e fly, use (in-C "code..."). Two more such
cases that come up all the time: C-function for linkage to functions written
directly in s7 style using in-C, and C-macro for macros in the C header file that
need to be wrapped in #ifdefs.
Here are some examples:
</p>

<pre class="indented">
;;; various math library functions
(c-define '((double j0 (double))
            (double j1 (double))
            (double erf (double))
            (double erfc (double))
            (double lgamma (double)))
          "m" "mat