open

The open static method can be used to open a file in read-only mode.

A File owns a resource, the file descriptor and takes care of closing the
file when it is droped.

  1. use std::error::Error;
  2. use std::fs::File;
  3. use std::io::prelude::*;
  4. use std::path::Path;
  6. fn main() {
  7.     // Create a path to the desired file
  8.     let path = Path::new("hello.txt");
  9.     let display = path.display();
  11.     // Open the path in read-only mode, returns `io::Result<File>`
  12.     let mut file = match File::open(&path) {
  13.         // The `description` method of `io::Error` returns a string that
  14.         // describes the error
  15.         Err(why) => panic!("couldn't open {}: {}", display,
  16.                                                    why.description()),
  17.         Ok(file) => file,
  18.     };
  20.     // Read the file contents into a string, returns `io::Result<usize>`
  21.     let mut s = String::new();
  22.     match file.read_to_string(&mut s) {
  23.         Err(why) => panic!("couldn't read {}: {}", display,
  24.                                                    why.description()),
  25.         Ok(_) => print!("{} contains:\n{}", display, s),
  26.     }
  28.     // `file` goes out of scope, and the "hello.txt" file gets closed
  29. }

Here’s the expected successful output:

  1. $ echo "Hello World!" > hello.txt
  2. $ rustc open.rs && ./open
  3. hello.txt contains:
  4. Hello World!

(You are encouraged to test the previous example under different failure
conditions: hello.txt doesn’t exist, or hello.txt is not readable,
etc.)