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Asked: 2026-06-11T22:55:53+00:00

I am working on a TCP server side app, which forwards data to a

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I am working on a TCP server side app, which forwards data to a client.

The problem I am facing is that I try to find out on my server side app if my client disconnected and which data was sent and which not.

My research showed that there are basically two ways to find that out:

  • 1) read from the socket and check if the FIN signal came back
  • 2) waiting for the sigpipe signal on the send call

The first solution doesn’t seem reliable to me, as I can’t guarantee that the client doesn’t send any random data and as such would make my test succeed even though it shouldn’t.

The problem with the second solution is that I only get the sigpipe after X following calls to send and as such can’t guarantee which data was really sent and which not. I read here on SO and on other sites, that the sigpipe is only supposed to come after the second call to send, I can reproduce that behavior if I only send and receive over localhost but not if I really use the network.

My question now is if it’s normal that X can vary and if yes which parameters I might look at to alter that behavior or if that is not reliable possible due to TCP nature.

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1 Answer

  1. Editorial Team

    TCP connection is bidirectional. A FIN from a client signals that the client won’t be sending any more data, but the data in the other direction (from the server to the client) can still be sent (if client does not reset the connection with the RST). The reliable way to detect the FIN from the client, is to read from the client socket (if you are using socket interface) until the read returns 0.

    TCP guarantees that if both ends terminate connection with a FIN that is acknowledged, all data that was exchanged within the connection, was received by the other side. If the connection is terminated with the RST, TCP by itself gives you no way to determine which data was successfully read by the other side. To do it, you need some application level mechanism, such as application level acknowledgements. But the best way would be to design your protocol in such a way, that connection, under normal circumstances, is always closed gracefully (FINs from both sides, no RSTs).

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