# How to correctly detect closed TCP connection

**URL:** <https://discourse.julialang.org/t/how-to-correctly-detect-closed-tcp-connection/92287>\
**Category:** New to Julia\
**Tags:** question, networking\
**Created:** [December 29, 2022, 7:54pm UTC](https://discourse.julialang.org/t/how-to-correctly-detect-closed-tcp-connection/92287 "2022-12-29T19:54:23Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![plukes](https://avatars.discourse-cdn.com/v4/letter/p/bbce88/32.png) [@plukes](https://discourse.julialang.org/u/plukes)\
**Post date:** [December 29, 2022, 7:54pm UTC](https://discourse.julialang.org/t/how-to-correctly-detect-closed-tcp-connection/92287/1 "2022-12-29T19:54:23Z")

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Hi,  
I have been trying to implement a simple TCP server that sends periodically data to a created connection. Based on the manual I can implement a minimal working example as follows:

```julia
function write_data(sock)
    while isopen(sock)
        write(sock,"hello\n")
        sleep(1)
    end
    println("socket closed")
end

errormonitor(@async begin
           server = listen(2001)
           while true
               sock = accept(server)
               println("Accepted connection")
               errormonitor(@async write_data(sock))
           end
       end)

```

This works as expected. When I connect from different process, different language, it correctly accepts the connection and starts transmitting the data.

The problem I am having is when I close the connection from the different process (in julia `close(sock)`) the `isopen(sock)` still returns true thus the data are being still transmitted. This [seems](https://github.com/JuliaLang/julia/issues/10601) to be a wanted behaviour.

I have read upon the need to use `eof(sock)` and tried to rewrite the function writing the data as

```julia
function write_data(sock)
    while isopen(sock) && !eof(sock)
        write(sock,"hello\n")
        sleep(1)
    end
    println("socket closed")
end

```

But then it does not send anything as the call to `eof(sock)` is blocking and the other end does not transmit anything.

What is the correct way to handle remote connection drop in such a loop where I want to only transmit data while the connection is active? Is there something such as non blocking eof?

Thank you

---

<div class="post-metadata">

**Author:** ![plukes](https://avatars.discourse-cdn.com/v4/letter/p/bbce88/32.png) [@plukes](https://discourse.julialang.org/u/plukes)\
**Post date:** [December 30, 2022, 10:04am UTC](https://discourse.julialang.org/t/how-to-correctly-detect-closed-tcp-connection/92287/2 "2022-12-30T10:04:53Z")

</div>

One possible solution I have figured out is to use `Channel` to asynchronously wait on the `eof`.

```julia
function is_socket_eof(channel::Channel, sock)
    put!(channel, eof(sock))
end

function is_socket_closed(sock, eof_monitor::Channel)
    return !isopen(sock) || (isready(eof_monitor) && take!(eof_monitor))
end

function write_data(sock)
    eof_monitor = Channel((ch) -> is_socket_eof(ch, sock))
    while !is_socket_closed(sock, eof_monitor)
        write(sock,"hello\n")
        sleep(1)
    end
    println("socket closed")
end

```

This seems to correctly detect if the socket was closed remotely.

I am unsure why I do have to put the `!isopen(sock)` to the `is_socket_closed` function. Without it, I have found out that the condition in the while `!is_socket_closed(...)` would result to true and yet ‘isopen(sock)’ would be false resulting in an error when writing to the socket. Maybe something connected with the asynchronicity which I don’t fully understand yet.

If someone would have an idea for better solution, it would be appreciated.

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<div class="post-metadata">

**Author:** ![phma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/phma/32/6576_2.png) [@phma](https://discourse.julialang.org/u/phma)\
**Post date:** [December 13, 2024, 10:06am UTC](https://discourse.julialang.org/t/how-to-correctly-detect-closed-tcp-connection/92287/3 "2024-12-13T10:06:47Z")

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I found another way which doesn’t block and seems to work: `isreadable(sock)`. It returns true while the socket is open and false when it’s closed by the other end.