Just a simple example:

--The  c file:

#include <stdio.h>

#include "lua.h"

#include "luaconf.h"

#include "lualib.h"

#include "lauxlib.h"

#include "math.h"

staticint l_sin (lua_State *L) {

double d = lua_tonumber(L,1);  /* get argument */

  lua_pushnumber(L, sin(d));  /* push result */

    return1;  /* number of results */

}

int main()

{

float pi =3.1415926;

float pidiv6 = pi /6;

float pidiv4 = pi /4;

float rt =0;

lua_State *L = luaL_newstate();

luaL_openlibs(L);

lua_pushcfunction(L, l_sin);//Lua know the address of l_sin in c context

lua_setglobal(L,"mysin"); //map l_sin to mysin will be called in lua context

if ( !luaL_dofile(L,"./cal.lua") ) {

printf("load cal.lua successful\n");

}else {

printf("error load file: %s\n", lua_tostring(L, -1));

       return -1;

}

lua_getglobal(L,"lsin");

lua_pushnumber(L, pidiv4);

if ( !lua_pcall(L,1, 1, 0) ) {

rt = lua_tonumber(L, -1);

}else {

printf("error : %s\n", lua_tostring(L, -1));

       return -1;

}

printf("rt = %f\n", rt);

return0;

}

print("start...")
function lsin ( angle )
return mysin(angle)
end
print("end...")

==output==

start...
end...
load cal.lua successful
rt = 0.707107

//Actually a more simpler way is to call mysin directly without cal.lua

lua_getglobal(L, "mysin");

lua_pushnumber(L, pidiv4);

if ( !lua_pcall(L, 1, 1, 0) ) {

rt = lua_tonumber(L, -1);

} else {

printf("error : %s\n", lua_tostring(L, -1));

        return -1;

}