Re: [PATCH v1 2/2] gdb: rely on the first non-exited thread TPID when reading Linux procfs files

[Matthieu Longo <matthieu.longo@arm.com>]

On 24/08/2026 16:29, Matthieu Longo wrote:
> On 20/08/2026 16:48, Simon Marchi wrote:
>> On 7/28/26 10:33 AM, Matthieu Longo wrote:
>>> +/* Return a PTID that identifies the current process and can be used to
>>> +   access procfs safely.
>>> +
>>> +   The returned PTID is that of the thread-group leader whenever it is
>>> +   still alive.  If the leader has already exited, the PTID of the first
>>> +   non-exited thread in the current inferior is returned instead.
>>> +   This ensures that the returned PTID always refers to a live thread
>>> +   whose procfs entries are present and populated.  */
>>> +static ptid_t
>>> +get_process_reference_ptid (bool verbose = false)
>>> +{
>>> +  /* Get the current thread.  */
>>> +  thread_info *thr = inferior_thread ();
>>> +
>>> +  /* Construct the PTID of the thread-group leader.  On Linux,
>>> +     the leader's LWP ID is equal to the process ID.  */
>>> +  ptid_t leader_ptid (thr->ptid.pid (), thr->ptid.pid ());
>>> +
>>> +  /* Use the thread-group leader if it is still alive.  Otherwise, use
>>> +     the first thread that has not exited.  */
>>> +  thread_info *leader_thr
>>> +    = current_inferior ()->find_thread (leader_ptid);
>>> +  ptid_t ptid = (leader_thr == nullptr
>>> +		 ? current_inferior ()->first_non_exited_thread ()
>>> +		 : leader_ptid);
>>> +
>>> +  if (!verbose)
>>> +    return ptid;
>>> +
>>> +  if (leader_thr != nullptr)
>>> +    gdb_printf (_("process %d\n"), leader_ptid.pid ());
>>> +  else
>>> +    gdb_printf (_("process %d [Note: information where gathered from LWP %ld " \
>>> +		  "as the thread-group leader (LWP=%ld) already exited.]\n"),
>>> +		ptid.pid (), ptid.lwp (), leader_ptid.lwp ());
>>
>> I don't think the printfs should be part of this getter function.  Move
>> it to a separate function called by whoever needs it.  Or just inline
>> it, it looks like there is just a single caller that needs it
>> (linux_info_proc).
>>
> 
> The idea is to inform the user if a fallback occurs.
> Now, if I move the printing to another function, I don't have the fetched information anymore, and
> cannot figure out easily if a fallback happened and using which thread.
> 
> It is ugly but I could not find better.
> Do you have a better approach ?
> > Matthieu

There is actually a way to figure it out by comparing ptid.pid () and ptid.lwp ().

Sent r2 addressing your comment.
https://inbox.sourceware.org/gdb-patches/20260824162155.467233-1-matthieu.longo@arm.com/T/#u

Matthieu