Question

I’m trying Lua and want to know how lua_State working
code and result:

state.c

#include <stdio.h>
#include "lua/src/lua.h"
#include "lua/src/lauxlib.h"
static void stackDump(lua_State *L){
    int i;
    int top = lua_gettop(L);
    for(i = 1; i<= top; i++) {
        int t = lua_type(L, i);
        switch(t){
        case LUA_TSTRING:
            printf("'%s'", lua_tostring(L, i));
            break;
        case LUA_TBOOLEAN:
            printf(lua_toboolean(L, i) ?"true":"false");
            break;
        case LUA_TNUMBER:
            printf("%g", lua_tonumber(L, i));
            break;
        default:
            printf("%s", lua_typename(L, t));
            break;
        }
        printf(" ");
    }
    printf("\n");
}

static int divide(struct lua_State *L){

    double a = lua_tonumber(L, 1);
    double b = lua_tonumber(L, 2);
    printf("%p\n", L);

    stackDump(L);

    int quot = (int)a / (int)b;
    int rem = (int)a % (int)b;

    lua_pushnumber(L, quot);
    lua_pushnumber(L, rem);

    stackDump(L);
    printf("---end div---\n");

    return 2;
}

int main(void){
    struct lua_State *L = lua_open();
    lua_pushboolean(L, 1);
    lua_pushnumber(L, 10);
    lua_pushnil(L);
    lua_pushstring(L, "hello");

    printf("%p\n", L);

    stackDump(L);

    lua_register(L, "div", divide);
    luaL_dofile(L, "div.lua");

    stackDump(L);
    lua_close(L);
    return 0;
}

div.lua
local c = div(20, 10)

0x100c009e0
true 10 nil ‘hello’
—start div—
0x100c009e0
20 10
20 10 2 0
—end div—
true 10 nil ‘hello’

I see lua_State in divide is the same with the main one, but they have different data in stack, How this be done ?

I know the best way to understand this is to read source code of Lua , maybe you can tell me where to find the right place.

Answer 1

It gets different data the same way anything gets different data: code changes the data inside of the object.

struct Object
{
  int val;
};

void more_stuff(Object *the_data)
{
  //the_data->val has 5 in it now.
}

void do_stuff(Object *the_data)
{
  int old_val = the_data->val;
  the_data->val = 5;
  more_stuff(the_data);
  the_data->val = old_val;
}

int main()
{
  Object my_data;
  my_data.val = 1;

  //my_data.val has 1.
  do_stuff(&my_data);
  //my_data.val still has 1.
}