File size

How can we get the size in bytes of a file in Rust? It is possible to read the file into a vector, and get the length of the vector, but this is inefficient.

Instead, we can use the filesystem metadata, which is part of the fs module in the standard library. Then we just access the len() function.

Example

Here we see a special method, get_file_length, that accesses the metadata struct for a file. If many parts of the metadata are needed, the same metadata struct could be used.

Part 1 We call metadata with a file name argument. With expect() we specify an error message if the file is not present.

Part 2 We use the try_into() method to cast the u64 returned by len() into a usize.

Part 3 We call our special function (get_file_length). If the file is present, it returns a usize indicating the byte count of the file.

use std::fs;

pub fn get_file_length(file: &str) -> usize {
    // Part 1: get metadata for file.
    let metadata = fs::metadata(file).expect("Need metadata");
    // Part 2: get length of file from metadata, and convert to usize.
    metadata.len().try_into().unwrap()
}

fn main() {
    // Part 3: call function that gets file length.
    let result = get_file_length("Cargo.toml");
    println!("FILE LENGTH: {}", result);
}
FILE LENGTH: 81

Summary

With the fs module, we can access metadata about a file. This is stored separately on the filesystem, so we do not need to process the file in its entirety.