Enable utsname on MacOS

[arcrose]

I'm doing a really quick experiment to try to invoke the uname function on macOS. Following the documentation, I wrote the code below.

extern crate nix;

fn main() {
    println!("Hello, world!");
    let name = nix::sys::utsname::uname();
    println!("
      sysname = {}
      nodename = {}
      release = {}
      version = {}
      machine = {}
    ", name.sysname(), name.nodename(), name.release(), name.version(), name.machine());
}

My Cargo.toml file points to version 0.8.1 of nix as a dependency. When I try to compile this program with cargo build I get the following error:

error[E0433]: failed to resolve. Could not find `utsname` in `sys`
 --> src/main.rs:5:16
  |
5 |     let name = nix::sys::utsname::uname();
  |                ^^^^^^^^^^^^^^^^^^^^^^^^ Could not find `utsname` in `sys`

I'm running this on macOS 10.12.4 with rustc 1.17.0.

[arcrose]

I see that utsname is actually only exported for Linux and Android targets. Would it not work on macOS?

[asomers]

@zsck try it and find out. If it does, submit a PR to enable it. Include in the PR a link to the relevant man page.

[arcrose]

I modified src/sys/mod.rs to export utsname on macOS like so:

#[cfg(any(target_os = "linux", target_os = "android", target_os = "macos"))]
pub mod utsname;

Now my program compiles and runs, however it terminates silently as soon as uname is called. The following println! doesn't produce output. If I do the following:

cargo run
echo $?

I can see that my program terminated with status code 1, so there's certainly some kind of error occurring.

Unfortunately I am not equipped to figure out how to resolve this issue.

[arcrose]

Interestingly, if I modify the wrapper for uname so that it looks like so:

pub fn uname() -> UtsName {
    unsafe {
        let mut ret: UtsName = mem::uninitialized();
        let retcode = ffi::uname(&mut ret as *mut UtsName);
        println!("RETURN CODE: {}", retcode);
        ret
    }
}

Then re-run my program, I now see the following output:

Hello, world!
RETURN CODE FROM uname: 0

      sysname = Darwin
      nodename = R�
      release =
      version =
      machine =

The value of nodename appears to change every time I run my program, and sometimes there are extra whitespace (newline) characters in the value of version.

[Susurrus]

@zsck Just wanted to point out that your documentation link is incorrect. It points to the docs for Linux (which is why that function exists). You really want to explore the docs here. Note the dropdown to select the Platform at the top of the page. This is common with multi-platform crates that have platform-specific functionality. Be sure to read the right docs!

As to the actual issue, the proper place to bring this up actually isn't here, but in libc. As you've seen, nix just provides a safe wrapper around uname, but the FFI is done by the libc crate. Please open an issue there about this (and be sure to link to this issue from there!). Once that's resolved we can expose it on MacOS if appropriate.

[arcrose]

I am not 100% sure if this is in fact a problem with libc. I wrote the following code using libc, and it seems to work just fine.

extern crate libc;

use std::mem;

fn main() {
    println!("Getting name");
    unsafe {
        let mut name: libc::utsname = mem::uninitialized();
        let ret_code = libc::uname(&mut name as *mut libc::utsname);
        println!("RETCODE = {}", ret_code);
        print!("version = ");
        for i in 0..name.version.len() {
            print!("{}", name.version[i] as u8 as char);
        }
    }
}

The invocation of libc::uname here is inspired directly by the way it's called in nix.

[Susurrus]

You didn't need to write code, just look at libc see and see where it's declared. What platforms do they support using it on?

[arcrose]

I don't see anything indicating that uname wouldn't be available on both macOS and Linux. Going through their documentation into the source, it appears there are no restrictions on which platforms uname is exported for.

I don't see how it's a problem of libc's if I'm able to use uname perfectly fine via libc, but I cannot using nix.

[asomers]

There seems to be an assumption that nix::sys::utsname has the same memory layout as libc::utsname. That assumption is probably true on Linux but not OSX. That's the reason that your first experiment didn't work.

[Susurrus]

@Somers You're probably right that the memory layout is incorrect in libc.

First off you can see that libc defines uname() for all unix platforms (@zsck it's easiest just to look through the hierarchy in the libc source like here). But utsname is defined per-platform (see here). And you can see that the OS X implements is here and that it's common among all BSDs that aren't Dragonfly.

[asomers]

@Susurrus My concern isn't that libc's definition for OSX's utsname is wrong, though that may be a problem too. My main concern is that nix's uname passes a pointer to nix's UtsName structure directly to an FFI uname function. The only way that can work is if nix's Utsname structure has the same layout as the structure that the OS's uname function expects. That's very unlikely to be true on all platforms.
https://github.com/nix-rust/nix/blob/master/src/sys/utsname.rs#L59

[Susurrus]

Oh, man, our utsname code is completely wrong. We shouldn't have FFI definitions in our code. And really we should have a newtype and then pass that to the FFI function. As long as we specify it's repr(C) we should be fine.

[Susurrus]

Sorry, not completely wrong, but definitely not well written.

[Susurrus]

@zsck Would you be interested in filing a PR for this? You'd instead create a newtype for UtsName that looks like of like the PtyMaster struct I added in #556. Then defer all FFI to the libc crate. That will likely fix the issue.

[arcrose]

I wish I could say that I would like to take care of this, but my use case for uname is very limited in scope and doesn't add a whole lot of value to the project I'm working on (for my employer).

[Susurrus]

This should be addressed by #739. @zsck please test out that PR and let me know if it works for you.