一文详解golang通过io包进行文件读写

// Go 1.19/src/io/io.go

type Reader interface {
	Read(p []byte) (n int, err error)
}

type Writer interface {
	Write(p []byte) (n int, err error)
}

// 读操作
func ReadAll(r Reader) ([]byte, error) {...}

// 写操作
func Copy(dst Writer, src Reader) (written int64, err error) {...}


func WriteString(w Writer, s string) (n int, err error) {...}

func ReadAll(r Reader) ([]byte, error) {
        // 新建字节数组，cap=512
	b := make([]byte, 0, 512)
        // 开启循环读取内容
	for {
                // 
		if len(b) == cap(b) {
			// Add more capacity (let append pick how much).
			b = append(b, 0)[:len(b)]
		}
                // 读取 reader中的内容，填充到对应部分
		n, err := r.Read(b[len(b):cap(b)])
		b = b[:len(b)+n]
                // 如果读到了结尾，可以返回数据
		if err != nil {
			if err == EOF {
				err = nil
			}
			return b, err
		}
	}
}

// io.Copy()
func Copy(dst Writer, src Reader) (written int64, err error) {
	return copyBuffer(dst, src, nil) // 实际通过调用此函数实现
}

func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
	if buf != nil && len(buf) == 0 { // 当buf非空或者长度0，panic
		panic("empty buffer in CopyBuffer")
	}
	return copyBuffer(dst, src, buf) // 继续往下调用
}


func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
	// If the reader has a WriteTo method, use it to do the copy.
	// Avoids an allocation and a copy.
	if wt, ok := src.(WriterTo); ok { // 源实现了writeTo就可以直接调用结束
		return wt.WriteTo(dst)
	}
	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
	if rt, ok := dst.(ReaderFrom); ok { // 目标实现了readFrom，也可以直接调用结束
		return rt.ReadFrom(src)
	}
	if buf == nil {
		size := 32 * 1024 
		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
			if l.N < 1 {
				size = 1
			} else {
				size = int(l.N)
			}
		}
		buf = make([]byte, size) // buf空时，创建buf
	}
        // 开启循环内容读取
	for {
		nr, er := src.Read(buf) // 从源中读取内容到buf中
		if nr > 0 {
			nw, ew := dst.Write(buf[0:nr]) // 目标w从buf中写入内容
			if nw < 0 || nr < nw {
				nw = 0
				if ew == nil {
					ew = errInvalidWrite
				}
			}
			written += int64(nw) // 更新写入长度
			if ew != nil { // 写入返回err，退出循环
				err = ew
				break
			}
			if nr != nw { // 读取与写入长度不等，退出
				err = ErrShortWrite
				break
			}
		}
		if er != nil { // 读err非空，退出
			if er != EOF {
				err = er
			}
			break
		}
	}
	return written, err // 返回写入长度及err
}

// io.WriteString()
func WriteString(w Writer, s string) (n int, err error) {
	if sw, ok := w.(StringWriter); ok { // 如w实现了 StringWriter，则调用相关方法实现写入
		return sw.WriteString(s)
	}
	return w.Write([]byte(s)) // 作为兜底，w对象是肯定实现了 Write() 方法，所以调用此方法实现写入
}

func ioRW()  {
	// read
	f, err := os.Open("D:\\demo1\\src\\demo23\\go-io\\file\\file.txt")
	defer f.Close()
	if err != nil {
		fmt.Printf("err: %s\n", err)
		return
	}
	b, err := io.ReadAll(f)
	if err != nil {
		fmt.Printf("err: %s\n", err)
		return
	}
	fmt.Printf("ioRW read content: \n%s\n", b)

	// write
	// 1.io.Copy()
	fw, err := os.Create("io_w.txt")
	defer fw.Close()
	n, err := io.Copy(fw, strings.NewReader(string(b)))
	if err != nil {
		fmt.Printf("err: %s\n", err)
		return
	}
	// 2.io.WriteString()
	fw_1, _ := os.Create("io_w_1.txt")
	defer fw_1.Close()
	n, err := io.WriteString(fw_1, string(b))
	if err != nil {
		fmt.Printf("err: %s\n", err)
		return
	}

	fmt.Printf("ioRW write size: %d\n", n)
}

// Deprecated: As of Go 1.16, this function simply calls io.ReadAll.
func ReadAll(r io.Reader) ([]byte, error) {
	return io.ReadAll(r)
}

// Deprecated: As of Go 1.16, this function simply calls os.ReadFile.
func ReadFile(filename string) ([]byte, error) {
	return os.ReadFile(filename)
}

// Deprecated: As of Go 1.16, this function simply calls os.WriteFile.
func WriteFile(filename string, data []byte, perm fs.FileMode) error {
	return os.WriteFile(filename, data, perm)
}

func ioutilRW()  {
	// read
	b, err := ioutil.ReadFile("D:\\demo1\\src\\demo23\\go-io\\file\\file.txt")
	if err != nil {
		fmt.Printf("err: %s\n", err)
		return
	}

	fmt.Printf("ioutilRW read content: \n%s\n", b)

	// write
	err = ioutil.WriteFile("ioutilRW_w.txt", b, 664)
	if err != nil {
		fmt.Printf("err: %s\n", err)
		return
	}
	fmt.Println("ioutilRW write err: ", err)
}