Threads cannot be implemented as a library

Hans Boehm has an excellent paper on this topic, "Threads Cannot be Implemented as a Library", https://dl.acm.org/doi/10.1145/1065010.1065042. (Yes, the full paper is free to read from the ACM! They've made the entire library publicly available during the pandemic.) Before you dismiss the paper based on the title, let me correct it and say it really should be "Threads should not be implemented as a library." Boehm's argument is that when you implement something like threads as a library, and the language itself is not aware of the semantics, lots of bad stuff happens.

> our official stance is not actually as strict as “no threads, period”, though in practice for many people it might amount to that. There is nothing stopping you from linking to pthreads or calling clone(2) to spin up threads in a Hare program, but the standard library explicitly provides no multi-threading support, synchronization primitives, or re-entrancy guarantees https://dl.acm.org/doi/10.1145/1065010.1065042

Another thing I don't understand is how can threads be a library in CL ? Threads cannot be implemented as a library https://www.hpl.hp.com/techreports/2004/HPL-2004-209.pdf https://dl.acm.org/doi/10.1145/1065010.1065042