Golang: Write a message to the current (see comments for a method for any console) console's input on Windows.

write_to_stdin_windows.go

// Used in https://github.com/SCP002/terminator.
// For POSIX, see: https://gist.github.com/SCP002/c7c3bf4aafd3e32e0dc0aa65dda2bf14.

package main

import (
	"errors"
	"os"
	"unsafe"

	"golang.org/x/sys/windows"
)

// Windows types.
//
// Inspired by https://github.com/Azure/go-ansiterm/blob/master/winterm/api.go.
type (
	// https://docs.microsoft.com/en-us/windows/console/input-record-str.
	inputRecord struct {
		eventType uint16
		keyEvent  keyEventRecord
	}

	// https://docs.microsoft.com/en-us/windows/console/key-event-record-str.
	keyEventRecord struct {
		keyDown         int32
		repeatCount     uint16
		virtualKeyCode  uint16
		virtualScanCode uint16
		unicodeChar     uint16
		controlKeyState uint32
	}
)

// Windows constants.
const (
	// https://docs.microsoft.com/en-us/windows/console/input-record-str#members.
	keyEvent uint16 = 0x0001
)

func main() {
	err := Write("Hello, World!\r\n")
	if err != nil {
		panic(err)
	}
}

// Write writes a message to the current console's input.
//
// To write to other console, see the implementation in:
//
// https://github.com/SCP002/terminator/blob/main/internal/proxy/answer/send.go.
func Write(msg string) error {
	k32 := windows.NewLazyDLL("kernel32.dll")

	inpRecList, err := StrToInputRecords(msg)
	if err != nil {
		return errors.New("Failed to convert string message to an array of inputRecord.")
	}
	k32Proc := k32.NewProc("WriteConsoleInputW")
	var written uint32 = 0
	var toWrite uint32 = uint32(len(inpRecList))
	r1, _, _ := k32Proc.Call(
		os.Stdin.Fd(),
		// Actually passing the whole slice. Must be [0] due the way syscall works.
		uintptr(unsafe.Pointer(&inpRecList[0])),
		uintptr(toWrite),
		// A pointer to the number of input records actually written. Not using it (placeholder).
		uintptr(unsafe.Pointer(&written)),
	)
	if r1 == 0 {
		return errors.New("Failed to write an array of inputRecord to the current console's input.")
	}
	return nil
}

// StrToInputRecords converts a string into a slice of inputRecord, see:
//
// https://docs.microsoft.com/en-us/windows/console/input-record-str.
func StrToInputRecords(msg string) ([]inputRecord, error) {
	records := []inputRecord{}
	utf16chars, err := windows.UTF16FromString(msg)
	if err != nil {
		return records, err
	}
	for _, char := range utf16chars {
		record := inputRecord{
			eventType: keyEvent,
			keyEvent: keyEventRecord{
				// 1 = TRUE, the key is pressed. Can omit key release events.
				keyDown:         1,
				repeatCount:     1,
				virtualKeyCode:  0,
				virtualScanCode: 0,
				unicodeChar:     char,
				controlKeyState: 0,
			},
		}
		records = append(records, record)
	}
	return records, nil
}