Re: RFC: A way to get a TID from a pthread_t handler

[Trevor Gross <tmgross@umich.edu>]

On Thu, Jan 11, 2024 at 1:23 PM Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 1/11/24 01:10, Trevor Gross via Libc-help wrote:
> > It seems like there isn't a good way to get the thread ID from a
> > pthread_t. If you are within the thread then you can call gettid(),
> > but there is no public API to get a spawned thread's TID after calling
> > pthread_create().
> >
> > This value is stored in the pthread, so a new public function could
> > just be a basic getter:
> >
> >     pid_t pthread_gettid(pthread_t threadid)
> >     {
> >       struct pthread *pd = (struct pthread *) threadid;
> >       return pd->tid;
> >     }
> >
> > Would an addition like this likely get accepted? Or is there anything
> > overlooked about an easier way to recover the spawned thread's TID?
> >
> > I could submit a patch, but require instruction on where this should live.

I appreciate the thorough response,

> This is much more complicated than it seems at first :-)
>
> Please read through:
> Bug 27880 - Please provide a pthread pid accessor
> https://sourceware.org/bugzilla/show_bug.cgi?id=27880
>
> Discussion here:
> https://inbox.sourceware.org/libc-alpha/6d79a213-0df2-be8e-3596-e010f366a34f@linaro.org/

Whoops, I didn't come across these in my search. Actually did not at
all come across pthread_getthreadid_np that is mentioned there, only
the similarly named pthread_gettid_np. Thanks for the links.

> I don't recommend this as a "first contribution" kind of patch.
>
> Are you looking to start with libc development?
>
> Are you looking for an initial project to work on?
>
> --
> Cheers,
> Carlos.
>

This was sort of a need-based suggestion that I am still interested
enough in looking into if there is a reasonable path forward. We were
discussing how to expose the OS TID, it was pointed out that this is
indirectly available via pthread_getcpuclockid ([1], [2]) but not
directly.

But indeed, it seems more complicated than originally thought :) I
will follow up in Florian's response.

- Trevor

[1]: https://github.com/bminor/glibc/blob/520b1df08de68a3de328b65a25b86300a7ddf512/nptl/pthread_getcpuclockid.c#L38
[2]: https://github.com/bminor/glibc/blob/520b1df08de68a3de328b65a25b86300a7ddf512/sysdeps/unix/sysv/linux/kernel-posix-cpu-timers.h#L23-L33