Re: nss_ldap broken

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_at_celabo.org / jvidrine_at_verio.net / nectar_at_freebsd.org