Why operate normally when bufio.NewWriter get os.Stdin in local

package main

import (
	"bufio"
	"os"
)

func main() {
	bw := bufio.NewWriter(os.Stdin)
	bw2 := bufio.NewWriter(os.Stdout)

	bw.WriteString("Hello, world 1\n")
	bw2.WriteString("Hello, world 2\n")

	bw.Flush()
	bw2.Flush()
}

This code show both string in a local environment.
But why does it work differently in different environments?

My local environment
OS : macOS 12.6
go : go1.19.2 darwin/amd64
ide : vscode

• on my local machine :

$ go run myworkspace/main/main.go
Hello, world 1
Hello, world 2

• on the playground :

# in the 'Output' section
---
Hello, world 2

Program exited.

The Go Playground
https://go.dev/play/p/PtoDwCZGggd

Scroll all the way down to "About the Playground"

> About the Playground
>
> The Go Playground runs the program inside a sandbox.
>
> There are limitations to the programs that can be run in the playground:
>
> The only communication a playground program has to the outside world is by writing to standard output and standard error.

The Go Playground discards writes to standard input.

Locally, os.Stdin is a file.

var (
	Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
	Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
	Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
)

> Stdin, Stdout, and Stderr are open Files pointing to the standard input, standard output, and standard error file descriptors.

The bottom line is : your program does not control what thingy will be used as Stdin and Stdout when it is started.

On your local machine, try running : echo "" | go run main.go or go run main.go < anyfile.

(also : you should check for errors in your go code)

It is a standard setup, when running a command from a terminal, that the shell uses the same file descriptor, which points to a read/write terminal, for stdin and stdout/stderr. With that setup, writing to stdin "works".

But you can't rely on it (you shouldn't, actually), and it turns out that the playground doesn't start its processes that way :

package main

import (
	"fmt"
	"os"
)

func main() {
	_, err := os.Stdin.Write([]byte("Hello\n"))
	if err != nil {
		fmt.Fprintln(os.Stderr, "*** error:", err)
	}
}

// Output:
// *** error: write /dev/stdin: bad file descriptor
//
// Program exited.

https://go.dev/play/p/NmxgOsK2ovp

see also :

https://stackoverflow.com/questions/37822580/why-is-it-possible-to-write-to-stdin

https://stackoverflow.com/questions/7383803/writing-to-stdin-and-reading-from-stdout-unix-linux-c-programming

(found by googling "writing to stdin")