reading data with Telnet client in go - go

I'm trying to read data from some devices via telnet protocol and below is my simple code.
I just want to print some meaningful results.
package main
import (
"fmt"
"github.com/reiver/go-telnet"
)
func main() {
conn, _ := telnet.DialTo("10.253.102.41:23")
fmt.Println(conn)
}
but this is what I got by this way:
&{0xc000006028 0xc000004720 0xc000040640}

It's obvious that it gets you &{0xc000006028 0xc000004720 0xc000040640} cause you are printing the connection object and it's the pointer address of that. If you want to print the data, you have to read it through connection using the Read method of the connection. Something like this:
b := make([]byte, 100)
n, err := conn.Read(b)
if err != nil {
// handle error
}
fmt.Println(string(b))

Related

Uses of io.ReadCloser

Could someone please explain (or/and share examples) when and why readers should to be closed explicitly, i.e. implement io.ReadCloser, not just io.Reader.
For example, when you are working with files, or any resource that should be closed to release the allocated resource (or memory for example for your resource, e.g. C code calling from Go).
You may use it when you have Read and Close methods, an example to show that you may use one common function to work with different types using io.ReadCloser:
package main
import (
"fmt"
"io"
"log"
"os"
)
func main() {
f, err := os.Open("./main.go")
if err != nil {
log.Fatal(err)
}
doIt(f)
doIt(os.Stdin)
}
func doIt(rc io.ReadCloser) {
defer rc.Close()
buf := make([]byte, 4)
n, err := rc.Read(buf)
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s\n", buf[:n])
}
Run and enter 12345 as an input, Output:
pack
12345
1234
See also:
Does Go automatically close resources if not explicitly closed?
It's for an explicit definition of Reader and Closer interface. So let's say you write some functionality that reads data, but you also want to close resource after doing it (again not to leak descriptors).
func ...(r io.ReaderCloser) {
defer r.Close()
... // some reading
}
anything you pass in it will need to have both interfaces defined, is it os.File or any custom struct, in this case, you are forcing client of your API to define Read and Close interfaces implementations, not just io.Reader.

How to reset or discard the buffered data

Not able to reset or discard the buffer.
I am trying to get the data over the serial port where I am getting data packet of some fixed length for every 10 seconds. I have an infinite for loop to receive the data packets continuously. After receiving the new data packet I am resetting the buffer but when I receive the next data packet, it overwrites the buffer and I get mixed data packet.
Let say I should receive packet abcdef continuously for every n second. But when I try the following code I am receiving packet bcdefa then after n second cdefab then defabc and so on
package main
import (
"bufio"
"log"
"time"
"github.com/tarm/serial"
)
func main() {
c := &serial.Config{Name: "/dev/ttyUSB0", Baud: 57600}
s, err := serial.OpenPort(c)
if err != nil {
log.Println(err)
return
}
for {
time.Sleep(time.Second / 2)
reader := bufio.NewReader(s)
pck, err := reader.Peek(46)
if err != nil {
log.Println(err)
}
go parse(pck)
reader.Reset(s)
}
}
How do I reset or discard the buffer data effectively so that I will receive the exact data packet.
bare in mind i cant check what i m saying here...
1/ you must not instantiate the bufio reader at each iteration
2/ bufio.Reader.Peek does NOT advance the reader
https://golang.org/pkg/bufio/#Reader.Peek
3/ Unless you get a malformed packet, i think you dont need to reset at all.
4/ Please indent your code at play.golang.org
5/ You are not checking the read error for termination
6/ All package i can found to work with serial ports in go exposes an instance of io.Reader, so it might be useless to use an additional bufio.Reader. I suspect you r using https://godoc.org/github.com/tarm/serial#OpenPort
This is probably not the definitive answer, but it should help.
package main
import (
"io"
"log"
"time"
)
func main() {
s, err := serial.OpenPort(c)
if err != nil {
log.Fatal(err)
}
pck := make([]byte, 46)
for {
time.Sleep(time.Second / 2)
n, err := s.Read(pck)
if err != nil {
if err == io.EOF {
break
}
log.Println(err)
}
pck = pck[:n]
go parse(pck)
}
}

Can`t figure out how to use buffers with binary web socket

everyone!
I'm trying to get my go code work with openstack serial console. It`s exposed via web socket. And i have problems with it.
I found gorrilla websocket lib (which is great) and took this example as a reference
With a few tweaks, now i have a code like this:
package main
import (
"log"
"net/url"
"os"
"os/signal"
"time"
"net/http"
"github.com/gorilla/websocket"
)
func main() {
DialSettings := &websocket.Dialer {
Proxy: http.ProxyFromEnvironment,
HandshakeTimeout: 45 * time.Second,
Subprotocols: []string{"binary",},
ReadBufferSize: 4096,
WriteBufferSize: 4096,
}
log.SetFlags(0)
interrupt := make(chan os.Signal, 1)
signal.Notify(interrupt, os.Interrupt)
u, _ := url.Parse("ws://172.17.0.64:6083/?token=d1763f2b-3466-424c-aece-6aeea2a733d5") //websocket url as it outputs from 'nova get-serial-console test' cmd
log.Printf("connecting to %s", u.String())
c, _, err := DialSettings.Dial(u.String(), nil)
if err != nil {
log.Fatal("dial:", err)
}
defer c.Close()
done := make(chan struct{})
go func() {
defer close(done)
for {
_, message, err := c.ReadMessage()
if err != nil {
log.Println("read:", err)
return
}
log.Printf("%s", message)
}
}()
c.WriteMessage(websocket.TextMessage, []byte("\n")) //just to force output to console
for {
select {
case <-done:
return
case <-interrupt:
log.Println("interrupt")
// Cleanly close the connection by sending a close message and then
// waiting (with timeout) for the server to close the connection.
err := c.WriteMessage(websocket.CloseMessage, websocket.FormatCloseMessage(websocket.CloseNormalClosure, ""))
if err != nil {
log.Println("write close:", err)
return
}
select {
case <-done:
case <-time.After(time.Second):
}
return
}
}
}
And i get output like this:
connecting to ws://172.17.0.64:6083/?token=d1763f2b-3466-424c-aece-6aeea2a733d5
CentOS Linux 7
(C
ore)
K
erne
l
3.10.0-862.el7.x86_64
o
n an
x
86_64
centos
-test login:
Total mess...
I think it's because i recieve just a chunks of bytes with no way to delimit them. I need some buffer to store them and when do something like bufio.ReadLine. But i'm not most experienced go programmer, and i run out of ideas how to do this. At the end i just need strings to work with.
The log package writes each log message on a separate line. If the log message does not end with a newline, then the log package will add one.
These extra newlines are garbling the output. To fix the output, replace the call to log.Printf("%s", message) with a function that does not add newlines to the output. Here are some options:
Write the message to stderr (same destination as default log package config):
os.Stderr.Write(message)
Write the message to stdout (a more conventional location to write program output):
os.Stdout.Write(message)

concurrency | goroutines | golang | buffered reader

Believe I am either misunderstanding how go routines work, how buffered readers work or both.
Expect an asynchronous execution of the goroutine ( a buffered reader with a for loop reading the buffer, waiting for a message from the server )
Try METHOD A to call go xyz() before the client dials the server; so xyz() creates the buffer and starts reading in the background. Then, the client dials the server; server sends message back; the client is reading the buffer so, it gets the message and prints to console
What Actually Happens the client to send the message to the server, but does not pick up anything on the buffer while reading for possible reply from server; so it is running concurrently because I know the for loop has not stopped, but it lets the next line of code execute ( client sending message to server ).
But When METHOD B I call xyz() NOT concurrently and after the client dials the server, all things work as expected. The client gets the message back from the server and prints to console.
METHOD A, we have the order :
///////////// steps 1 and 2 are in the goroutine called by go xyz()
creates the buffered reader
for loop -- reading the buffer for message from the server -- print out
client dials the server
go xyz(conn, p)
fmt.Fprintf(conn, "Give me a hash to work on ...")
METHOD B, we have the order :
///////////// steps 2 and 3 are in the goroutine called by xyz()
client dials the server
creates buffered reader
for loop -- reading the buffer for message from the server -- print out
fmt.Fprintf(conn, "Give me a hash to work on ...")
xyz(conn, p)
client.go
package main
import (
"fmt"
"net"
"bufio"
)
func xyz(conn net.Conn, p []byte) {
rd := bufio.NewReader(conn)
for {
_, err := rd.Read(p)
if err == nil {
fmt.Printf("SERVER : %s\n", p)
} else {
fmt.Printf("Some error %v\n", err)
}
}
}
func main() {
p := make([]byte, 2048)
conn, err := net.Dial("udp", "127.0.0.1:1234")
if err != nil {
fmt.Printf("Some error %v", err)
return
}
go xyz(conn, p)
fmt.Fprintf(conn, "Give me a hash to work on ...")
}
server.go
package main
import (
"fmt"
"net"
)
func sendResponse(conn *net.UDPConn, addr *net.UDPAddr, hash string) {
_,err := conn.WriteToUDP([]byte("Hello, here is the hash - " + hash), addr)
if err != nil {
fmt.Printf("Couldn't send response %v", err)
}
}
func main() {
hash := "36";
p := make([]byte, 2048)
addr := net.UDPAddr{
Port: 1234,
IP: net.ParseIP("127.0.0.1"),
}
ser, err := net.ListenUDP("udp", &addr)
if err != nil {
fmt.Printf("Some error %v\n", err)
return
}
for {
_, remoteaddr, err := ser.ReadFromUDP(p)
fmt.Printf("CLIENT : %v : %s\n", remoteaddr, p)
if err != nil {
fmt.Printf("Some error %v", err)
continue
}
go sendResponse(ser, remoteaddr, hash)
}
}
The Go Programming Language Specification
Go statements
A "go" statement starts the execution of a function call as an
independent concurrent thread of control, or goroutine, within the
same address space.
... unlike with a regular call, program execution does not wait for
the invoked function to complete.
client.go starts the goroutine xyz and then keeps going to the end of the main function which terminates the program. The program doesn't wait for the xyz goroutine to run or finish.

Reading from stdin in golang

I'm trying to read from Stdin in Golang as I'm trying to implement a driver for Erlang. I have the following code:
package main
import (
"fmt"
"os"
"bufio"
"time"
)
func main() {
go func() {
stdout := bufio.NewWriter(os.Stdin)
p := []byte{121,100,125,'\n'}
stdout.Write(p)
}()
stdin := bufio.NewReader(os.Stdin)
values := make([]byte,4,4)
for{
fmt.Println("b")
if read_exact(stdin) > 0 {
stdin.Read(values)
fmt.Println("a")
give_func_write(values)
}else{
continue
}
}
}
func read_exact(r *bufio.Reader) int {
bits := make([]byte,3,3)
a,_ := r.Read(bits)
if a > 0 {
r.Reset(r)
return 1
}
return -1
}
func give_func_write(a []byte) bool {
fmt.Println("Yahu")
return true
}
However it seems that the give_func_write is never reached. I tried to start a goroutine to write to standard input after 2 seconds to test this.
What am I missing here?
Also the line r.Reset(r). Is this valid in go? What I tried to achieve is simply restart the reading from the beginning of the file. Is there a better way?
EDIT
After having played around I was able to find that the code is stuck at a,_ := r.Read(bits) in the read_exact function
I guess that I will need to have a protocol in which I send a \n to
make the input work and at the same time discard it when reading it
No, you don't. Stdin is line-buffered only if it's bound to terminal. You can run your program prog < /dev/zero or cat file | prog.
bufio.NewWriter(os.Stdin).Write(p)
You probably don't want to write to stdin. See "Writing to stdin and reading from stdout" for details.
Well, it's not particular clear for me what you're trying to achieve. I'm assuming, that you just want to read data from stdin by fixed-size chunks. Use io.ReadFull for this. Or if you want to use buffers, you can use Reader.Peek or Scanner to ensure, that specific number of bytes is available. I've changed your program to demonstrate the usage of io.ReadFull:
package main
import (
"fmt"
"io"
"time"
)
func main() {
input, output := io.Pipe()
go func() {
defer output.Close()
for _, m := range []byte("123456") {
output.Write([]byte{m})
time.Sleep(time.Second)
}
}()
message := make([]byte, 3)
_, err := io.ReadFull(input, message)
for err == nil {
fmt.Println(string(message))
_, err = io.ReadFull(input, message)
}
if err != io.EOF {
panic(err)
}
}
You can easily split it in two programs and test it that way. Just change input to os.Stdin.

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