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

[Trevor Gross <tmgross@umich.edu>]

On Thu, Jan 11, 2024 at 4:32 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Trevor Gross via Libc-help:
>
> > 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?
>
> We should probably make a copy of the TID inside pthread_create and
> return that from pthread_gettid, instead of pd->tid that might be
> cleared by the kernel upon thread exit.  This way, pthread_gettid
> remains valid until pthread_join is called.
>
> Thanks,
> Florian
>

As in, store a second pid_t value within pthread? This sounds doable,
I think that would sidestep a lot of the problems listed in [1]. Maybe
it could also be used other places after a INVALID_TD_P check, such as
at [2], to still return something somewhat valid if the check returns
a false positive.

Adding Adhemerval based on the thread, did you ever prototype an
implementation for this? (pthread_gettid_np, [1])

- Trevor

[1]: https://inbox.sourceware.org/libc-alpha/6d79a213-0df2-be8e-3596-e010f366a34f@linaro.org/
[2]: https://github.com/bminor/glibc/blob/520b1df08de68a3de328b65a25b86300a7ddf512/nptl/pthread_getcpuclockid.c#L38