Indeed, there is a problem with local variables. The example relies on using global variables. If you want multiple threads executing the same function, it is not going to work. Also there is no solution for calling other functions (and have them switch in the middle). A long time ago, I worked on an implementation that does not have these limitations. See: http://www.iwriteiam.nl/Ha_cmt.html
> It could as well rely on local variables with static lifetime, though.
For those following along, static local variables are the same thing as static global variables[1], C just enforces a scoping restriction as to who can access it at compile time. Most compilers will emit the same BSS section entries for both (though they will mangle the symbol name). It's basically compiler syntactic sugar for most compilers (though I'm sure the actual standard itself allows for some weird exotic use cases where they get handled completely differently).
[1] Static global variables are symbols that are restricted to a compilation unit (as opposed to regular global variables that can be linked to from another compilation unit). If you run `nm` on them, they'll all have a lower case letter indicating it's type.
