Deeply understand the io.CopyN function in the Go language documentation to copy files with a limited number of bytes

In-depth understanding of the io.CopyN function in the Go language documentation to implement file copy with a limited number of bytes

The io package in the Go language provides many functions for processing input Output stream functions and methods. One of the very useful functions is io.CopyN, which can copy files with a limited number of bytes. This article will provide an in-depth understanding of this function and provide specific code examples.

First, let us understand the basic definition of the io.CopyN function. Its definition is as follows:

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

The function of this function is to copy data from the source Reader to the target Writer, copying up to n bytes. The number of bytes copied will not exceed n. If the data in the source Reader is less than n bytes, copying will stop at the end of the source Reader.

To use the io.CopyN function to copy files, you first need to open the source file and the target file. You can use the os.Open function to open a source file and the os.Create function to create a target file. The following is a specific code example:

package main

import (
    "io"
    "log"
    "os"
)

func main() {
    srcFile, err := os.Open("source.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer srcFile.Close()

    dstFile, err := os.Create("destination.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer dstFile.Close()

    written, err := io.CopyN(dstFile, srcFile, 1024) // 限定复制1KB字节
    if err != nil {
        log.Fatal(err)
    }

    log.Printf("Copied %d bytes.", written)
}

In the above code, we first opened the source file named source.txt through the os.Open function, and created a file named destination.txt through the os.Create function. Target file. Then, we call the io.CopyN function to copy the data in the source file to the target file, up to 1024 bytes (i.e. 1KB). After the copy is complete, we print out the number of bytes copied.

It should be noted that we use the defer statement to ensure that the open file is closed before the program ends. This is a commonly used file handling pattern to avoid resource leaks.

In the above code example, we limit the maximum copy to 1KB bytes. If you want to copy more or less bytes, just modify the third parameter passed to the io.CopyN function.

In summary, the io.CopyN function can limit the number of bytes copied during the file copying process, which is very practical. Through an in-depth understanding of the io.CopyN function in the Go language documentation and demonstration using specific code examples, we hope that readers will have a clear understanding of this function and can flexibly apply it in actual development.

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