From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/5982 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: C threads, v. 6.2 Date: Sat, 30 Aug 2014 09:43:36 +0200 Message-ID: <1409384616.4476.231.camel@eris.loria.fr> References: <1409254089.4476.137.camel@eris.loria.fr> <20140828200029.GA12888@brightrain.aerifal.cx> <1409261653.4476.142.camel@eris.loria.fr> <20140828215641.GD12888@brightrain.aerifal.cx> <1409268337.4476.151.camel@eris.loria.fr> <20140828233809.GE12888@brightrain.aerifal.cx> <1409298999.4476.163.camel@eris.loria.fr> <1409299363.4476.164.camel@eris.loria.fr> <20140829155744.GH12888@brightrain.aerifal.cx> <1409338871.4476.206.camel@eris.loria.fr> <20140830053036.GJ12888@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; 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C3d0WolfKhTp6clRyhcg04xtGCHt1Dh/rExRamYWGyxXa7xn1yYuvkAwP7fClg0Zhx+qr/rM7d+Yesc73nHD5OW7Zh478tRzPxACH/iRa245duzkAdsJfMuy8aXm/FUFlpeWWOm2WZidI9WapaUlFIaTlQ4jfR6/vO9D2NE32LTeYnZOUmnA6PAohx9+qnTy1NnrP/ Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.4.4-3 Xref: news.gmane.org gmane.linux.lib.musl.general:5982 Archived-At: --=-qm97BLiLpqDludLaOz01 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: quoted-printable Am Samstag, den 30.08.2014, 01:30 -0400 schrieb Rich Felker: > On Fri, Aug 29, 2014 at 09:01:11PM +0200, Jens Gustedt wrote: > Unless the intent is to permanently have namespace violations, mtx_t > must be defined at some point such that it does not have > pthread_mutex_t as its C++ ABI "struct tag". It could have mtx_t > (because the specific name is reserved), or something like __mtx_t > (with a name in a general reserved namespace). This requires being a > different type from pthread_mutex_t. Are we master of that decision, or do we have to coordinate that with other C libraries? > > There is basically one base choice to make: > >=20 > > - we decide if pthread_mutex_t and mtx_t are seen as two different > > types or not for any application that includes both headers >=20 > This is not a choice; it's mandated by the fact that our > pthread_mutex_t has a "struct tag" (in C++) that's a namespace > violation for use as the tag for mtx_t. We could probably also find a trick that has us clean on the C side, and have namespace violation just in a C++ context :) > However, by the C rules, they're only "different types" when they're > both visible in the same translation unit. To a translation unit where > only one is visible, since the typedef name is not actually part of > the type, just an alias, both are structures without tags, and the one > that is visible is _the same type_ as whichever one it needs to be to > make the code correct. >=20 > I don't see any problem if an application has both types visible in > one of it's TUs, since no "aliasing" takes place on the app side. The > tagless structure "struct { union {...} __u; }" (whichever instance of > it) is simply zero-initialized on the application TU side. On the > implementation side, functions like pthread_mutex_trylock access a > tagless structure "struct { union {...} __u; }", of which they have > only one defined: the one referenced by the pthread_mutex_t typedef. As I said, on the side of the current C thread implementation that needs a thorough revision to be sure that none of the TU sees two types. I'll look into that. > > (This should be made independent of the question if we silently use > > the same hidden type, or similar structured type, under the hood.) > >=20 > > For C this choice is not so relevant, since all interfaces are just > > pointers to struct, so they are interchangeble, and this helps for the > > implementation. > >=20 > > For C++ this is not the same because "type" for them means *typename*, > > defined in addition that is determined in some subtle and not so > > obvious way. > >=20 > > For backward compatibility, the C++ ABI seems to dictate that there > > must be at least one such type that is called pthread_mutex_t. So we > > have to keep the type with that typename for them, it is as simple as > > that. > >=20 > > Now in a C++ context that choice above boils down to the question > >=20 > > - is mtx_t a typedef to pthread_mutex_t or is it a proper type? > >=20 > > If we want it to be a proper type (for which I would argue, I think) > > we have to think of ways to make C++ believe that the two types are > > different, even if we use the same implementation underneath. >=20 > Yes, because of the namespace, C++ has to believe the types are > different. But the (C) implementation of the functions is not subject > to C++ rules about types; it's not C++ code. Thus I think everything > is fine. >=20 > If you really still think there's a problem, I still have one trick > I've mentioned before that makes it a 100% non-problem: never using > the pthread_mutex_t or mtx_t type at all internally, but instead using > the type of their first member. I believe I could make this work with > only a few lines of source-level changes, no change to the output > code, and minimal ugliness. Let me know if you still have doubts > whether the above analysis I gave is correct, and if so, I'll give my > trick a try. So let us talk through this, I suspose the main change that you would do for that is to change the accessor macros such that they have the additional indirection. I can see that this would easily work for the pthread TU. For the C thread TU, what would be the mechanics for them to call one of the (aliased) pthread functions? Jens --=20 :: INRIA Nancy Grand Est ::: AlGorille ::: ICube/ICPS ::: :: ::::::::::::::: office Strasbourg : +33 368854536 :: :: :::::::::::::::::::::: gsm France : +33 651400183 :: :: ::::::::::::::: gsm international : +49 15737185122 :: :: http://icube-icps.unistra.fr/index.php/Jens_Gustedt :: --=-qm97BLiLpqDludLaOz01 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iEYEABECAAYFAlQBgKgACgkQD9PoadrVN+Jn1gCeP7oTE9TRj5fWHJ8h0evOu0mH 4F0An3nlH8LjiyMvU+o8MXTq/Fhbuc23 =zKMQ -----END PGP SIGNATURE----- --=-qm97BLiLpqDludLaOz01--