Because functions in C and Lua work so differently, exposing a C function to Lua can get a bit tricky. All C functions that Lua can call must follow the signature of lua_CFunction, which is defined in lua.h as the following:
typedef int (*lua_CFunction) (lua_State *L);
This function takes only one argument, the lua_State. The return value of the function is an integer. This integer is the number of elements that the function pushed onto the stack as return values.
Lua has multiple stacks—each C function called from Lua has its own stack and does not share the global stack.
Let's take for example a simple C function that returns the magnitude of a three-dimensional vector. In C, the code for doing so might look something like the following:
double Vec3Magnitude(double x, double y, double z) {
double dot = x * x + y * y + z * z;
if...
