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

Question

I need to write to file descriptor 3. I have been searching for it but the documentation is quite poor. The only thing that I have found is the use of libc libr

Answer 1

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");
    }
}