Are you thinking of C++ issues with constructors? I don’t see anything wrong with the use of static variables in this single file C program. This isn’t a library; it’s a standalone web server.
I am thinking of multithreading. Mutable static variables pretty much destroy any possibility of multithreading without a major refactor. Test harnessing is an issue too.
But if this only ever wants to be an app binary, I guess it's sort of okay.
This program isn't meant to be multithreaded, it's meant to be multi-/process/. The difference is that each instance of the application has its own independent address space. So that's one address space per instance. And because of that there are no threading issues. Also as a result any communication between different processes has to be explicitly defined.
Making this program multi-threaded would be a big mistake because then you couldn't use any of the HTTP proxies out there to monitor for connections and hand them off to this program.
This is arguably a much better architecture from a safety/security standpoint then trying to spin multiple threads each sharing an address space. It forces you to use OS-provided mechanisms for shared state rather than simply manipulating the memory to share information.
But if this only ever wants to be an app binary, I guess it's sort of okay.