Involved Source Files 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.scan.go
Code Examples
package main
import (
"bufio"
"fmt"
"os"
"strings"
)
func main() {
scanner := bufio.NewScanner(strings.NewReader("gopher"))
for scanner.Scan() {
fmt.Println(len(scanner.Bytes()) == 6)
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "shouldn't see an error scanning a string")
}
}
package main
import (
"bufio"
"fmt"
"strconv"
"strings"
)
func main() {
// 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)
}
}
package main
import (
"bufio"
"bytes"
"fmt"
"os"
"strings"
)
func main() {
onComma := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
i := bytes.IndexByte(data, ',')
if i == -1 {
if !atEOF {
return 0, nil, nil
}
// If we have reached the end, return the last token.
return 0, data, bufio.ErrFinalToken
}
// If the token is "STOP", stop the scanning and ignore the rest.
if string(data[:i]) == "STOP" {
return i + 1, nil, bufio.ErrFinalToken
}
// Otherwise, return the token before the comma.
return i + 1, data[:i], nil
}
const input = "1,2,STOP,4,"
scanner := bufio.NewScanner(strings.NewReader(input))
scanner.Split(onComma)
for scanner.Scan() {
fmt.Printf("Got a token %q\n", scanner.Text())
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "reading input:", err)
}
}
package main
import (
"bufio"
"fmt"
"os"
"strings"
)
func main() {
// Comma-separated list; last entry is empty.
const input = "1,2,3,4,"
scanner := bufio.NewScanner(strings.NewReader(input))
// Define a split function that separates on commas.
onComma := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
for i := 0; i < len(data); i++ {
if data[i] == ',' {
return i + 1, data[:i], nil
}
}
if !atEOF {
return 0, nil, nil
}
// There is one final token to be delivered, which may be the empty string.
// Returning bufio.ErrFinalToken here tells Scan there are no more tokens after this
// but does not trigger an error to be returned from Scan itself.
return 0, data, bufio.ErrFinalToken
}
scanner.Split(onComma)
// Scan.
for scanner.Scan() {
fmt.Printf("%q ", scanner.Text())
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "reading input:", err)
}
}
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
scanner := bufio.NewScanner(os.Stdin)
for scanner.Scan() {
fmt.Println(scanner.Text()) // Println will add back the final '\n'
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "reading standard input:", err)
}
}
package main
import (
"bufio"
"fmt"
"os"
"strings"
)
func main() {
// An artificial input source.
const input = "Now is the winter of our discontent,\nMade glorious summer by this sun of York.\n"
scanner := bufio.NewScanner(strings.NewReader(input))
// Set the split function for the scanning operation.
scanner.Split(bufio.ScanWords)
// Count the words.
count := 0
for scanner.Scan() {
count++
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "reading input:", err)
}
fmt.Printf("%d\n", count)
}
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
w := bufio.NewWriter(os.Stdout)
fmt.Fprint(w, "Hello, ")
fmt.Fprint(w, "world!")
w.Flush() // Don't forget to flush!
}
package main
import (
"bufio"
"os"
"strconv"
)
func main() {
w := bufio.NewWriter(os.Stdout)
for _, i := range []int64{1, 2, 3, 4} {
b := w.AvailableBuffer()
b = strconv.AppendInt(b, i, 10)
b = append(b, ' ')
w.Write(b)
}
w.Flush()
}
Package-Level Type Names (total 5)
/* sort by: | */
Reader implements buffering for an io.Reader object. Buffered returns the number of bytes that can be read from the current buffer. 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. 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 [Reader.UnreadByte] or [Reader.UnreadRune] call from succeeding
until the next read operation. 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).
If the underlying [Reader] can return a non-zero count with io.EOF,
then this Read method can do so as well; see the [io.Reader] docs. ReadByte reads and returns a single byte.
If no byte is available, returns an 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. ReadLine is a low-level line-reading primitive. Most callers should use
[Reader.ReadBytes]('\n') or [Reader.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 [Reader.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. 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. 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
[Reader.ReadBytes] or ReadString instead.
ReadSlice returns err != nil if and only if line does not end in delim. 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. Reset discards any buffered data, resets all state, and switches
the buffered reader to read from r.
Calling Reset on the zero value of [Reader] initializes the internal buffer
to the default size.
Calling b.Reset(b) (that is, resetting a [Reader] to itself) does nothing. Size returns the size of the underlying buffer in bytes. 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, [Reader.Peek], [Reader.Discard], and [Reader.WriteTo] are not
considered read operations. UnreadRune unreads the last rune. If the most recent method called on
the [Reader] was not a [Reader.ReadRune], [Reader.UnreadRune] returns an error. (In this
regard it is stricter than [Reader.UnreadByte], which will unread the last byte
from any read operation.) WriteTo implements io.WriterTo.
This may make multiple calls to the [Reader.Read] method of the underlying [Reader].
If the underlying reader supports the [Reader.WriteTo] method,
this calls the underlying [Reader.WriteTo] without buffering.
*Reader : compress/flate.Reader
*Reader : image/jpeg.Reader
*Reader : io.ByteReader
*Reader : io.ByteScanner
*Reader : io.Reader
*Reader : io.RuneReader
*Reader : io.RuneScanner
*Reader : io.WriterTo
func NewReader(rd io.Reader) *Reader
func NewReaderSize(rd io.Reader, size int) *Reader
func net/http/httputil.(*ClientConn).Hijack() (c net.Conn, r *Reader)
func net/http/httputil.(*ServerConn).Hijack() (net.Conn, *Reader)
func NewReadWriter(r *Reader, w *Writer) *ReadWriter
func go/internal/gcimporter.FindExportData(r *Reader) (hdr string, size int, err error)
func net/http.ReadRequest(b *Reader) (*http.Request, error)
func net/http.ReadResponse(r *Reader, req *http.Request) (*http.Response, error)
func net/http/httputil.NewClientConn(c net.Conn, r *Reader) *httputil.ClientConn
func net/http/httputil.NewProxyClientConn(c net.Conn, r *Reader) *httputil.ClientConn
func net/http/httputil.NewServerConn(c net.Conn, r *Reader) *httputil.ServerConn
func net/textproto.NewReader(r *Reader) *textproto.Reader
ReadWriter stores pointers to a [Reader] and a [Writer].
It implements [io.ReadWriter].Reader*ReaderWriter*Writer Available returns how many bytes are unused in the buffer. AvailableBuffer returns an empty buffer with b.Available() capacity.
This buffer is intended to be appended to and
passed to an immediately succeeding [Writer.Write] call.
The buffer is only valid until the next write operation on b. 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. Flush writes any buffered data to the underlying [io.Writer]. 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 [Reader.UnreadByte] or [Reader.UnreadRune] call from succeeding
until the next read operation. 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).
If the underlying [Reader] can return a non-zero count with io.EOF,
then this Read method can do so as well; see the [io.Reader] docs. ReadByte reads and returns a single byte.
If no byte is available, returns an 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. ReadFrom implements [io.ReaderFrom]. If the underlying writer
supports the ReadFrom method, this calls the underlying ReadFrom.
If there is buffered data and an underlying ReadFrom, this fills
the buffer and writes it before calling ReadFrom. ReadLine is a low-level line-reading primitive. Most callers should use
[Reader.ReadBytes]('\n') or [Reader.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 [Reader.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. 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. 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
[Reader.ReadBytes] or ReadString instead.
ReadSlice returns err != nil if and only if line does not end in delim. 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. 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, [Reader.Peek], [Reader.Discard], and [Reader.WriteTo] are not
considered read operations. UnreadRune unreads the last rune. If the most recent method called on
the [Reader] was not a [Reader.ReadRune], [Reader.UnreadRune] returns an error. (In this
regard it is stricter than [Reader.UnreadByte], which will unread the last byte
from any read operation.) 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. WriteByte writes a single byte. WriteRune writes a single Unicode code point, returning
the number of bytes written and any 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. WriteTo implements io.WriterTo.
This may make multiple calls to the [Reader.Read] method of the underlying [Reader].
If the underlying reader supports the [Reader.WriteTo] method,
this calls the underlying [Reader.WriteTo] without buffering.
ReadWriter : compress/flate.Reader
ReadWriter : image/jpeg.Reader
ReadWriter : internal/bisect.Writer
ReadWriter : io.ByteReader
ReadWriter : io.ByteScanner
ReadWriter : io.ByteWriter
ReadWriter : io.Reader
ReadWriter : io.ReaderFrom
ReadWriter : io.ReadWriter
ReadWriter : io.RuneReader
ReadWriter : io.RuneScanner
ReadWriter : io.StringWriter
ReadWriter : io.Writer
ReadWriter : io.WriterTo
func NewReadWriter(r *Reader, w *Writer) *ReadWriter
func net/http.Hijacker.Hijack() (net.Conn, *ReadWriter, error)
func net/http.(*ResponseController).Hijack() (net.Conn, *ReadWriter, error)
Scanner provides a convenient interface for reading data such as
a file of newline-delimited lines of text. Successive calls to
the [Scanner.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. [Scanner.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 [Scanner.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. 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 must be less than the larger of max and cap(buf).
If max <= cap(buf), [Scanner.Scan] will use this buffer only and do no allocation.
By default, [Scanner.Scan] uses an internal buffer and sets the
maximum token size to [MaxScanTokenSize].
Buffer panics if it is called after scanning has started. Bytes returns the most recent token generated by a call to [Scanner.Scan].
The underlying array may point to data that will be overwritten
by a subsequent call to Scan. It does no allocation. Err returns the first non-EOF error that was encountered by the [Scanner]. Scan advances the [Scanner] to the next token, which will then be
available through the [Scanner.Bytes] or [Scanner.Text] method. It returns false when
there are no more tokens, either by reaching the end of the input or an error.
After Scan returns false, the [Scanner.Err] method will return any error that
occurred during scanning, except that if it was [io.EOF], [Scanner.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. Split sets the split function for the [Scanner].
The default split function is [ScanLines].
Split panics if it is called after scanning has started. Text returns the most recent token generated by a call to [Scanner.Scan]
as a newly allocated string holding its bytes.
func NewScanner(r io.Reader) *Scanner
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. If that error is [ErrFinalToken], scanning
stops with no error. A non-nil token delivered with [ErrFinalToken]
will be the last token, and a nil token with [ErrFinalToken]
immediately stops the scanning.
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.
func (*Scanner).Split(split SplitFunc)
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 [Writer.Flush], will return the error.
After all data has been written, the client should call the
[Writer.Flush] method to guarantee all data has been forwarded to
the underlying [io.Writer]. Available returns how many bytes are unused in the buffer. AvailableBuffer returns an empty buffer with b.Available() capacity.
This buffer is intended to be appended to and
passed to an immediately succeeding [Writer.Write] call.
The buffer is only valid until the next write operation on b. Buffered returns the number of bytes that have been written into the current buffer. Flush writes any buffered data to the underlying [io.Writer]. ReadFrom implements [io.ReaderFrom]. If the underlying writer
supports the ReadFrom method, this calls the underlying ReadFrom.
If there is buffered data and an underlying ReadFrom, this fills
the buffer and writes it before calling ReadFrom. Reset discards any unflushed buffered data, clears any error, and
resets b to write its output to w.
Calling Reset on the zero value of [Writer] initializes the internal buffer
to the default size.
Calling w.Reset(w) (that is, resetting a [Writer] to itself) does nothing. Size returns the size of the underlying buffer in bytes. 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. WriteByte writes a single byte. WriteRune writes a single Unicode code point, returning
the number of bytes written and any 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.
*Writer : internal/bisect.Writer
*Writer : io.ByteWriter
*Writer : io.ReaderFrom
*Writer : io.StringWriter
*Writer : io.Writer
func NewWriter(w io.Writer) *Writer
func NewWriterSize(w io.Writer, size int) *Writer
func NewReadWriter(r *Reader, w *Writer) *ReadWriter
func net/textproto.NewWriter(w *Writer) *textproto.Writer
Package-Level Functions (total 10)
NewReader returns a new [Reader] whose buffer has the default size.
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].
NewReadWriter allocates a new [ReadWriter] that dispatches to r and w.
NewScanner returns a new [Scanner] to read from r.
The split function defaults to [ScanLines].
NewWriter returns a new [Writer] whose buffer has the default size.
If the argument io.Writer is already a [Writer] with large enough buffer size,
it returns the underlying [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].
ScanBytes is a split function for a [Scanner] that returns each byte as a token.
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.
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.
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.
ErrFinalToken is a special sentinel error value. It is intended to be
returned by a Split function to indicate that the scanning should stop
with no error. If the token being delivered with this error is not nil,
the token is the last token.
The value is useful to stop processing early or when it is necessary to
deliver a final empty token (which is different from a nil 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.
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.
The pages are generated with Goldsv0.6.9-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
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