get width

main.go

// ref: https://github.com/buger/goterm

package main

import (
	"fmt"
	"os"
	"runtime"
	"syscall"
	"time"
	"unsafe"
)

type winsize struct {
	Row    uint16
	Col    uint16
	Xpixel uint16
	Ypixel uint16
}

func main() {
	for {
		for i := 0; i < getWidth(); i++ {
			fmt.Print("#")
		}
		fmt.Print("\r")
		time.Sleep(500 * time.Millisecond)
	}
}

func getWidth() (w int) {
	ws := new(winsize)

	var _TIOCGWINSZ int64

	switch runtime.GOOS {
	case "linux":
		_TIOCGWINSZ = 0x5413
	case "darwin":
		_TIOCGWINSZ = 1074295912
	}

	r1, _, errno := syscall.Syscall(syscall.SYS_IOCTL,
		uintptr(syscall.Stdin),
		uintptr(_TIOCGWINSZ),
		uintptr(unsafe.Pointer(ws)),
	)

	if int(r1) == -1 {
		fmt.Println("Error:", os.NewSyscallError("GetWinsize", errno))
	}

	return int(ws.Col)
}