Update file

There are several ways to "update a file".

In one case we open the file for reading read the whole file into memory then separately open the file for writing and write the content.

Another way is to open the file for both reading and writing at the same time, read the content, rewind the file pointer, truncate the file to 0 and then write the new content.

In this example we open the file using the options by setting the 3 flags:

• read

• write

• create

examples/files/update-file/src/main.rs

use std::io::Seek;
use std::io::SeekFrom;
use std::env;
use std::process::exit;
use std::error::Error;
use std::fs::File;
use std::io::Read;
use std::io::Write;

const SIZE: usize = 10;

fn main() -> Result<(), Box<dyn Error>> {
    let args = env::args().collect::<Vec<String>>();
    if args.len() != 2 {
        eprintln!("Usage: {} TEXT", args[0]);
        exit(1);
    }

    let filename = "message.txt";

    let mut file =  File::options().read(true).write(true).create(true).open(filename)?;

    let mut content = String::new();
    let mut buffer = [0; SIZE];
    loop {
        let size = file.read(&mut buffer)?;
        println!("Read: {size}");
        if size < SIZE {
            break;
        }
        content.push_str(&String::from_utf8(buffer[0..size].to_vec())?);
    }

    println!("old: {content:?}");

    file.seek(SeekFrom::Start(0))?;
    file.set_len(0)?; // truncate

    file.write_all(args[1].as_bytes())?;

    Ok(())

}