为什么在我的Go程序中使用stderr和ioutil.ReadAll获得“坏文件描述符”

Question

The command "psql" should throw an error, and I am trying to read stderr and print it in the Go program. I use ioutil.ReadAll to read the data from stderr, and stdout.

Unfortunately it is not reading from stderr at all. ioutil.ReadAll returns an error, which is not the error I am expecting.

The error I get is

read |0: bad file descriptor

Here is the code.

package main

import (
        "fmt"
        "os/exec"
        "io/ioutil"
)

func main() {
        cmd := exec.Command("psql")
        stdout, err := cmd.StdoutPipe()
        if err != nil {
                fmt.Printf("Error: %s", err)
        }
        stderr, err := cmd.StderrPipe()
        if err != nil {
                fmt.Printf("Error: %s", err)
        }
        err = cmd.Start()
        if err != nil {
                fmt.Printf("Start error %s",err)
        }

        d := cmd.Wait()
        if d != nil {
                fmt.Println(d)
        }

        stdo,g := ioutil.ReadAll(stdout)
        stde,f := ioutil.ReadAll(stderr)

        if g != nil {
                fmt.Println(g)
        }

        if f !=nil {
                fmt.Println(f)
        }

        fmt.Printf("Standard err is %s 
", stde)
        fmt.Printf("Standard out is %s 
",stdo)
}

Answer 1

I found that through experimentation that I am getting the error, due to the fact that I am calling

   stdo,g := ioutil.ReadAll(stdout)
   stde,f := ioutil.ReadAll(stderr)

after

 d := cmd.Wait()

so what happens is the stdout, stderr pipe get closed after the cmd.Wait() returns.

Here are the code comments for the cmd.StderrPipe()

// StderrPipe returns a pipe that will be connected to the command's
// standard error when the command starts.
// The pipe will be closed automatically after Wait sees the command exit.

So obviously we can't read stdout and stderr after they get closed.

We cannot read them before the command starts either. So we have to put them in between start and wait.

Here is code that fixes that.

package main

import (
        "fmt"
        "os/exec"
        "io/ioutil"
)

func main() {
        cmd := exec.Command("psql")
        stdout, err := cmd.StdoutPipe()
        if err != nil {
                fmt.Printf("Error: %s", err)
        }
        stderr, err := cmd.StderrPipe()
        if err != nil {
                fmt.Printf("Error: %s", err)
        }
        err = cmd.Start()
        if err != nil {
                fmt.Printf("Start error %s",err)
        }

        stdo,g := ioutil.ReadAll(stdout)
        stde,f := ioutil.ReadAll(stderr)

        d := cmd.Wait()

        if d != nil {
                fmt.Println(d)
        }

        if g != nil {
                fmt.Println(g)
        }

        if f !=nil {
                fmt.Println(f)
        }

        fmt.Printf("Standard err is %s 
", stde)
        fmt.Printf("Standard out is %s 
",stdo)
}