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

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, Write},    os::unix::io::FromRawFd,};fn main() -> io::Result<()> {    let mut f = unsafe { File::from_raw_fd(3) };    write!(&mut f, "Hello, world!")?;    Ok(())}

$ target/debug/example 3> /tmp/output$ cat /tmp/outputHello, world!

from_raw_fd is unsafe because there's no guarantee that the file descriptor is valid or who is actually responsible for that file descriptor.

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

The libc crate is "just" a wrapping library to interface between C and Rust so to know how to use a function one should read the manual of the C function, there are many source for that, here one for fdopen():

The fdopen() function associates a stream with the existing file descriptor, fd. The mode of the stream (one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the file descriptor. The file position indicator of the new stream is set to that belonging to fd, and the error and end-of-file indicators are cleared. Modes "w" or "w+" do not cause truncation of the file. The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed. The result of applying fdopen() to a shared memory object is undefined.

Basic use is so:

use libc::fdopen;use std::ffi::CString;fn main() {    let mode = CString::new("w").unwrap();    unsafe {        let _ = fdopen(3, mode.as_ptr());    }}

To use it, you could use fwrite():

The function fwrite() writes nmemb elements of data, each size bytes long, to the stream pointed to by stream, obtaining them from the location given by ptr.

So, complete example:

use libc::{c_void, fdopen, fwrite};use std::ffi::CString;fn main() {    let mode = CString::new("w").unwrap();    let file = unsafe {        let file = fdopen(3, mode.as_ptr());        if file.is_null() {            panic!("can't open file");        }        file    };    let welcome = "Hello world!";    let result = unsafe { fwrite(welcome.as_ptr() as *const c_void, 1, welcome.len(), file) };    if result != welcome.len() {        panic!("write not successful");    }}