Lua C API - loading multiple files with variables and functions of the same names

Question

Say I have two Lua files which I'll be using from the standard Lua C API, that share a common library:

common.lua

function printHello(name)
    print("Hello from " .. name)
end

file1.lua

require "common"
local scriptName = "file1"

function doSomething()
    printHello(scriptName)
end

file2.lua

require "common"
local scriptName = "file2"

function doSomething()
    printHello(scriptName)
end

Now say I want to have both file*.lua files share the same lua_State. Without changing any of the Lua code, how can I load the files in way such that I can call a specific doSomething()?

Is there a way I can move "everything" from the loaded files (functions, variables, tables) it into a global table within the lua_State using the script-name (or whatever) as the key? Also, is there a way I can do this such that file1.lua and file2.lua can share the "in memory" version of common.lua?

I am using Lua 5.1.

Thanks!

The way you'd do this changed a lot between Lua 5.1 and 5.2. What version of Lua are you using exactly? — Joseph Sible-Reinstate Monica, May 08 '20 at 16:53
@JosephSible-ReinstateMonica 5.1. I've updated the question as well. — Addy, May 08 '20 at 16:54
Your life will be much easier if you define local functions and return them instead of defining global variables and relying on games with the global environment. — lhf, May 08 '20 at 17:00
You can give separate `_G` to each `file*.lua` by implementing your own `require`. Both `_G1` and `_G2` would inherit standard Lua globals and globals created by `common.lua` — Egor Skriptunoff, May 08 '20 at 17:00
@JosephSible-ReinstateMonica Coming back to this a couple months later, how would this be done in Lua 5.2? I can create a new question if you'd prefer. — Addy, Aug 28 '20 at 22:07

Joseph Sible-Reinstate Monica · Accepted Answer · 2020-05-10T03:02:47.483

Here's how you do it in pure Lua 5.1:

file1_env = setmetatable({}, {__index = _G})
local file1_chunk = loadfile('file1.lua')
setfenv(file1_chunk, file1_env)
file1_chunk()

file2_env = setmetatable({}, {__index = _G})
local file2_chunk = loadfile('file2.lua')
setfenv(file2_chunk, file2_env)
file2_chunk()

file1_env.doSomething() -- prints "Hello from file1"
file2_env.doSomething() -- prints "Hello from file2"

What's going on is you're changing the environment that each file's chunk runs in, so instead of putting their doSomething functions in the global environment and thus stomping each other, they go in their own local environment (which uses a metatable so they can use things that are in the global environment like require and print). And as requested, common.lua only has to run once, as you'll see if you put something like printHello('common') at the end of it.

If you want to do this from C, all of the functionality I used can be straightforwardly converted to the C API, like this:

#include <lua5.1/lua.h>
#include <lua5.1/lualib.h>
#include <lua5.1/lauxlib.h>

int main(void) {
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);

    /* stack is empty */
    lua_createtable(L, 0, 1);
    /* -1: file1_env */
    lua_createtable(L, 0, 1);
    /* -2: file1_env, -1: file1_env_mt */
    lua_pushvalue(L, LUA_GLOBALSINDEX);
    /* -3: file1_env, -2: file1_env_mt, -1: _G */
    lua_setfield(L, -2, "__index");
    /* -2: file1_env, -1: file1_env_mt */
    lua_setmetatable(L, -2);
    /* -1: file1_env */
    luaL_loadfile(L, "file1.lua");
    /* -2: file1_env, -1: file1_chunk */
    lua_pushvalue(L, -2);
    /* -3: file1_env, -2: file1_chunk, -1: file1_env */
    lua_setfenv(L, -2);
    /* -2: file1_env, -1: file1_chunk */
    lua_call(L, 0, 0);
    /* -1: file1_env */
    lua_setglobal(L, "file1_env");
    /* stack is empty */
    lua_createtable(L, 0, 1);
    /* -1: file2_env */
    lua_createtable(L, 0, 1);
    /* -2: file2_env, -1: file2_env_mt */
    lua_pushvalue(L, LUA_GLOBALSINDEX);
    /* -3: file2_env, -2: file2_env_mt, -1: _G */
    lua_setfield(L, -2, "__index");
    /* -2: file2_env, -1: file2_env_mt */
    lua_setmetatable(L, -2);
    /* -1: file2_env */
    luaL_loadfile(L, "file2.lua");
    /* -2: file2_env, -1: file2_chunk */
    lua_pushvalue(L, -2);
    /* -3: file2_env, -2: file2_chunk, -1: file2_env */
    lua_setfenv(L, -2);
    /* -2: file2_env, -1: file2_chunk */
    lua_call(L, 0, 0);
    /* -1: file2_env */
    lua_setglobal(L, "file2_env");
    /* stack is empty */
    lua_getglobal(L, "file1_env");
    /* -1: file1_env */
    lua_getfield(L, -1, "doSomething");
    /* -2: file1_env, -1: file1_env.doSomething */
    lua_call(L, 0, 0);
    /* -1: file1_env */
    lua_pop(L, 1);
    /* stack is empty */
    lua_getglobal(L, "file2_env");
    /* -1: file2_env */
    lua_getfield(L, -1, "doSomething");
    /* -2: file2_env, -1: file2_env.doSomething */
    lua_call(L, 0, 0);
    /* -1: file2_env */
    lua_pop(L, 1);
    /* stack is empty */

    lua_close(L);
    return 0;
}

In the C API, should I save the corresponding `file1_env` and `file2_env1` into a global table so I can retrieve them as needed? — Addy, May 08 '20 at 17:40
I don't understand how `local file1_env = setmetatable({}, {__index = _G})` and `local file1_env = setmetatable({}, {__index = _G})` translate to the C API? How would I load a file into a variable via the C API? — Addy, May 09 '20 at 11:41

Lua C API - loading multiple files with variables and functions of the same names

1 Answers1