Get proper file creation (birth) time on Linux

Questions / Help
1

File.stat gives me a pretty useless value for ctime. As documented it is “the last time the file or the inode was changed“.
Using stat via bash gives me a correct creation time via the Birth field.

I understand why that’s the case, as Elixir just calls out to :file.read_file_info and the birth field is a somewhat recent thing in Linux and I could not find a way to read it yet from Erlang.

Does anyone know of a way to get that field without shelling out to bash? Is there maybe a package that has implemented a NIF for that?

Or do I need to write my own NIF?
If so, Rust seems to get this one right: Metadata in std::fs - Rust

1 Like
2

I don’t know a particular NIF for you; but I’m wondering about the “shelling out to bash part”. I don’t know the harm in just using System.cmd

    {result, _exit} = System.cmd("stat", ["/home/cam/hello"])

    %{"Birth" => birth} =
      result
      |> String.split("\n")
      |> Enum.reject(&(&1 == ""))
      |> Map.new(fn line ->
        [key, value] =
          line
          |> String.trim()
          |> String.split(":", parts: 2)

        {key, value |> String.trim()}
      end)

    assert "2025-09-06 10:12:29.200959233 -0400" == birth
3

Thanks. But I need to get this not just for a single file, but as part of a file scanner.
Performance is key. I haven’t benchmarked yet, but I doubt actually calling stat + parsing is a viable option for me.

4

Huh, apparently they added a new syscall (statx) with this information. It’s also safe for 2038.

Does Erlang use that call underneath? If not, maybe it should going forward?

5

It does not, it calls fstat. It looks like on Windows the ctime is the same as Rust’s created.

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6

Yeah, I wouldn’t call 2017 new, but I guess new enough that it’s not in all standard libraries yet.
Python also doesn’t have it, but ruby, node, rust and zig do.

Anyway, I went ahead with the plan and created a small rustler NIF:

Now I just need to figure out how to publish this to hex such that a user doesn’t need to compile the thing?
Looks like rustler_precompiled is the way to go.

2 Likes
7

I was quite surprised Python didn’t have it; they usually have everything. Apparently there has been an open issue/PR for several years now. Strangely it seems to have become active again recently.

A nif seems like the way to go but maybe also consider opening an Erlang/OTP issue as this seems like something that should be updated to me. At least before 2038 :slight_smile:

1 Like
8

Package is published: file_statx | Hex :tada:

(Setting up rustler_precompiled in CI took me so long, I hate setting up CI stuff…)

Done :+1:

1 Like
9

The issue for future reference (if only the WWW could have had backlinks).