If you compile with optimization and NDEBUG is defined, then the whole assert() is replaced with ((void) 0) and your code is not executed at all.
Don't assert() on pthread_join() return value. If thread was not started yet, then pthread_join() will fail, but it's ok for us.
(cherry picked from commit d0b797e62c, 21a84dcec9cdf25657bba5c221aac9d4bb81d8b3, 8ed37a1f36e1b1266728647e715f95a28df3f900)
Ivan Kluchnikov