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Package bufio

import "bufio"
Overview
Index
Examples

Overview ▾

Package bufio implements buffered I/O. It wraps an io.Reader or io.Writer object, creating another object (Reader or Writer) that also implements the interface but provides buffering and some help for textual I/O.

Index ▾

Constants
Variables
func ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error)
func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error)
func ScanRunes(data []byte, atEOF bool) (advance int, token []byte, err error)
func ScanWords(data []byte, atEOF bool) (advance int, token []byte, err error)
type ReadWriter
    func NewReadWriter(r *Reader, w *Writer) *ReadWriter
type Reader
    func NewReader(rd io.Reader) *Reader
    func NewReaderSize(rd io.Reader, size int) *Reader
    func (b *Reader) Buffered() int
    func (b *Reader) Discard(n int) (discarded int, err error)
    func (b *Reader) Peek(n int) ([]byte, error)
    func (b *Reader) Read(p []byte) (n int, err error)
    func (b *Reader) ReadByte() (byte, error)
    func (b *Reader) ReadBytes(delim byte) ([]byte, error)
    func (b *Reader) ReadLine() (line []byte, isPrefix bool, err error)
    func (b *Reader) ReadRune() (r rune, size int, err error)
    func (b *Reader) ReadSlice(delim byte) (line []byte, err error)
    func (b *Reader) ReadString(delim byte) (string, error)
    func (b *Reader) Reset(r io.Reader)
    func (b *Reader) UnreadByte() error
    func (b *Reader) UnreadRune() error
    func (b *Reader) WriteTo(w io.Writer) (n int64, err error)
type Scanner
    func NewScanner(r io.Reader) *Scanner
    func (s *Scanner) Buffer(buf []byte, max int)
    func (s *Scanner) Bytes() []byte
    func (s *Scanner) Err() error
    func (s *Scanner) Scan() bool
    func (s *Scanner) Split(split SplitFunc)
    func (s *Scanner) Text() string
type SplitFunc
type Writer
    func NewWriter(w io.Writer) *Writer
    func NewWriterSize(w io.Writer, size int) *Writer
    func (b *Writer) Available() int
    func (b *Writer) Buffered() int
    func (b *Writer) Flush() error
    func (b *Writer) ReadFrom(r io.Reader) (n int64, err error)
    func (b *Writer) Reset(w io.Writer)
    func (b *Writer) Write(p []byte) (nn int, err error)
    func (b *Writer) WriteByte(c byte) error
    func (b *Writer) WriteRune(r rune) (size int, err error)
    func (b *Writer) WriteString(s string) (int, error)

Package files

bufio.go scan.go

Constants

const (
        // MaxScanTokenSize is the maximum size used to buffer a token
        // unless the user provides an explicit buffer with Scan.Buffer.
        // The actual maximum token size may be smaller as the buffer
        // may need to include, for instance, a newline.
        MaxScanTokenSize = 64 * 1024
)

Variables

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")
)

Errors returned by Scanner.

var (
        ErrTooLong         = errors.New("bufio.Scanner: token too long")
        ErrNegativeAdvance = errors.New("bufio.Scanner: SplitFunc returns negative advance count")
        ErrAdvanceTooFar   = errors.New("bufio.Scanner: SplitFunc returns advance count beyond input")
)

ErrFinalToken is a special sentinel error value. It is intended to be returned by a Split function to indicate that the token being delivered with the error is the last token and scanning should stop after this one. After ErrFinalToken is received by Scan, scanning stops with no error. The value is useful to stop processing early or when it is necessary to deliver a final empty token. One could achieve the same behavior with a custom error value but providing one here is tidier. See the emptyFinalToken example for a use of this value.

var ErrFinalToken = errors.New("final token")

func ScanBytes

ScanBytes is a split function for a Scanner that returns each byte as a token.

func ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error)

func ScanLines

ScanLines is a split function for a Scanner that returns each line of text, stripped of any trailing end-of-line marker. The returned line may be empty. The end-of-line marker is one optional carriage return followed by one mandatory newline. In regular expression notation, it is `\r?\n`. The last non-empty line of input will be returned even if it has no newline.

func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error)

func ScanRunes

ScanRunes is a split function for a Scanner that returns each UTF-8-encoded rune as a token. The sequence of runes returned is equivalent to that from a range loop over the input as a string, which means that erroneous UTF-8 encodings translate to U+FFFD = "\xef\xbf\xbd". Because of the Scan interface, this makes it impossible for the client to distinguish correctly encoded replacement runes from encoding errors.

func ScanRunes(data []byte, atEOF bool) (advance int, token []byte, err error)

func ScanWords

ScanWords is a split function for a Scanner that returns each space-separated word of text, with surrounding spaces deleted. It will never return an empty string. The definition of space is set by unicode.IsSpace.

func ScanWords(data []byte, atEOF bool) (advance int, token []byte, err error)

type ReadWriter

ReadWriter stores pointers to a Reader and a Writer. It implements io.ReadWriter.

type ReadWriter struct {
        *Reader
        *Writer
}

func NewReadWriter

NewReadWriter allocates a new ReadWriter that dispatches to r and w.

func NewReadWriter(r *Reader, w *Writer) *ReadWriter

type Reader

Reader implements buffering for an io.Reader object.

type Reader struct {
        // contains filtered or unexported fields
}

func NewReader

NewReader returns a new Reader whose buffer has the default size.

func NewReader(rd io.Reader) *Reader

func NewReaderSize

NewReaderSize returns a new Reader whose buffer has at least the specified size. If the argument io.Reader is already a Reader with large enough size, it returns the underlying Reader.

func NewReaderSize(rd io.Reader, size int) *Reader

func (*Reader) Buffered

Buffered returns the number of bytes that can be read from the current buffer.

func (b *Reader) Buffered() int

func (*Reader) Discard

Discard skips the next n bytes, returning the number of bytes discarded.

If Discard skips fewer than n bytes, it also returns an error. If 0 <= n <= b.Buffered(), Discard is guaranteed to succeed without reading from the underlying io.Reader.

func (b *Reader) Discard(n int) (discarded int, err error)

func (*Reader) Peek

Peek returns the next n bytes without advancing the reader. The bytes stop being valid at the next read call. If Peek returns fewer than n bytes, it also returns an error explaining why the read is short. The error is ErrBufferFull if n is larger than b's buffer size.

func (b *Reader) Peek(n int) ([]byte, error)

func (*Reader) Read

Read reads data into p. It returns the number of bytes read into p. The bytes are taken from at most one Read on the underlying Reader, hence n may be less than len(p). At EOF, the count will be zero and err will be io.EOF.

func (b *Reader) Read(p []byte) (n int, err error)

func (*Reader) ReadByte

ReadByte reads and returns a single byte. If no byte is available, returns an error.

func (b *Reader) ReadByte() (byte, error)

func (*Reader) ReadBytes

ReadBytes reads until the first occurrence of delim in the input, returning a slice containing the data up to and including the delimiter. If ReadBytes encounters an error before finding a delimiter, it returns the data read before the error and the error itself (often io.EOF). ReadBytes returns err != nil if and only if the returned data does not end in delim. For simple uses, a Scanner may be more convenient.

func (b *Reader) ReadBytes(delim byte) ([]byte, error)

func (*Reader) ReadLine

ReadLine is a low-level line-reading primitive. Most callers should use ReadBytes('\n') or ReadString('\n') instead or use a Scanner.

ReadLine tries to return a single line, not including the end-of-line bytes. If the line was too long for the buffer then isPrefix is set and the beginning of the line is returned. The rest of the line will be returned from future calls. isPrefix will be false when returning the last fragment of the line. The returned buffer is only valid until the next call to ReadLine. ReadLine either returns a non-nil line or it returns an error, never both.

The text returned from ReadLine does not include the line end ("\r\n" or "\n"). No indication or error is given if the input ends without a final line end. Calling UnreadByte after ReadLine will always unread the last byte read (possibly a character belonging to the line end) even if that byte is not part of the line returned by ReadLine.

func (b *Reader) ReadLine() (line []byte, isPrefix bool, err error)

func (*Reader) ReadRune

ReadRune reads a single UTF-8 encoded Unicode character and returns the rune and its size in bytes. If the encoded rune is invalid, it consumes one byte and returns unicode.ReplacementChar (U+FFFD) with a size of 1.

func (b *Reader) ReadRune() (r rune, size int, err error)

func (*Reader) ReadSlice

ReadSlice reads until the first occurrence of delim in the input, returning a slice pointing at the bytes in the buffer. The bytes stop being valid at the next read. If ReadSlice encounters an error before finding a delimiter, it returns all the data in the buffer and the error itself (often io.EOF). ReadSlice fails with error ErrBufferFull if the buffer fills without a delim. Because the data returned from ReadSlice will be overwritten by the next I/O operation, most clients should use ReadBytes or ReadString instead. ReadSlice returns err != nil if and only if line does not end in delim.

func (b *Reader) ReadSlice(delim byte) (line []byte, err error)

func (*Reader) ReadString

ReadString reads until the first occurrence of delim in the input, returning a string containing the data up to and including the delimiter. If ReadString encounters an error before finding a delimiter, it returns the data read before the error and the error itself (often io.EOF). ReadString returns err != nil if and only if the returned data does not end in delim. For simple uses, a Scanner may be more convenient.

func (b *Reader) ReadString(delim byte) (string, error)

func (*Reader) Reset

Reset discards any buffered data, resets all state, and switches the buffered reader to read from r.

func (b *Reader) Reset(r io.Reader)

func (*Reader) UnreadByte

UnreadByte unreads the last byte. Only the most recently read byte can be unread.

func (b *Reader) UnreadByte() error

func (*Reader) UnreadRune

UnreadRune unreads the last rune. If the most recent read operation on the buffer was not a ReadRune, UnreadRune returns an error. (In this regard it is stricter than UnreadByte, which will unread the last byte from any read operation.)

func (b *Reader) UnreadRune() error

func (*Reader) WriteTo

WriteTo implements io.WriterTo.

func (b *Reader) WriteTo(w io.Writer) (n int64, err error)

type Scanner

Scanner provides a convenient interface for reading data such as a file of newline-delimited lines of text. Successive calls to the Scan method will step through the 'tokens' of a file, skipping the bytes between the tokens. The specification of a token is defined by a split function of type SplitFunc; the default split function breaks the input into lines with line termination stripped. Split functions are defined in this package for scanning a file into lines, bytes, UTF-8-encoded runes, and space-delimited words. The client may instead provide a custom split function.

Scanning stops unrecoverably at EOF, the first I/O error, or a token too large to fit in the buffer. When a scan stops, the reader may have advanced arbitrarily far past the last token. Programs that need more control over error handling or large tokens, or must run sequential scans on a reader, should use bufio.Reader instead.

type Scanner struct {
        // contains filtered or unexported fields
}

Example (Custom)

Use a Scanner with a custom split function (built by wrapping ScanWords) to validate 32-bit decimal input.

Code:

// An artificial input source.
    const input = "1234 5678 1234567901234567890"
    scanner := bufio.NewScanner(strings.NewReader(input))
    // Create a custom split function by wrapping the existing ScanWords function.
    split := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
            advance, token, err = bufio.ScanWords(data, atEOF)
            if err == nil && token != nil {
                    _, err = strconv.ParseInt(string(token), 10, 32)
            }
            return
    }
    // Set the split function for the scanning operation.
    scanner.Split(split)
    // Validate the input
    for scanner.Scan() {
            fmt.Printf("%s\n", scanner.Text())
    }

    if err := scanner.Err(); err != nil {
            fmt.Printf("Invalid input: %s", err)
    }
    

Output:

1234
5678
Invalid input: strconv.ParseInt: parsing "1234567901234567890": value out of range

Example (EmptyFinalToken)

Use a Scanner with a custom split function to parse a comma-separated list with an empty final value.

"1" "2" "3" "4" ""

Example (Lines)

The simplest use of a Scanner, to read standard input as a set of lines.

Example (Words)

Use a Scanner to implement a simple word-count utility by scanning the input as a sequence of space-delimited tokens.

15

func NewScanner

NewScanner returns a new Scanner to read from r. The split function defaults to ScanLines.

func NewScanner(r io.Reader) *Scanner

func (*Scanner) Buffer

Buffer sets the initial buffer to use when scanning and the maximum size of buffer that may be allocated during scanning. The maximum token size is the larger of max and cap(buf). If max <= cap(buf), Scan will use this buffer only and do no allocation.

By default, Scan uses an internal buffer and sets the maximum token size to MaxScanTokenSize.

Buffer panics if it is called after scanning has started.

func (s *Scanner) Buffer(buf []byte, max int)

func (*Scanner) Bytes

Bytes returns the most recent token generated by a call to Scan. The underlying array may point to data that will be overwritten by a subsequent call to Scan. It does no allocation.

func (s *Scanner) Bytes() []byte

func (*Scanner) Err

Err returns the first non-EOF error that was encountered by the Scanner.

func (s *Scanner) Err() error

func (*Scanner) Scan

Scan advances the Scanner to the next token, which will then be available through the Bytes or Text method. It returns false when the scan stops, either by reaching the end of the input or an error. After Scan returns false, the Err method will return any error that occurred during scanning, except that if it was io.EOF, Err will return nil. Scan panics if the split function returns 100 empty tokens without advancing the input. This is a common error mode for scanners.

func (s *Scanner) Scan() bool

func (*Scanner) Split

Split sets the split function for the Scanner. The default split function is ScanLines.

Split panics if it is called after scanning has started.

func (s *Scanner) Split(split SplitFunc)

func (*Scanner) Text

Text returns the most recent token generated by a call to Scan as a newly allocated string holding its bytes.

func (s *Scanner) Text() string

type SplitFunc

SplitFunc is the signature of the split function used to tokenize the input. The arguments are an initial substring of the remaining unprocessed data and a flag, atEOF, that reports whether the Reader has no more data to give. The return values are the number of bytes to advance the input and the next token to return to the user, plus an error, if any. If the data does not yet hold a complete token, for instance if it has no newline while scanning lines, SplitFunc can return (0, nil, nil) to signal the Scanner to read more data into the slice and try again with a longer slice starting at the same point in the input.

If the returned error is non-nil, scanning stops and the error is returned to the client.

The function is never called with an empty data slice unless atEOF is true. If atEOF is true, however, data may be non-empty and, as always, holds unprocessed text.

type SplitFunc func(data []byte, atEOF bool) (advance int, token []byte, err error)

type Writer

Writer implements buffering for an io.Writer object. If an error occurs writing to a Writer, no more data will be accepted and all subsequent writes will return the error. After all data has been written, the client should call the Flush method to guarantee all data has been forwarded to the underlying io.Writer.

type Writer struct {
        // contains filtered or unexported fields
}

Example

Hello, world!

func NewWriter

NewWriter returns a new Writer whose buffer has the default size.

func NewWriter(w io.Writer) *Writer

func NewWriterSize

NewWriterSize returns a new Writer whose buffer has at least the specified size. If the argument io.Writer is already a Writer with large enough size, it returns the underlying Writer.

func NewWriterSize(w io.Writer, size int) *Writer

func (*Writer) Available

Available returns how many bytes are unused in the buffer.

func (b *Writer) Available() int

func (*Writer) Buffered

Buffered returns the number of bytes that have been written into the current buffer.

func (b *Writer) Buffered() int

func (*Writer) Flush

Flush writes any buffered data to the underlying io.Writer.

func (b *Writer) Flush() error

func (*Writer) ReadFrom

ReadFrom implements io.ReaderFrom.

func (b *Writer) ReadFrom(r io.Reader) (n int64, err error)

func (*Writer) Reset

Reset discards any unflushed buffered data, clears any error, and resets b to write its output to w.

func (b *Writer) Reset(w io.Writer)

func (*Writer) Write

Write writes the contents of p into the buffer. It returns the number of bytes written. If nn < len(p), it also returns an error explaining why the write is short.

func (b *Writer) Write(p []byte) (nn int, err error)

func (*Writer) WriteByte

WriteByte writes a single byte.

func (b *Writer) WriteByte(c byte) error

func (*Writer) WriteRune

WriteRune writes a single Unicode code point, returning the number of bytes written and any error.

func (b *Writer) WriteRune(r rune) (size int, err error)

func (*Writer) WriteString

WriteString writes a string. It returns the number of bytes written. If the count is less than len(s), it also returns an error explaining why the write is short.

func (b *Writer) WriteString(s string) (int, error)