call_lua.c

#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
#include "math.h"

int myCfunc(lua_State *L) {
  printf ("    START running C redis (LUA)\n");
  double d = lua_tonumber(L, 1);
  lua_pushnumber(L, d * d);
  printf ("    END   running C redis (LUA)\n");
  return 1;
}

int main() {
    lua_State *L = lua_open();
    luaL_openlibs(L);
    lua_register(L, "redis", myCfunc);

    if (luaL_loadfile(L, "ccall.lua") || lua_pcall(L, 0, 0, 0)) {
        printf("0: error: %s", lua_tostring(L, -1));
        return -1;
    }

    lua_getglobal(L, "squarer");
    if(!lua_isfunction(L, -1)) {
        printf("function not found\n");
        lua_pop(L,1);
        return -1;
    }
    lua_pushnumber(L, 12);   /* push 1st argument */

    /* do the call (2 arguments, 1 result) */
    printf("START: calling from C main()\n");
    if (lua_pcall(L, 1, 1, 0) != 0) {
        printf("error running function `f': %s\n",lua_tostring(L, -1));
        return -1;
    }
    printf("END:   calling from C main()\n");

    /* retrieve result */
    if (!lua_isnumber(L, -1)) {
        printf("function `redis' must return a number\n");
        return -1;
    }
    double z = lua_tonumber(L, -1);
    printf("Result: %f\n",z);
    lua_pop(L, 1);
    lua_close(L);

    return 0;
}

ccall.lua

function squarer(x)
  io.write("  lua level -> START call C\n");
  sum = redis(x);
  io.write("  lua level -> END   call C\n");
  return sum;
end

output

START: calling from C main()
  lua level -> START call C
    START running C redis (LUA)
    END   running C redis (LUA)
  lua level -> END   call C
END:   calling from C main()
Result: 144.000000

Redisql lua embedding proof of concept

#!/bin/bash

echo gcc -I/usr/include/lua5.1/ -L/usr/local/lib/ -llua -o call_lua{,.c} -llua -ldl -lm
gcc -I/usr/include/lua5.1/ -L/usr/local/lib/ -llua -o call_lua{,.c} -llua -ldl