How to know TCP connection is closed in net package?

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?

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() {
  ...

---

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

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"

After struggling for a while on this, here is a POSIX solution that uses MSG_PEEK to prevent draining the buffer and causing race conditions. This lets you check the whether the READ half of a TCP socket is still open from another goroutine:

func connCheck(conn net.Conn) error {
    var sysErr error = nil
    rc, err := conn.(syscall.Conn).SyscallConn()
    if err != nil { return err }
    err = rc.Read(func(fd uintptr) bool {
        var buf []byte = []byte{0}
        n, _, err := syscall.Recvfrom(int(fd), buf, syscall.MSG_PEEK | syscall.MSG_DONTWAIT)
        switch {
        case n == 0 && err == nil:
            sysErr = io.EOF
        case err == syscall.EAGAIN || err == syscall.EWOULDBLOCK:
            sysErr = nil
        default:
            sysErr = err
        }
        return true
    })
    if err != nil { return err }

    return sysErr
}

This is based on the above mysql#connCheck, but that one does a 1-byte read syscall, which can potentially conflict with other goroutines attempting to read a stream.

April 2019 status:

Reading the threads and posts on https://github.com/golang/go/issues/15735

There is a general lack of official support to detect whether a server closed a TCP connection if the server does not write on the connection (reading will fail, but writing won't).

There is a solution provided by https://github.com/methane that works only on Linux and is expensive as it does some allocations -

This is available here: https://github.com/go-sql-driver/mysql/blob/master/conncheck.go

I found this to be working but not being cross platform is blocking me from adopting it -

My five cents, I figured it is better to write to the connection and get an error, thanks for the answers above, here is my working solution

func isTCPWorking(c net.Conn) bool {
   _, err := c.Write([]byte("OK"))
   if err != nil {
	  c.Close() // close if problem
	  return false
   }
   return true
}

        _, err := conn.Read(make([]byte, 0))
		if err!=io.EOF{
			// this connection is invalid
			logger.W("conn closed....",err)

		}else{
			byt, _:= ioutil.ReadAll(conn);
		}