He calls it a C trick but the actual trick is implemented in asm. It would be a cool trick if he did it purely in C. Especially today, "all the world's not x86".

(And I used to be a gcc maintainer for m68k and i860. I'm quite familiar with frame pointers and their non-use.)

Btw: you can do this trick without resorting to assembly - start by taking the address of a local variable.

That depends entirely on the calling convention - potentially you can take two words down from the first argument, but that is no guarantee, and would not suffice for the didactic goals this article attempts to accomplish.

Worth knowing that this also describes some of the mechanics of a relatively common C vulnerability, and a tactic used by some content protection schemes.

There is no special call stack manipulation in the CL condition system. It's built on top of dynamic scoping and normal catch/throw unwinds; see http://www.pipeline.com/~hbaker1/MetaCircular.html for block/return-from on top of catch/throw.

Indeed, but the Common Lisp condition system exposes it and wraps it elegantly.

I've always wondered: if I implement Common Lisp in C or Fortran, what does that say about Common Lisp?

Reminds me of the good old days programming in TCL.

What does the mentioned switch statement do?

My guess would be that it triggers a compiler error because of the missing braces {}. However, reading http://en.cppreference.com/w/c/language/switch and https://msdn.microsoft.com/en-us/library/66k51h7a.aspx, I am starting to wonder about that. Both pages claim that switch(expression) can be followed by any statement, not just compound ones.

That seems to make it legal C. It also would make

  switch(i) x += 1;

and possibly even

  switch(i) foo: x += 1;

valid (but nonsensical) C. Chances are that the standard contains special wording that prohibits the latter two, though. If not, this is useful to know for the obfuscated C contest (more so if you can jump to that label)

It does not, GCC compiles the example perfectly fine.

It is equivalent to:

  if (x == 1 || x == 2 || x == 3 || x == 4)
    printf("What the fuck?");

quite an insightful article. and, no, i do now understand frame pointers. ;)