Re: gdb show thread names

[Pedro Alves <palves@redhat.com>]

On 6/15/20 10:28 PM, Jonny Grant wrote:
> 
> 
> On 15/06/2020 22:12, Pedro Alves wrote:
>> On 6/15/20 9:53 PM, Jonny Grant wrote:
>>>
>>>
>>> On 15/06/2020 17:21, Philippe Waroquiers wrote:
>>>> On Mon, 2020-06-15 at 16:51 +0100, Pedro Alves wrote:
>>>>>
>>>>> Starting program: /home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.threads/names/names 
>>>>> [Thread debugging using libthread_db enabled]
>>>>> Using host libthread_db library "/lib64/libthread_db.so.1".
>>>>> [New Thread 0x7ffff74b8700 (LWP 24171) "main"]
>>>>> [New Thread 0x7ffff6cb7700 (LWP 24172) "main"]
>>>>> [New Thread 0x7ffff64b6700 (LWP 24173) "main"]
>>>>>
>>>>> Thread 1 "main" hit Breakpoint 1, all_threads_ready () at /home/pedro/gdb/binutils-gdb/src/gdb/testsuite/gdb.threads/names.c:51
>>>>> 51      }
>>>>> (gdb) info threads 
>>>>>   Id   Target Id                                  Frame 
>>>>> * 1    Thread 0x7ffff7fb5740 (LWP 24170) "main"   all_threads_ready () at /home/pedro/gdb/binutils-gdb/src/gdb/testsuite/gdb.threads/names.c:51
>>>>>   2    Thread 0x7ffff74b8700 (LWP 24171) "carrot" 0x00007ffff7bc89aa in futex_wait (private=0, expected=4, futex_word=0x7fffffffd604) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
>>>>>   3    Thread 0x7ffff6cb7700 (LWP 24172) "potato" 0x00007ffff7bc89aa in futex_wait (private=0, expected=4, futex_word=0x7fffffffd604) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
>>>>>   4    Thread 0x7ffff64b6700 (LWP 24173) "celery" 0x00007ffff7bc89aa in futex_wait (private=0, expected=4, futex_word=0x7fffffffd604) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
>>>>> (gdb) 
>>>>>
>>>>> I.e., printing the thread name when the thread is created
>>>>> looks more confusing than helpful to me.
>>>> Yes, that is confusing.
>>>>
>>>> And for the following events, when I tried, the patch was far to be ready 
>>>> e.g. for the exit events, it gives (for the above):
>>>>   (gdb) c
>>>>   Continuing.
>>>>   [Thread 0x7ffff743d700 (LWP 22783) exited]
>>>>   [Thread 0x7ffff7c3e700 (LWP 22782) exited]
>>>>   [Thread 0x7ffff7c3f740 (LWP 22778) "main" exited]
>>>>
>>>> So, unclear why there is no carrot, potato or celery in the 2 exited threads
>>>> but "main" is present.
>>>> (and sometimes there is no names in any exited event).
>>>>
>>>> So, when I looked at it, it needed quite some more work ...
>>>>
>>>> Philippe
>>>>
>>>
>>> Hi, Maybe it is more complicated than it is worth after all.
>>> Although, I did think new threads inherited the process executable name, rather than the main() symbol.
>>
>> It's not the main() symbol, it's the name of the parent thread.
>> The testcase does:
>>
>>   pthread_setname_np (pthread_self (), "main");
>>
>> on the main thread before spawning the other threads.
>>
>> So a child thread of "carrot" would be called "carrot" too by
>> default, until it changed its name.
>>
>> If we removed that pthread_setname_np call on the main thread, then
>> the main thread's name would default to the process executable name
>> indeed.
>>
>>
>> If we included the thread id in these notifications instead, I think it
>> would be quite useful.  Like, we could have:
>>
>>  [Thread 1.2 (0x7ffff74b8700 (LWP 13984)) created]
>>  [Thread 1.2 (0x7ffff74b8700 (LWP 13984)) exited]
> 
> 
> I'm just looking at my original email, I saw present behaviour is :-
> 
> [Thread 0x7fff695e9700 (LWP 3580240) exited]
> [New Thread 0x7fff98ff9700 (LWP 3580609)]
> 
> May I ask which do you refer to as the 'thread id'?
> I know on the Linux kernel there is gettid syscall, but they don't correspond to the pthread_self() handle.

The "Id" reported in the first column of "info threads".

The "INF.THR" format is what GDB shows when you have multiple inferiors:

(gdb) add-inferior 
[New inferior 2]
Added inferior 2 on connection 1 (native)
(gdb) info threads 
  Id   Target Id                                 Frame 
* 1.1  Thread 0x7ffff7fb5740 (LWP 7275) "names"  all_threads_ready () at /home/pedro/gdb/binutils-gdb/src/gdb/testsuite/gdb.threads/names.c:62
  1.2  Thread 0x7ffff74b8700 (LWP 7279) "carrot" 0x00007ffff7bc89aa in futex_wait (private=0, expected=4, futex_word=0x7fffffffd604) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
  1.3  Thread 0x7fffeffff700 (LWP 7281) "potato" 0x00007ffff7bc89aa in futex_wait (private=0, expected=4, futex_word=0x7fffffffd604) at ../sysdeps/unix/sysv/linux/futex-internal.h:61


So if we only have one inferior, it could be shown as:

  [Thread 2 (0x7ffff74b8700 (LWP 13984)) created]
  [Thread 2 (0x7ffff74b8700 (LWP 13984)) exited]
          ^       ^                ^
          |       |                |
          |       |                \-- kernel tid (gettid syscal)
          |       |
          |       \--- pthread_t (pthread_get_self() handle)
          |
           \-- gdb thread id ("info threads", "thread TID", etc.)

Thanks,
Pedro Alves