Documentation

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

    Examples

    Constants

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

    Variables

    View Source
    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")
    )
    View Source
    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")
    	ErrBadReadCount    = errors.New("bufio.Scanner: Read returned impossible count")
    )

      Errors returned by Scanner.

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

        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.

        Functions

        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.

                Types

                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) Discard

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

                              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 (*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.

                                Calling Peek prevents a UnreadByte or UnreadRune call from succeeding until the next read operation.

                                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. The bytes are taken from at most one Read on the underlying Reader, hence n may be less than len(p). To read exactly len(p) bytes, use io.ReadFull(b, p). At EOF, the count will be zero and err will be io.EOF.

                                  func (*Reader) ReadByte

                                  func (b *Reader) ReadByte() (byte, 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) ([]byte, 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. 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 (*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) (string, 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) Size

                                                func (b *Reader) Size() int

                                                  Size returns the size of the underlying buffer in bytes.

                                                  func (*Reader) UnreadByte

                                                  func (b *Reader) UnreadByte() error

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

                                                    UnreadByte returns an error if the most recent method called on the Reader was not a read operation. Notably, Peek is not considered a read operation.

                                                    func (*Reader) UnreadRune

                                                    func (b *Reader) UnreadRune() error

                                                      UnreadRune unreads the last rune. If the most recent method called on the Reader 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. This may make multiple calls to the Read method of the underlying Reader. If the underlying reader supports the WriteTo method, this calls the underlying WriteTo without buffering.

                                                        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.

                                                            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.

                                                              Output:
                                                              
                                                              "1" "2" "3" "4" ""
                                                              
                                                              Example (Lines)

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

                                                                Output:
                                                                
                                                                
                                                                Example (Words)

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

                                                                  Output:
                                                                  
                                                                  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) Buffer

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

                                                                      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 (*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.

                                                                        Example

                                                                          Return the most recent call to Scan as a []byte.

                                                                          Output:
                                                                          
                                                                          true
                                                                          

                                                                          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. Scan panics if the split function returns too many empty tokens without advancing the input. This is a common error mode for scanners.

                                                                              func (*Scanner) Split

                                                                              func (s *Scanner) Split(split SplitFunc)

                                                                                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 (*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, if any, plus an error, if any.

                                                                                    Scanning stops if the function returns an error, in which case some of the input may be discarded.

                                                                                    Otherwise, the Scanner advances the input. If the token is not nil, the Scanner returns it to the user. If the token is nil, the Scanner reads more data and continues scanning; if there is no more data--if atEOF was true--the Scanner returns. If the data does not yet hold a complete token, for instance if it has no newline while scanning lines, a 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.

                                                                                    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, and Flush, 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
                                                                                      Output:
                                                                                      
                                                                                      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. If the underlying writer supports the ReadFrom method, and b has no buffered data yet, this calls the underlying ReadFrom without buffering.

                                                                                                  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) Size

                                                                                                    func (b *Writer) Size() int

                                                                                                      Size returns the size of the underlying buffer in bytes.

                                                                                                      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.

                                                                                                              Source Files