Dear All,

    As you read kernel code, you occasionally encounter strange-looking C idioms. 
It's a good idea to share explanations of these :)

    So here are a few random links.

    The "do {} while(0)" -- https://gcc.gnu.org/onlinedocs/cpp/Swallowing-the-Semicolon.html, explained at the source.

    The token paste operator ## and variadic macros https://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html

      A story of how ## was used to obfuscate a backdoor in a cryptocoin:
      http://earlz.net/view/2016/01/16/0717/analyzing-the-56-million-exploit-and-cryptsys-security
        It used this trick to make source code look innocuous:
          +#define S_ORDER(a,b,c,d) b##a##d##c
          +#define CLine S_ORDER(I,F,E,L)
          +#define CRead S_ORDER(p,po,n,e)
        and then used CLine, CRead, etc.

    The stringification operator #: https://gcc.gnu.org/onlinedocs/cpp/Stringification.html

    String concatenation makes it all work: https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html

    As usual, there's a fun StackOverflow discussion (see the answer with 
BAD_STRINGIFY and BAD_PASTE, and see if it works for you via "gcc -E"):
http://stackoverflow.com/questions/216875/what-are-the-applications-of-the-preprocessor-operator-and-gotchas-to-conside

    Such is the C preprocessor. And if you think it scary, consider the 
fact that C++ templates are Turing-complete: any computable function can 
be computed at _compile-time_ in C++:
http://matt.might.net/articles/c++-template-meta-programming-with-lambda-calculus/

    Cheers,

--Sergey