Transfert content of a file



I’m trying to transfert the content of a file in another file (created by the application)

fn openfile(file: String, name: &str) -> Result<File, io::Error> {
    let mut f = File::open(file)?;
    let mut e = File::open(name)?;
    let mut s = String::new();
    f.read_to_string(&mut s)?;
    println!("{}\n\n", s);
    match e.write_all(&mut s.as_bytes()) {
        Err(why) => {
            panic!("couldn't write to {}: {}", name, why.description())
        Ok(_) => println!("successfully wrote to {}", name),


The println print correctly the content of a file but the write_all function panic.
“other os error”.

How can I do what I want to do ?
Another question (while i’m here), did I use the question mark operator correcty ?

Thanks for the help :slight_smile:


open is for reading only. You probably want create for the file you’re writing to.


Instead of doing the read followed by the write, you could use std::io::copy :


omg i’m a dumb, I was doing it like in C.
Thanks for the information


If all you’re doing is copying a file, you should actually use std::fs::copy because it copies a lot more than just the contents, including permissions and NTFS file streams. Also it can be significantly faster (in particular on Windows).


Two points that aren’t relevant when using the proper functions, but nevertheless:

  • for arbitary files, don’t read them into a string (it requires UTF-8), instead use read_to_end into a Vec
  • instead of using why.description() format just why which should give you a more detailed error message (Error::description(), because it has to return a string slice, is kind of a last resort for when you don’t want to call code that may allocate).