如何从文件中删除最后N个字节

Question

I want to delete thelast N bytes from file in Go, My mean resize file without create new temp file and after block copy rename temp file.

I want to do this is read-write mode look like:

    // Read Write Mode
    file, err := os.OpenFile("C:\\Test.zip", os.O_RDWR, 0644)
    if err != nil {
        return err
    }

For example in .NET look like this:

FileInfo fi = new FileInfo("filename");
FileStream fs = fi.Open(FileMode.Open);

long bytesToDelete = 5000;
fs.SetLength (Math.Max(0, fi.Length - bytesToDelete));

fs.Close();

Answer 1

Actually, this is already implemented is the os.Truncate() function. But this function takes the new size. So to use this, you have to first get the size of the file. For that, you may use os.Stat().

Wrapping it into a function:

func truncateFile(name string, bytesToRemove int64) error {
    fi, err := os.Stat(name)
    if err != nil {
        return err
    }

    return os.Truncate(name, fi.Size()-bytesToRemove)
}

Using it to remove the last 5000 bytes:

if err := truncateFile("C:\\Test.zip", 5000); err != nil {
    fmt.Println("Error:", err)
}

Another alternative is to use the File.Truncate() method for that. If we have an os.File, we may also use File.Stat() to get its size.

This is how it would look like:

func truncateFile(name string, bytesToRemove int64) error {
    f, err := os.OpenFile(name, os.O_RDWR, 0644)
    if err != nil {
        return err
    }
    defer f.Close()

    fi, err := f.Stat()
    if err != nil {
        return err
    }

    return f.Truncate(fi.Size() - bytesToRemove)
}

Using it is the same. This may be preferable if we're working on a file (we have it opened) and we have to truncate it. But in that case you'd want to pass os.File instead of its name to truncateFile().

Note: if you try to remove more bytes than the file currently has, truncateFile() will return an error.