In particular, std::thread constructor's (the non-default one) has a workaround against LTO optimizing the pthread call away: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-...

> Parallelism without pthread

To get __atomic_add_dispatch to work, looks like one is expected to ensure pthread_create is referenced. One way to do it without creating a pthread or std::thread, is to do it outside LTO'd files, or like they did above.

> > It is possible to create threads by using the OS syscalls bypassing completely the requirement of pthead

As the other person said, it is impratical to do so, and it's easier to just reimplement gthread and pthread functions to be hooks (some toolchains do this).