In-depth understanding of the io.CopyBuffer function in the Go language documentation to implement buffered file copying

The standard library of Go language provides many functions related to IO operations, among which there is an io.CopyBuffer function that can realize buffered file copying. In this article, we will deeply understand the implementation principle of the io.CopyBuffer function and provide specific code examples.

1. Function introduction

The signature of the io.CopyBuffer function is as follows:

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

The function of this function is to copy the data in src to dst, and use buf as buffer. The return value of the function is the number of bytes copied and any errors that may have occurred.

For src and dst variables, they are both interface types and implement the Reader and Writer interfaces respectively. For a detailed introduction to these two interfaces, please refer to the relevant content in the Go language documentation.

2. Function implementation principle

The implementation of the io.CopyBuffer function is relatively simple, mainly using for loops and buf to control the copy process.

The specific implementation steps are as follows:

1. Check whether buf is empty. If it is empty, create a byte array of 8192 bytes as the default buffer.
2. Use a for loop to read buf bytes from src into the buffer each time, and then write the data in the buffer to dst until all the data in src is read.
3. If an error occurs during reading or writing, the error is returned immediately.
4. Return the number of bytes copied.

The following is the specific implementation code of the io.CopyBuffer function:

func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
    if buf == nil {
        buf = make([]byte, 8192)
    }
    for {
        nr, er := src.Read(buf)
        if nr > 0 {
            nw, ew := dst.Write(buf[0:nr])
            if nw > 0 {
                written += int64(nw)
            }
            if ew != nil {
                err = ew
                break
            }
            if nr != nw {
                err = ErrShortWrite
                break
            }
        }
        if er != nil {
            if er != io.EOF {
                err = er
            }
            break
        }
    }
    return written, err
}

3. Code example

The following is a simple code example that demonstrates how to use io The .CopyBuffer function copies one file into another file:

package main

import (
    "fmt"
    "io"
    "os"
)

func main() {
    srcFile, err := os.Open("test.txt")
    if err != nil {
        fmt.Println("Open source file error:", err)
        return
    }
    defer srcFile.Close()

    dstFile, err := os.Create("test-copy.txt")
    if err != nil {
        fmt.Println("Create dest file error:", err)
        return
    }
    defer dstFile.Close()

    buf := make([]byte, 1024)
    _, err = io.CopyBuffer(dstFile, srcFile, buf)
    if err != nil {
        fmt.Println("Copy file error:", err)
        return
    }

    fmt.Println("Copy file success!")
}

In the above example, we open the test.txt file and copy it into the test-copy.txt file. Use the make function to create a buffer of size 1024 bytes, and then pass the buffer in as the third parameter of the io.CopyBuffer function.

4. Summary

The io.CopyBuffer function is a very useful function that can copy data with buffering while avoiding the overhead of multiple system calls. This article details the implementation principle of the io.CopyBuffer function and provides code examples. For applications that need to copy large amounts of data, using the io.CopyBuffer function can effectively improve the performance of the program.

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