recv() function is mentioned in the answer. The recv() function is not only used to receive data, but its return value also contains important information about the connection status. The recv() function returns 0 when the peer has completed an ordered shutdown (i.e., four waves) and there is no more data to receive. This is the important basis for the application layer to determine that the Socket connection has been disconnected. Therefore, by monitoring the return value of the recv() function, the application layer can detect whether the Socket connection has been disconnected. It should be noted that recv() returns -1 and sets errno to indicate that an error has occurred and further judgment is needed based on the specific value of errno. This is different from the normal shutdown state after four waves are completed. In this way, the application layer can monitor the connection status without directly relying on the four wave details inside the operating system.

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