How can I read from a specific raw file descriptor in Rust?

Question

Editor's note: This question is for a version of Rust prior to 1.0. Some answers have been updated to cover Rust 1.0 or up, but not all.

I am writing a systemd socket activated service in Rust. My process is being handed an open file descriptor by systemd.

Are there any Rust IO functions that take a raw file descriptor?

I'm using a Rust nightly before Rust 1.0.

Answer 1

As of Rust 1.1, you can use FromRawFd to create a File from a specific file descriptor, but only on UNIX-like operating systems:

use std::{
    fs::File,
    io::{self, Read},
    os::unix::io::FromRawFd,
};

fn main() -> io::Result<()> {
    let mut f = unsafe { File::from_raw_fd(3) };
    let mut input = String::new();
    f.read_to_string(&mut input)?;

    println!("I read: {}", input);

    Ok(())
}

$ cat /tmp/output
Hello, world!
$ target/debug/example 3< /tmp/output
I read: Hello, world!

from_raw_fd is unsafe:

This function is also unsafe as the primitives currently returned have the contract that they are the sole owner of the file descriptor they are wrapping. Usage of this function could accidentally allow violating this contract which can cause memory unsafety in code that relies on it being true.

The created File will assume ownership of the file descriptor: when the File goes out of scope, the file descriptor will be closed. You can avoid this by using either IntoRawFd or mem::forget.

See also:

• How do I write to a specific raw file descriptor from Rust?