Vfrdtyky

Today when I try to send 100M data to my server (a very simple TCP server also written in Golang), I found that the TCPConn.Write method returns 104857600 and nil error and then I close the socket. But my server only receives very little data. I think it is because Write method works in async mode, so although the method returns 104857600, only a little data is sent to the server. So I want to know whether there is a way to set the Write work in sync mode or how to detect whether all data is sent to the server from the socket.

The code is as follows:
server:

const ListenAddress = "192.168.0.128:8888"

func main() {



    var l net.Listener

    var err error

    l, err = net.Listen("tcp", ListenAddress)

    if err != nil {

        fmt.Println("Error listening:", err)

        os.Exit(1)

    }

    defer l.Close()

    fmt.Println("listen on " + ListenAddress)

    for {

        conn, err := l.Accept()

        if err != nil {

            fmt.Println("Error accepting: ", err)

            os.Exit(1)

        }

        //logs an incoming message

        fmt.Printf("Received message %s -> %s n", conn.RemoteAddr(), conn.LocalAddr())

        // Handle connections in a new goroutine.

        go handleRequest(conn)

    }

}

func handleRequest(conn net.Conn) {

    defer conn.Close()

    rcvLen := 0

    rcvData := make(byte,20 * 1024 * 1024) // 20M

    for {

        l , err := conn.Read(rcvData)

        if err != nil {

            fmt.Printf("%v", err)

            return

        }

        rcvLen += l

        fmt.Printf("recv: %drn", rcvLen)



        conn.Write(rcvData[:l])

    }

}

Client:

conn, err := net.Dial("tcp", "192.168.0.128:8888")

if err != nil {

    fmt.Println(err)

    os.Exit(-1)

}

defer conn.Close()

data := make(byte, 500 * 1024 * 1024)

len, err := conn.Write(data)

fmt.Println("send len: ", len)

The output of the client:

send len:  524288000

The output of the server:

listen on 192.168.0.128:8888

Received message 192.168.0.2:50561 -> 192.168.0.128:8888 

recv: 166440

recv: 265720

EOF

I know if I can make the client wait for a while by SetLinger method, the data will be all sent to the server before the socket is closed. But I want to find a way to make the socket send all data before returns without calling SetLinger(). Thanks in advance. Please excuse my poor English.

Did you poll the socket before trying to write?

Behind the socket is your operating system's tcp stack. When writing on a socket, you push bytes to the send buffer. Your operating system then self determines when and how to send. If the receiving end has no buffer space.available in their receice buffer, your sending end knows this and will not put any more information in the send buffer.

Make sure your send buffer has enough space for whatever you are trying to send next. This is done by polling the socket. This method is usually called Socket.Poll. I.recommend you ccheck the golang docs for the exact usage.

You are not handling the error returned by conn.Read correctly. From the docs (emphasis mine):

When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read. It may return the (non-nil) error from the same call or return the error (and n == 0) from a subsequent call. [...]

Callers should always process the n > 0 bytes returned before considering the error err. Doing so correctly handles I/O errors that happen after reading some bytes and also both of the allowed EOF behaviors.

Note that you are re-inventing io.Copy (albeit with an excessive buffer size). Your server code can be rewritten as:

func handleRequest(conn net.Conn) {

    defer conn.Close()

    n, err := io.Copy(conn, conn)

}