On Thu, Nov 16, 2023 at 7:07 PM Bruno Haible <bruno@clisp.org> wrote:

> David Edelsohn wrote:
> > >     ibm-clang links against libpthread.a as well:
> > >     $ ldd /opt/IBM/openxlC/17.1.1/bin/.ibm-clang.orig
> > >     /opt/IBM/openxlC/17.1.1/bin/.ibm-clang.orig needs:
> > >          /usr/lib/libpthreads.a(shr_xpg5_64.o)
> > >          /usr/opt/zlibNX/lib/libz.a(libz.so.1)
> > >          /usr/lib/libcurses.a(shr42_64.o)
> > >          /usr/lib/libiconv.a(shr4_64.o)
> > >          /usr/lib/libc++.a(shr_64.o)
> > >          /usr/lib/libc++abi.a(libc++abi.so.1)
> > >          /usr/lib/libc.a(shr_64.o)
> > >          /usr/lib/libpthreads.a(_shr_xpg5_64.o)
> > >          /usr/lib/libc++.a(libc++.so.1)
> > >          /usr/lib/libunwind.a(libunwind.so.1)
> > >          /usr/lib/libc.a(_shr_64.o)
> > >          /unix
> > >          /usr/lib/libcrypt.a(shr_64.o)
> > >
> >
> > I have asked the IBM Clang team why ibm-clang depends on libpthreads.
>
> The reason is that
>   - For a library, it is a normal expectation nowadays that it is
>     multithread-safe.
>   - Making a library multithread-safe (without major hacks) means to do
>     locking or to call pthread_once / call_once in some places.
>   - The ISO C 11 threading functions in libc have some drawbacks compared
>     to the pthread functions. [1] So most developer prefer to rely on the
>     POSIX threads API.
>   - Since AIX does not have the POSIX mutex functions in libc and does not
>     support weak symbols like in ELF, this means a dependency to
>     pthread_mutex_lock or pthread_once.
>   - Accordingly, in the list of libraries above, 3 libraries need pthread*
>     symbols:
>
> $ nm -X 64 /usr/lib/libc++abi.a | grep ' U ' | grep pthread_mutex
> pthread_mutex_lock   U           -
> pthread_mutex_unlock U           -
> $ nm -X 64 /usr/lib/libc++.a | grep ' U ' | grep pthread_mutex
> pthread_mutex_destroy U           -
> pthread_mutex_init   U           -
> pthread_mutex_lock   U           -
> pthread_mutex_trylock U           -
> pthread_mutex_unlock U           -
> pthread_mutexattr_destroy U           -
> pthread_mutexattr_init U           -
> pthread_mutexattr_settype U           -
> $ nm -X 64 /usr/opt/zlibNX/lib/libz.a | grep ' U ' | grep pthread_mutex
> pthread_mutex_destroy U           -
> pthread_mutex_init   U           -
> pthread_mutex_lock   U           -
> pthread_mutex_unlock U           -
>

There are ibm_clang and ibm_clang_r (previous xlc and xlc_r) to compile
with and without thread safe.   If IBM Clang team
chose to only provide a thread safe version of libc++, okay, but that
doesn't seem like a fundamental requirement.
zlibNX is another can of worms.

David