Well, strtok could use thread local variables to store intermediate state, to make it threadsafe while maintaining the same API. Not saying this is a good idea, but technically it would work, no?
You could, but that would change the behaviour of existing programs. It might well be that there are well-defined programs out there that use strtok across separate, but properly synchronized, threads.
This is why it's crucial to get APIs right first time.
