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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) Peek(n int) ([]byte, error)
    func (b *Reader) Read(p []byte) (n int, err error)
    func (b *Reader) ReadByte() (c byte, err error)
    func (b *Reader) ReadBytes(delim byte) (line []byte, err 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) (line string, err 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) 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 (
        // Maximum size used to buffer a token. 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")
)
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")
)

Errors returned by Scanner.

func ScanBytes

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

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

func ScanLines

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

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 ScanRunes

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

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 ScanWords

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

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.

type ReadWriter

type ReadWriter struct {
        *Reader
        *Writer
}

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

func NewReadWriter

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

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

type Reader

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

Reader implements buffering for an io.Reader object.

func NewReader

func NewReader(rd io.Reader) *Reader

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

func NewReaderSize

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

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 (*Reader) Buffered

func (b *Reader) Buffered() int

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

func (*Reader) Peek

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

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 (*Reader) Read

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

Read reads data into p. It returns the number of bytes read into p. It calls Read at most once 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 (*Reader) ReadByte

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

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

func (*Reader) ReadBytes

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

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 (*Reader) ReadLine

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

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.

func (*Reader) ReadRune

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

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 (*Reader) ReadSlice

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

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 (*Reader) ReadString

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

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 (*Reader) Reset

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

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

func (*Reader) UnreadByte

func (b *Reader) UnreadByte() error

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

func (*Reader) UnreadRune

func (b *Reader) UnreadRune() error

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 (*Reader) WriteTo

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

WriteTo implements io.WriterTo.

type Scanner

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

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.

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 (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

func NewScanner(r io.Reader) *Scanner

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

func (*Scanner) Bytes

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

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 (*Scanner) Err

func (s *Scanner) Err() error

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

func (*Scanner) Scan

func (s *Scanner) Scan() bool

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.

func (*Scanner) Split

func (s *Scanner) Split(split SplitFunc)

Split sets the split function for the Scanner. If called, it must be called before Scan. The default split function is ScanLines.

func (*Scanner) Text

func (s *Scanner) Text() string

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

type SplitFunc

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

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 Writer

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

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.

Example

Hello, world!

func NewWriter

func NewWriter(w io.Writer) *Writer

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

func NewWriterSize

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

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 (*Writer) Available

func (b *Writer) Available() int

Available returns how many bytes are unused in the buffer.

func (*Writer) Buffered

func (b *Writer) Buffered() int

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

func (*Writer) Flush

func (b *Writer) Flush() error

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

func (*Writer) ReadFrom

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

ReadFrom implements io.ReaderFrom.

func (*Writer) Reset

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

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

func (*Writer) Write

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

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 (*Writer) WriteByte

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

WriteByte writes a single byte.

func (*Writer) WriteRune

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

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

func (*Writer) WriteString

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

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.