Zwlin's Blog

[笔记] Go Package io

2020/11/02

Package io

Overview

Package io provides basic interfaces to I/O primitives. Its primary job is to wrap existing implementations of such primitives, such as those in package os, into shared public interfaces that abstract the functionality, plus some other related primitives.

Because these interfaces and primitives wrap lower-level operations with various implementations, unless otherwise informed clients should not assume they are safe for parallel execution.

Main Interface

io.Reader

Reader is the interface that wraps the basic Read method.

Read reads up to len(p) bytes into p. It returns the number of bytes read (0 <= n <= len(p)) and any error encountered. Even if Read returns n < len(p), it may use all of p as scratch space during the call. If some data is available but not len(p) bytes, Read conventionally returns what is available instead of waiting for more.

When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read. It may return the (non-nil) error from the same call or return the error (and n == 0) from a subsequent call. An instance of this general case is that a Reader returning a non-zero number of bytes at the end of the input stream may return either err == EOF or err == nil. The next Read should return 0, EOF.

Callers should always process the n > 0 bytes returned before considering the error err. Doing so correctly handles I/O errors that happen after reading some bytes and also both of the allowed EOF behaviors.

Implementations of Read are discouraged from returning a zero byte count with a nil error, except when len(p) == 0. Callers should treat a return of 0 and nil as indicating that nothing happened; in particular it does not indicate EOF.

Implementations must not retain p.

io.Writer

Writer is the interface that wraps the basic Write method.

Write writes len(p) bytes from p to the underlying data stream. It returns the number of bytes written from p (0 <= n <= len(p)) and any error encountered that caused the write to stop early. Write must return a non-nil error if it returns n < len(p). Write must not modify the slice data, even temporarily.

Implementations must not retain p.

io.Closer

Closer is the interface that wraps the basic Close method.

The behavior of Close after the first call is undefined. Specific implementations may document their own behavior.

io.Seeker

Seeker is the interface that wraps the basic Seek method.

Seek sets the offset for the next Read or Write to offset, interpreted according to whence: SeekStart means relative to the start of the file, SeekCurrent means relative to the current offset, and SeekEnd means relative to the end. Seek returns the new offset relative to the start of the file and an error, if any.

Seeking to an offset before the start of the file is an error. Seeking to any positive offset is legal, but the behavior of subsequent I/O operations on the underlying object is implementation-dependent.

Relations

Relations

For io.Reader

ByteReader & ByteScanner & RuneReader & RuneScanner

ByteReader is the interface that wraps the ReadByte method.

ReadByte reads and returns the next byte from the input or any error encountered. If ReadByte returns an error, no input byte was consumed, and the returned byte value is undefined.

ReadByte provides an efficient interface for byte-at-time processing. A Reader that does not implement ByteReader can be wrapped using bufio.NewReader to add this method.

ByteScanner is the interface that adds the UnreadByte method to the basic ReadByte method.

UnreadByte causes the next call to ReadByte to return the same byte as the previous call to ReadByte. It may be an error to call UnreadByte twice without an intervening call to ReadByte.

Example
 1r1 := strings.NewReader("ABCDEEF")
 2r2 := strings.NewReader("ABCDEEF")
 3fmt.Println("Unread")
 4
 5bfr:=bufio.NewReader(r1)
 6for b,err := bfr.ReadByte();err !=io.EOF;b,err = bfr.ReadByte(){
 7    if err!=nil{
 8        fmt.Println(err)
 9        break
10    }
11    if b == 'C'{
12        bfr.UnreadByte()
13        break
14    }
15    fmt.Printf("%q ",b)
16}
17fmt.Println()
18b , _ := bfr.ReadByte()
19fmt.Printf("%q\n",b)
20
21fmt.Println("Do not Unread")
22
23bfr = bufio.NewReader(r2)
24for b,err := bfr.ReadByte();err !=io.EOF;b,err = bfr.ReadByte(){
25    if err!=nil{
26        fmt.Println(err)
27        break
28    }
29    if b == 'C'{
30        //bfr.UnreadByte()
31        break
32    }
33    fmt.Printf("%q ",b)
34}
35fmt.Println()
36b , _ = bfr.ReadByte()
37fmt.Printf("%q\n",b)
38
39//output:
40//Unread
41//'A' 'B'
42//'C'
43//Do not Unread
44//'A' 'B'
45//'D'

ReaderAt

ReaderAt is the interface that wraps the basic ReadAt method.

ReadAt reads len(p) bytes into p starting at offset off in the underlying input source. It returns the number of bytes read (0 <= n <= len(p)) and any error encountered.

When ReadAt returns n < len(p), it returns a non-nil error explaining why more bytes were not returned. In this respect, ReadAt is stricter than Read.

Even if ReadAt returns n < len(p), it may use all of p as scratch space during the call. If some data is available but not len(p) bytes, ReadAt blocks until either all the data is available or an error occurs. In this respect ReadAt is different from Read.

If the n = len(p) bytes returned by ReadAt are at the end of the input source, ReadAt may return either err == EOF or err == nil.

If ReadAt is reading from an input source with a seek offset, ReadAt should not affect nor be affected by the underlying seek offset.

Clients of ReadAt can execute parallel ReadAt calls on the same input source.

Implementations must not retain p.

WriteTo

WriterTo is the interface that wraps the WriteTo method.

WriteTo writes data to w until there’s no more data to write or when an error occurs. The return value n is the number of bytes written. Any error encountered during the write is also returned.

The Copy function uses WriterTo if available.

PipeReader

A PipeReader is the read half of a pipe.

func (*PipeReader) Close
1func (r *PipeReader) Close() error

Close closes the reader; subsequent writes to the write half of the pipe will return the error ErrClosedPipe.

func (*PipeReader) CloseWithError
1func (r *PipeReader) CloseWithError(err error) error

CloseWithError closes the reader; subsequent writes to the write half of the pipe will return the error err.

CloseWithError never overwrites the previous error if it exists and always returns nil.

func (*PipeReader) Read
1func (r *PipeReader) Read(data []byte) (n int, err error)

Read implements the standard Read interface: it reads data from the pipe, blocking until a writer arrives or the write end is closed. If the write end is closed with an error, that error is returned as err; otherwise err is EOF.

For io.Writer

ByteWriter

ByteWriter is the interface that wraps the WriteByte method.

StringWriter

StringWriter is the interface that wraps the WriteString method.

PipeWriter

A PipeWriter is the write half of a pipe.

func (*PipeWriter) Close
1func (w *PipeWriter) Close() error

Close closes the writer; subsequent reads from the read half of the pipe will return no bytes and EOF.

func (*PipeWriter) CloseWithError
1func (w *PipeWriter) CloseWithError(err error) error

CloseWithError closes the writer; subsequent reads from the read half of the pipe will return no bytes and the error err, or EOF if err is nil.

CloseWithError never overwrites the previous error if it exists and always returns nil.

func (*PipeWriter)Write
1func (w *PipeWriter) Write(data []byte) (n int, err error)

Write implements the standard Write interface: it writes data to the pipe, blocking until one or more readers have consumed all the data or the read end is closed. If the read end is closed with an error, that err is returned as err; otherwise err is ErrClosedPipe.

WriteAt

WriterAt is the interface that wraps the basic WriteAt method.

WriteAt writes len(p) bytes from p to the underlying data stream at offset off. It returns the number of bytes written from p (0 <= n <= len(p)) and any error encountered that caused the write to stop early. WriteAt must return a non-nil error if it returns n < len(p).

If WriteAt is writing to a destination with a seek offset, WriteAt should not affect nor be affected by the underlying seek offset.

Clients of WriteAt can execute parallel WriteAt calls on the same destination if the ranges do not overlap.

Implementations must not retain p.

ReadFrom

ReaderFrom is the interface that wraps the ReadFrom method.

ReadFrom reads data from r until EOF or error. The return value n is the number of bytes read. Any error except io.EOF encountered during the read is also returned.

The Copy function uses ReaderFrom if available.

Extended Interface

ReadCloser

ReadCloser is the interface that groups the basic Read and Close methods.

ReadSeeker

ReadSeeker is the interface that groups the basic Read and Seek methods.

ReadWriter

ReadWriter is the interface that groups the basic Read and Write methods.

ReadWriteCloser

ReadWriteCloser is the interface that groups the basic Read, Write and Close methods.

ReadWriteSeeker

ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods.

SectionReader

SectionReader implements Read, Seek, and ReadAt on a section of an underlying ReaderAt.

Example
 1package main
 2
 3import (
 4	"io"
 5	"log"
 6	"os"
 7	"strings"
 8)
 9
10func main() {
11	r := strings.NewReader("some io.Reader stream to be read\n")
12	s := io.NewSectionReader(r, 5, 17)
13
14	if _, err := io.Copy(os.Stdout, s); err != nil {
15		log.Fatal(err)
16	}
17
18}
19//output:
20//io.Reader stream

WriteCloser

WriteCloser is the interface that groups the basic Write and Close methods.

WriteSeeker

WriteSeeker is the interface that groups the basic Write and Seek methods.

Functions

Copy

1func Copy(dst Writer, src Reader) (written int64, err error)

Copy copies from src to dst until either EOF is reached on src or an error occurs. It returns the number of bytes copied and the first error encountered while copying, if any.

A successful Copy returns err == nil, not err == EOF. Because Copy is defined to read from src until EOF, it does not treat an EOF from Read as an error to be reported.

If src implements the WriterTo interface, the copy is implemented by calling src.WriteTo(dst). Otherwise, if dst implements the ReaderFrom interface, the copy is implemented by calling dst.ReadFrom(src).

CopyBuffer

1func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error)

CopyBuffer is identical to Copy except that it stages through the provided buffer (if one is required) rather than allocating a temporary one. If buf is nil, one is allocated; otherwise if it has zero length, CopyBuffer panics.

If either src implements WriterTo or dst implements ReaderFrom, buf will not be used to perform the copy.

CopyN

1func CopyN(dst Writer, src Reader, n int64) (written int64, err error)

CopyN copies n bytes (or until an error) from src to dst. It returns the number of bytes copied and the earliest error encountered while copying. On return, written == n if and only if err == nil.

If dst implements the ReaderFrom interface, the copy is implemented using it.

Pipe

1func Pipe() (*PipeReader, *PipeWriter)

Pipe creates a synchronous in-memory pipe. It can be used to connect code expecting an io.Reader with code expecting an io.Writer.

Reads and Writes on the pipe are matched one to one except when multiple Reads are needed to consume a single Write. That is, each Write to the PipeWriter blocks until it has satisfied one or more Reads from the PipeReader that fully consume the written data. The data is copied directly from the Write to the corresponding Read (or Reads); there is no internal buffering.

It is safe to call Read and Write in parallel with each other or with Close. Parallel calls to Read and parallel calls to Write are also safe: the individual calls will be gated sequentially.

ReadAtLeast

1func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error)

ReadAtLeast reads from r into buf until it has read at least min bytes. It returns the number of bytes copied and an error if fewer bytes were read. The error is EOF only if no bytes were read. If an EOF happens after reading fewer than min bytes, ReadAtLeast returns ErrUnexpectedEOF. If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer. On return, n >= min if and only if err == nil. If r returns an error having read at least min bytes, the error is dropped.

ReadFull

1func ReadFull(r Reader, buf []byte) (n int, err error)

ReadFull reads exactly len(buf) bytes from r into buf. It returns the number of bytes copied and an error if fewer bytes were read. The error is EOF only if no bytes were read. If an EOF happens after reading some but not all the bytes, ReadFull returns ErrUnexpectedEOF. On return, n == len(buf) if and only if err == nil. If r returns an error having read at least len(buf) bytes, the error is dropped.

WriteString

1func WriteString(w Writer, s string) (n int, err error)

WriteString writes the contents of the string s to w, which accepts a slice of bytes. If w implements StringWriter, its WriteString method is invoked directly. Otherwise, w.Write is called exactly once.

References