Lua, get clock time in msecs

now.c

/*
  Usage:
  
  $ cc -fPIC -c now.c -I/usr/include/luaX.Y -O3
  $ cc now.o -shared -o now.so
  
  In Lua:
  
  > now = require 'now'
  > now.now_ms()
  > print(now.now_ms())
  1466176843794  # Msecs elapsed since Unix epoch
*/

#include <time.h>
#include <sys/time.h>

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

static int now_ms(lua_State *L) {
    struct timespec ts;
    clock_gettime(CLOCK_REALTIME, &ts);
    long s = ts.tv_sec;
    long ns = ts.tv_nsec;
    long ms = s * 1000;
    ms += ns / 1000000;
    lua_pushnumber(L, ms);
    return 1;
}

static const struct luaL_Reg now[] = {
    {"now_ms", now_ms},
    {NULL, NULL}
};

int luaopen_now(lua_State *L) {
    lua_newtable(L);
    lua_pushcfunction(L, now_ms);
    lua_setfield(L, -2, "now_ms");
    return 1;
}