How to know TCP connection is closed in net package?

Question

I'm implementing a small TCP server. How do I know if one of my clients closed? Should I just try to read or write and check if err is nil?

Answer 1

That thread "Best way to reliably detect that a TCP connection is closed", using net.Conn for 'c' (also seen in utils/ping.go or locale-backend/server.go or many other instances):

one := make([]byte, 1) c.SetReadDeadline(time.Now()) if _, err := c.Read(one); err == io.EOF {   l.Printf(logger.LevelDebug, "%s detected closed LAN connection", id)   c.Close()   c = nil } else {   var zero time.Time   c.SetReadDeadline(time.Now().Add(10 * time.Millisecond)) } 

For detecting a timeout, it suggests:

if neterr, ok := err.(net.Error); ok && neterr.Timeout() {   ... 

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Update 2019: tuxedo25 mentions in the comments:

In go 1.7+, zero byte reads return immediately and will never return an error.
You must read at least one byte.

See commit 5bcdd63 and go issue 15735

net: don't return io.EOF from zero byte reads

Answer 2

Just try to read from it, and it will throw an error if it's closed. Handle gracefully if you wish!

For risk of giving away too much:

func Read(c *net.Conn, buffer []byte) bool {     bytesRead, err := c.Read(buffer)     if err != nil {         c.Close()         log.Println(err)         return false     }     log.Println("Read ", bytesRead, " bytes")     return true } 

Here is a nice introduction to using the net package to make a small TCP "chat server":

"Golang Away: TCP Chat Server"