[Bug 251112] Compiling C++ with asan fails by default because libclang_rt.asan-x86_64.so uses symbol pthread_attr_get_np but doesn't link to libpthread.so

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=251112

--- Comment #9 from Andrew Stitcher <astitcher at apache.org> ---
(In reply to Konstantin Belousov from comment #8)
I think I understand the libpthread/libc interaction a bit better now: It seems
that the only way to get pthread_create() is to actually link libpthread. So
any executable that actually has multiple threads will necessarily have
libpthread linked in. This is what I was missing before when you were talking
about pthread stubs - that is no stub for pthread_create() in libc.

It looks like there are stubs (weak symbols) in libc for the other relevant
pthread symbols used in libclang_rt.asan (even the ones that are documented to
require linking with -lpthread) so I think it would make sense to have a stub
for pthread_attr_get_np() which returns the ESRCH error (this is the documented
return if it cannot find the thread id which I think is necessarily the case if
there are no threads!). In the case of libclang_rt.asan I think if it did get
called somehow - which looks to be impossible - the return value is checked and
should abort the run.

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