Append mode

Learn append mode with OpenOptions.

Append mode

To add to the end of an existing file, OpenOptions is used. Which operations are allowed is stated one by one:

use std::fs::OpenOptions;
use std::io::Write;

let mut f = OpenOptions::new()
    .append(true)    // add to the end
    .create(true)    // create if missing
    .open("gunluk.txt")?;
writeln!(f, "yeni kayıt")?;

It's ideal for log files: each run adds new lines without deleting the earlier ones.

Options

OpenOptions is the basis of the other opening methods:

File::open(y)    // = OpenOptions::new().read(true).open(y)
File::create(y)  // = .write(true).create(true).truncate(true)

OpenOptions::new().write(true).create_new(true).open(y)
// opens only if the file does NOT exist; AlreadyExists error otherwise

create_new is safe against race conditions where two programs try to create the same file at the same time.

Exercise Log file

Write "a\n" to gunluk.txt in the temp folder with fs::write. Then open it twice in append mode with OpenOptions and add the line "b". Read the file and print its lines joined with |.

Show example solution

use std::env;
use std::fs::{self, OpenOptions};
use std::io::{self, Write};

fn main() -> io::Result<()> {
    let yol = env::temp_dir().join("gunluk.txt");
    fs::write(&yol, "a\n")?;
    for _ in 0..2 {
        let mut f = OpenOptions::new().append(true).create(true).open(&yol)?;
        writeln!(f, "b")?;
    }
    let icerik = fs::read_to_string(&yol)?;
    let s: Vec<&str> = icerik.lines().collect();
    println!("{}", s.join("|"));
    Ok(())
}

Test yourself

What does <code>OpenOptions::new().append(true).open(..)</code> do if the file doesn't exist?
- Does nothing
- Returns a NotFound error; create(true) is needed
- Creates the file
- Panics
Show answer

Returns a NotFound error; create(true) is needed — In OpenOptions every behaviour must be asked for explicitly; the default is nothing.

Append

let mut f = OpenOptions::new()
    .____(true)
    .create(true)
    .open("gunluk.txt")?;

Show answer

let mut f = OpenOptions::new()
    .append(true)
    .create(true)
    .open("gunluk.txt")?;

Method and equivalent

File::open

read(true)
File::create

write + create + truncate
append(true)

Appends
create_new(true)

Error if it exists

Next lesson: BufReader

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