even if a writer is waiting.  As long as there are readers, the writer will be starved.
This is intended as the write lock analog of PTHREAD_RWLOCK_PREFER_READER_NP.  But see BUGS.
to by attr in the pointer pref.
pthread_rwlockattr_setkind_np() returns a non-zero error number.
EINVAL pref specifies an unsupported value.
These functions are non-standard GNU extensions; hence the suffix "_np" (non-portable) in the names.