Date: Mon, 29 Mar 2004 21:43:15 -0600 From: "Jacques A. Vidrine" <nectar@FreeBSD.org> To: Daniel Eischen <eischen@vigrid.com> Cc: Sean McNeil <sean@mcneil.com> Subject: Re: nss_ldap broken Message-ID: <20040330034315.GC5998@madman.celabo.org> In-Reply-To: <Pine.GSO.4.10.10403292214380.12734-100000@pcnet5.pcnet.com> References: <20040330023247.GA5637@madman.celabo.org> <Pine.GSO.4.10.10403292214380.12734-100000@pcnet5.pcnet.com>
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On Mon, Mar 29, 2004 at 10:31:56PM -0500, Daniel Eischen wrote:
> On Mon, 29 Mar 2004, Jacques A. Vidrine wrote:
>
> > On Fri, Mar 26, 2004 at 05:51:02PM -0500, Daniel Eischen wrote:
> > > I think I made a comment about how you should always
> > > prefix _pthread_foo() calls with 'if (__isthreaded)'.
> >
> > Yes, I'm sure you did. My recollection was that it was an
> > optimization only, but it seems either I misunderstood or my
> > recollection is poor (or both) :-)
>
> I had no idea that libpthread would be loaded then unloaded.
Me neither :-)
> > (2) When a threading library is loaded (by any cause? DT_NEEDED?
> > dlopen RTLD_GLOBAL? dlopen RTLD_LOCAL?), __isthreaded is set
> > to 1
>
> No, __isthreaded is only set to 1 (non-zero) when the first
> thread (other than main) is created. But the library is
> auto-initialized and that's when libc's jump table is
> filled.
Gotcha, thanks for the clarification.
> > (3) When a threading library is unloaded, __isthreaded is reset to 0
>
> No, once threaded always threaded. There's really no going
> back.
So e.g. dlopen(libpthread) ... dlclose(libpthread) is not supported?
And following from that, dlopen(object_which_depends_on_libpthread) ...
dlclose(object_which_depends_on_libpthread) may also not be supported?
[...]
> I'm unsure how nss_ldap was built to depend on libpthread (or
> any threads library). I built it from ports and 'ldd' didn't
> report any dependency on a threads library.
Interesting. Rebuilt a few days ago after updating to OpenLDAP 2.1:
% ldd /usr/local/lib/nss_ldap.so.1
/usr/local/lib/nss_ldap.so.1:
libldap.so.2 => /usr/local/lib/libldap.so.2 (0x2815f000)
liblber.so.2 => /usr/local/lib/liblber.so.2 (0x28189000)
libpthread.so.1 => /usr/lib/libpthread.so.1 (0x28194000)
libssl.so.3 => /usr/lib/libssl.so.3 (0x281b7000)
libcrypto.so.3 => /lib/libcrypto.so.3 (0x281e4000)
> An example of how to build a library that is thread-safe
> but doesn't bring in the threads library is libgcc. It uses
> weak references (not definitions, like you see in our libc
> and libpthread) to the necessary locking functions and
> pthread_create. For instance:
>
> #pragma weak pthread_create
> #pragma weak pthread_mutex_lock
> #pragma weak pthread_mutex_unlock
> ...
>
> static void
> foo(void)
> {
> ...
> if (pthread_create != NULL) {
> pthread_mutex_lock(&foo_lock);
> ...
> pthread_mutex_unlock(&foo_lock);
> }
> ...
> }
>
> By making the pthread_* references weak, you don't need to be linked
> to the threads library; they will be NULL if it is not present.
> But if an application is linked to the threads library, then
> those references won't be NULL. There may be a little more
> fu to it, but that's the general idea.
Huh. Can/Should something like this be hidden within <pthread.h>, I
wonder?
Cheers,
--
Jacques Vidrine / nectar@celabo.org / jvidrine@verio.net / nectar@freebsd.org
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