// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// 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.
package io import ( ) // Seek whence values. const ( SeekStart = 0 // seek relative to the origin of the file SeekCurrent = 1 // seek relative to the current offset SeekEnd = 2 // seek relative to the end ) // ErrShortWrite means that a write accepted fewer bytes than requested // but failed to return an explicit error. var ErrShortWrite = errors.New("short write") // errInvalidWrite means that a write returned an impossible count. var errInvalidWrite = errors.New("invalid write result") // ErrShortBuffer means that a read required a longer buffer than was provided. var ErrShortBuffer = errors.New("short buffer") // EOF is the error returned by Read when no more input is available. // (Read must return EOF itself, not an error wrapping EOF, // because callers will test for EOF using ==.) // Functions should return EOF only to signal a graceful end of input. // If the EOF occurs unexpectedly in a structured data stream, // the appropriate error is either ErrUnexpectedEOF or some other error // giving more detail. var EOF = errors.New("EOF") // ErrUnexpectedEOF means that EOF was encountered in the // middle of reading a fixed-size block or data structure. var ErrUnexpectedEOF = errors.New("unexpected EOF") // ErrNoProgress is returned by some clients of an Reader when // many calls to Read have failed to return any data or error, // usually the sign of a broken Reader implementation. var ErrNoProgress = errors.New("multiple Read calls return no data or error") // 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. type Reader interface { Read(p []byte) (n int, err error) } // 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. type Writer interface { Write(p []byte) (n int, err error) } // 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. type Closer interface { Close() error } // 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. type Seeker interface { Seek(offset int64, whence int) (int64, error) } // ReadWriter is the interface that groups the basic Read and Write methods. type ReadWriter interface { Reader Writer } // ReadCloser is the interface that groups the basic Read and Close methods. type ReadCloser interface { Reader Closer } // WriteCloser is the interface that groups the basic Write and Close methods. type WriteCloser interface { Writer Closer } // ReadWriteCloser is the interface that groups the basic Read, Write and Close methods. type ReadWriteCloser interface { Reader Writer Closer } // ReadSeeker is the interface that groups the basic Read and Seek methods. type ReadSeeker interface { Reader Seeker } // ReadSeekCloser is the interface that groups the basic Read, Seek and Close // methods. type ReadSeekCloser interface { Reader Seeker Closer } // WriteSeeker is the interface that groups the basic Write and Seek methods. type WriteSeeker interface { Writer Seeker } // ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods. type ReadWriteSeeker interface { Reader Writer Seeker } // 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 EOF encountered during the read is also returned. // // The Copy function uses ReaderFrom if available. type ReaderFrom interface { ReadFrom(r Reader) (n int64, err error) } // 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. type WriterTo interface { WriteTo(w Writer) (n int64, err error) } // 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. type ReaderAt interface { ReadAt(p []byte, off int64) (n int, err error) } // 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. type WriterAt interface { WriteAt(p []byte, off int64) (n int, err error) } // 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. type ByteReader interface { ReadByte() (byte, error) } // 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. type ByteScanner interface { ByteReader UnreadByte() error } // ByteWriter is the interface that wraps the WriteByte method. type ByteWriter interface { WriteByte(c byte) error } // RuneReader is the interface that wraps the ReadRune method. // // ReadRune reads a single UTF-8 encoded Unicode character // and returns the rune and its size in bytes. If no character is // available, err will be set. type RuneReader interface { ReadRune() (r rune, size int, err error) } // RuneScanner is the interface that adds the UnreadRune method to the // basic ReadRune method. // // UnreadRune causes the next call to ReadRune to return the same rune // as the previous call to ReadRune. // It may be an error to call UnreadRune twice without an intervening // call to ReadRune. type RuneScanner interface { RuneReader UnreadRune() error } // StringWriter is the interface that wraps the WriteString method. type StringWriter interface { WriteString(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. func ( Writer, string) ( int, error) { if , := .(StringWriter); { return .WriteString() } return .Write([]byte()) } // 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. func ( Reader, []byte, int) ( int, error) { if len() < { return 0, ErrShortBuffer } for < && == nil { var int , = .Read([:]) += } if >= { = nil } else if > 0 && == EOF { = ErrUnexpectedEOF } return } // 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. func ( Reader, []byte) ( int, error) { return ReadAtLeast(, , len()) } // 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. func ( Writer, Reader, int64) ( int64, error) { , = Copy(, LimitReader(, )) if == { return , nil } if < && == nil { // src stopped early; must have been EOF. = EOF } return } // 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). func ( Writer, Reader) ( int64, error) { return copyBuffer(, , nil) } // 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. func ( Writer, Reader, []byte) ( int64, error) { if != nil && len() == 0 { panic("empty buffer in CopyBuffer") } return copyBuffer(, , ) } // copyBuffer is the actual implementation of Copy and CopyBuffer. // if buf is nil, one is allocated. func copyBuffer( Writer, Reader, []byte) ( int64, error) { // If the reader has a WriteTo method, use it to do the copy. // Avoids an allocation and a copy. if , := .(WriterTo); { return .WriteTo() } // Similarly, if the writer has a ReadFrom method, use it to do the copy. if , := .(ReaderFrom); { return .ReadFrom() } if == nil { := 32 * 1024 if , := .(*LimitedReader); && int64() > .N { if .N < 1 { = 1 } else { = int(.N) } } = make([]byte, ) } for { , := .Read() if > 0 { , := .Write([0:]) if < 0 || < { = 0 if == nil { = errInvalidWrite } } += int64() if != nil { = break } if != { = ErrShortWrite break } } if != nil { if != EOF { = } break } } return , } // LimitReader returns a Reader that reads from r // but stops with EOF after n bytes. // The underlying implementation is a *LimitedReader. func ( Reader, int64) Reader { return &LimitedReader{, } } // A LimitedReader reads from R but limits the amount of // data returned to just N bytes. Each call to Read // updates N to reflect the new amount remaining. // Read returns EOF when N <= 0 or when the underlying R returns EOF. type LimitedReader struct { R Reader // underlying reader N int64 // max bytes remaining } func ( *LimitedReader) ( []byte) ( int, error) { if .N <= 0 { return 0, EOF } if int64(len()) > .N { = [0:.N] } , = .R.Read() .N -= int64() return } // NewSectionReader returns a SectionReader that reads from r // starting at offset off and stops with EOF after n bytes. func ( ReaderAt, int64, int64) *SectionReader { return &SectionReader{, , , + } } // SectionReader implements Read, Seek, and ReadAt on a section // of an underlying ReaderAt. type SectionReader struct { r ReaderAt base int64 off int64 limit int64 } func ( *SectionReader) ( []byte) ( int, error) { if .off >= .limit { return 0, EOF } if := .limit - .off; int64(len()) > { = [0:] } , = .r.ReadAt(, .off) .off += int64() return } var errWhence = errors.New("Seek: invalid whence") var errOffset = errors.New("Seek: invalid offset") func ( *SectionReader) ( int64, int) (int64, error) { switch { default: return 0, errWhence case SeekStart: += .base case SeekCurrent: += .off case SeekEnd: += .limit } if < .base { return 0, errOffset } .off = return - .base, nil } func ( *SectionReader) ( []byte, int64) ( int, error) { if < 0 || >= .limit-.base { return 0, EOF } += .base if := .limit - ; int64(len()) > { = [0:] , = .r.ReadAt(, ) if == nil { = EOF } return , } return .r.ReadAt(, ) } // Size returns the size of the section in bytes. func ( *SectionReader) () int64 { return .limit - .base } // TeeReader returns a Reader that writes to w what it reads from r. // All reads from r performed through it are matched with // corresponding writes to w. There is no internal buffering - // the write must complete before the read completes. // Any error encountered while writing is reported as a read error. func ( Reader, Writer) Reader { return &teeReader{, } } type teeReader struct { r Reader w Writer } func ( *teeReader) ( []byte) ( int, error) { , = .r.Read() if > 0 { if , := .w.Write([:]); != nil { return , } } return } // Discard is an Writer on which all Write calls succeed // without doing anything. var Discard Writer = discard{} type discard struct{} // discard implements ReaderFrom as an optimization so Copy to // io.Discard can avoid doing unnecessary work. var _ ReaderFrom = discard{} func (discard) ( []byte) (int, error) { return len(), nil } func (discard) ( string) (int, error) { return len(), nil } var blackHolePool = sync.Pool{ New: func() interface{} { := make([]byte, 8192) return & }, } func (discard) ( Reader) ( int64, error) { := blackHolePool.Get().(*[]byte) := 0 for { , = .Read(*) += int64() if != nil { blackHolePool.Put() if == EOF { return , nil } return } } } // NopCloser returns a ReadCloser with a no-op Close method wrapping // the provided Reader r. func ( Reader) ReadCloser { return nopCloser{} } type nopCloser struct { Reader } func (nopCloser) () error { return nil } // ReadAll reads from r until an error or EOF and returns the data it read. // A successful call returns err == nil, not err == EOF. Because ReadAll is // defined to read from src until EOF, it does not treat an EOF from Read // as an error to be reported. func ( Reader) ([]byte, error) { := make([]byte, 0, 512) for { if len() == cap() { // Add more capacity (let append pick how much). = append(, 0)[:len()] } , := .Read([len():cap()]) = [:len()+] if != nil { if == EOF { = nil } return , } } }