Question

I want to be able to fully communicate with some programs after spawning them from Golang program. What I already have is spawning process and talking through pipes based on last line read from stdout:

package main

import (
    "fmt"
    "io"
    "log"
    "os/exec"
    "strings"
)

var stdinPipe io.WriteCloser
var stdoutPipe io.ReadCloser
var err error

func main() {
    cmd := &exec.Cmd{
        Path: "/Users/seba/Projects/go/src/bootstrap/in",
        Args: []string{"program"},
    }

    stdinPipe, err = cmd.StdinPipe()

    if err != nil {
        log.Fatal(err)
    }

    stdoutPipe, err = cmd.StdoutPipe()

    if err != nil {
        log.Fatal(err)
    }

    err = cmd.Start()

    if err != nil {
        log.Fatal(err)
    }

    var stdoutLines []string
    go stdoutManage(stdoutLines, stdoutController)

    cmd.Wait()
}

// TODO: imporove as in io.Copy
func stdoutManage(lines []string, manager func(string)) {
    buf := make([]byte, 32*1024)

    for {
        nr, err := stdoutPipe.Read(buf)

        if nr > 0 {
            thelines := strings.Split(string(buf), "\n")
            for _, l := range thelines {
                manager(l)
                lines = append(lines, l)
            }
        }

        buf = make([]byte, 32*1024) // clear buf

        if err != nil {
            break
        }
    }
}

However this approach have problems with programs clearing terminal output and programs which somehow buffer it’s stdin or don’t use stdin at all (don’t know if it’s possible).

So the question: is there a portable way of talking with programs (it can be non-Golang solution)?

Answer 1

Problems like this are usually to do with the C library which changes its default buffering mode depending on exactly what stdin / stdout / stderr are.

If stdout is a terminal then buffering is automatically set to line buffered, else it is set to buffered.

This is relevant to you because when you run the programs through a pipe they aren’t connected to a terminal and so will have buffering which messes up this sort of use.

To fix, you need to use a pseudo tty which pretends to be a terminal but acts just like a pipe. Here is a library implementing the pty interface which I haven’t actually tried but it looks like it does the right thing!