golang/go · error

bufio: invalid use of UnreadRune

Error message

bufio: invalid use of UnreadRune

What it means

Error "bufio: invalid use of UnreadRune" thrown in golang/go.

Source

Thrown at src/bufio/bufio.go:24

// object, creating another object (Reader or Writer) that also implements
// the interface but provides buffering and some help for textual I/O.
package bufio

import (
	"bytes"
	"errors"
	"io"
	"strings"
	"unicode/utf8"
)

const (
	defaultBufSize = 4096
)

var (
	ErrInvalidUnreadByte = errors.New("bufio: invalid use of UnreadByte")
	ErrInvalidUnreadRune = errors.New("bufio: invalid use of UnreadRune")
	ErrBufferFull        = errors.New("bufio: buffer full")
	ErrNegativeCount     = errors.New("bufio: negative count")
)

// Buffered input.

// Reader implements buffering for an io.Reader object.
// A new Reader is created by calling [NewReader] or [NewReaderSize];
// alternatively the zero value of a Reader may be used after calling [Reader.Reset]
// on it.
type Reader struct {
	buf          []byte
	rd           io.Reader // reader provided by the client
	r, w         int       // buf read and write positions
	err          error
	lastByte     int // last byte read for UnreadByte; -1 means invalid
	lastRuneSize int // size of last rune read for UnreadRune; -1 means invalid
}

View on GitHub (pinned to b6b368adc5)

When it happens

Trigger: Thrown at src/bufio/bufio.go:24 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/4aa8477c8ff7cca5. Report an issue: GitHub.