[newlib-cygwin] Cygwin: Set threadnames with SetThreadDescription()

Jon Turney jon.turney@dronecode.org.uk
Sun Jul 31 15:03:13 GMT 2022

On 29/07/2022 19:28, Corinna Vinschen wrote:
> On Jul 29 15:14, Jon Turney wrote:
>> On 29/07/2022 12:58, Corinna Vinschen wrote:
>>> Hi Jon,
>>> On Jul 29 11:01, Jon TURNEY via Cygwin-cvs wrote:
>>>> https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=d4689b99c68628d9ec2fc1ac7884906ddbf6a2fc
>>>> commit d4689b99c68628d9ec2fc1ac7884906ddbf6a2fc
>>>> Author: Jon Turney <jon.turney@dronecode.org.uk>
>>>> Date:   Thu May 19 17:27:39 2022 +0100
>>>>       Cygwin: Set threadnames with SetThreadDescription()
>>>>       [...]
>>>> +      /* SetThreadDescription only exists in a wide-char version, so we must
>>>> +	 convert threadname to wide-char.  The encoding of threadName is
>>>> +	 unclear, so use UTF8 until we know better. */
>>>> +      int bufsize = MultiByteToWideChar (CP_UTF8, 0, threadName, -1, NULL, 0);
>>>> +      WCHAR buf[bufsize];
>>>> +      bufsize = MultiByteToWideChar (CP_UTF8, 0, threadName, -1, buf, bufsize);
>>> I think this is wrong.  The function should use stock mbstowcs instead
>>> to get the externally used encoding.  Think of SetThreadName called with
>>> program_invocation_short_name in pthread::thread_init_wrapper, or called
>>> from pthread_setname_np with an externally provided thread name.  This
>>> thread name will use the locale of the application code it's called by.
>> I'm not sure.
>> The linux manpage for pthread_setname_np() says "The thread name is a
>> meaningful C language string", which I think means it's ASCII-encoded, not
>> locale-encoded.
> I think this only means, it's a NUL-terminated string. "Meaningful" is
> just trying to nudge developers into using meaningful names, not
> something like "blurb".

Oh yeah, that reading makes more sense!

Still I think the threadname is just really just an opaque NULL 
terminated byte sequence which you can get back with pthread_getname_np().

If there are other mechanisms which make that threadname available to 
other processes (which might have a different locale), it's unclear how 
the encoding is supposed to be handled...

>> (The solaris manpage explicitly says that the thread name is utf8 encoded)
> Ok, that's an interesting point.
>> The encoding for program_invocation_short_name was also unclear to me.
>> (It's the same as argv[0], so I guess it's in whatever encoding the
>> filesystem uses, which doesn't have to match the process locale encoding)
>> Expecting this function to work with non-ASCII names seems optimistic :)
> Well, for Linux it's certainly just an arbitrary, NUL-terminated byte
> stream, but yeah, it's certainly the only portable way to expect
> the portable codeset.
> Anyway, feel free to just keep the code as is.  We're typically using
> UTF-8 anyway and people switching to one of the legacy codesets are
> supposed to know what they are doing.

Yes, I think I'll leave this as is until someone complains! :)

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