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Mark Harrison
Mark Harrison

Reputation: 304662

Go: bidirectional communication with another process?

(note) not a dupe of Go Inter-Process Communication which is asking about System V IPC. (end note)

Using os/exec, how do I interactively communicate with another process? I'd like to get fd's for the process's stdin and stdout, and write to and read from the process using those fds.

Most examples I have found involve running another process and then slurping the resulting output.

Here's the python equivalent of what I'm looking for. 

p = subprocess.Popen("cmd", stdin=subprocess.PIPE, stdout=subprocess.PIPE)
(child_stdin, child_stdout) = (p.stdin, p.stdout)

As a tangible example, consider opening a pipe to dc, sending the line 12 34 +p and receiving the line 46.

(update)

func main() {
  cmd := exec.Command("dc")
  stdin, err := cmd.StdinPipe()
  must(err)
  stdout, err := cmd.StdoutPipe()
  must(err)

  err = cmd.Start()
  must(err)

  fmt.Fprintln(stdin, "2 2 +p")

  line := []byte{}
  n, err := stdout.Read(line)

  fmt.Printf("%d :%s:\n", n, line)
}

I see by strace that dc is receiving and answering as expected:

[pid  8089] write(4, "12 23 +p\n", 9 <unfinished ...>
...
[pid  8095] <... read resumed> "12 23 +p\n", 4096) = 9
...
[pid  8095] write(1, "35\n", 3 <unfinished ...>

but I don't seem to be getting the results back into my calling program:

0 ::

(update)

As per the accepted answer, my problem was not allocating the string to receive the response. Changing to line := make([]byte, 100) fixed everything.

Upvotes: 1

Views: 4095

Answers (1)

Mr_Pink
Mr_Pink

Reputation: 109443

An exec.Cmd has fields for the process stdin, std, and stderr which you can assign.

    // Stdin specifies the process's standard input.
    // If Stdin is nil, the process reads from the null device (os.DevNull).
    // If Stdin is an *os.File, the process's standard input is connected
    // directly to that file.
    // Otherwise, during the execution of the command a separate
    // goroutine reads from Stdin and delivers that data to the command
    // over a pipe. In this case, Wait does not complete until the goroutine
    // stops copying, either because it has reached the end of Stdin
    // (EOF or a read error) or because writing to the pipe returned an error.
    Stdin io.Reader

    // Stdout and Stderr specify the process's standard output and error.
    //
    // If either is nil, Run connects the corresponding file descriptor
    // to the null device (os.DevNull).
    //
    // If Stdout and Stderr are the same writer, at most one
    // goroutine at a time will call Write.
    Stdout io.Writer
    Stderr io.Writer

If you want a pre-made pipe to connected to any of those, you can use the *Pipe() methods

func (c *Cmd) StderrPipe() (io.ReadCloser, error)
func (c *Cmd) StdinPipe() (io.WriteCloser, error)
func (c *Cmd) StdoutPipe() (io.ReadCloser, error)

A basic example using the dc program (sans error checking):

cmd := exec.Command("dc")
stdin, _ := cmd.StdinPipe()
stdout, _ := cmd.StdoutPipe()
cmd.Start()

stdin.Write([]byte("12 34 +p\n"))

out := make([]byte, 1024)
n, _ := stdout.Read(out)

fmt.Println("OUTPUT:", string(out[:n]))

// prints "OUTPUT: 46"

Upvotes: 5

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