Best way to omit Lua standard libraries?

Question

What is the best way to remove or omit a Lua standard library package? For example remove the os library functions in a particular environment. The project in question is building Lua from the source files so I can edit the source, although I would rather do it through the API if possible.

Answer 1

See the file luaconf.h in the source kit for easy access to most compile-time configuration such as the actual type used for lua_Number.

See the file linit.c in the source kit for the list of core libraries that are loaded by calling luaL_openlibs().

Common practice is to copy that file to your application's source, and modify it to suit your needs, calling that copy's luaL_openlibs() in place of the core version. If you are compiling Lua privately and not linking to one of the pre-built binaries of the library, then you can find a method to do the equivalent that suits your needs.

Of course, you also don't need to compile or link to the sources for any library (such as os, found in loslib.c) that you choose to leave out of luaL_openlibs().

The only library that you probably can't leave out completely is the base library that provides things like pairs(), ipairs(), pcall(), tostring(), and lots more that can be really inconvenient to do without. When porting to an environment where some of these are problematic, it is usually a good idea to look closely at its implementation in lbaselib.c and either trim features from it or reimplement them to suit your needs.

Edit:

Another approach to including a different list of libraries in the interpreter is to not call luaL_openlibs() at all. Although provided as a convenience, like all of the auxiliary library, luaL_openlibs() is not mandatory. Instead, explicitly open just the libraries you want.

Chapter 5 of the reference manual talks about this:

To have access to these libraries, the C host program should call the luaL_openlibs function, which opens all standard libraries. Alternatively, it can open them individually by calling luaopen_base (for the basic library), luaopen_package (for the package library), luaopen_string (for the string library), luaopen_table (for the table library), luaopen_math (for the mathematical library), luaopen_io (for the I/O library), luaopen_os (for the Operating System library), and luaopen_debug (for the debug library). These functions are declared in lualib.h and should not be called directly: you must call them like any other Lua C function, e.g., by using lua_call.

That last sentence is occasionally the source of trouble, since older versions of Lua did not have that restriction. Each of the individual module's luaopen_xxx() functions follows the same protocol used by the require function. It should be passed a single argument: a string containing the name by which the module is known. The exception is the base module, which is passed an empty string because it has no actual name.

Here's a function that creates a new Lua state and opens only the base and package libraries:

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

lua_State *CreateBasicLua() {
    lua_State *L;

    L = luaL_newstate();
    if (L) {
        lua_pushcfunction(L, luaopen_base);
        lua_pushstring(L, "");
        lua_call(L, 1, 0);
        lua_pushcfunction(L, luaopen_package);
        lua_pushstring(L, LUA_LOADLIBNAME);
        lua_call(L, 1, 0);
    }
    return L;
}

It returns the new lua_State on success, or NULL on failure.

Answer 2

Let's say you only want to open the base and package libraries. In Lua 5.2, the most concise way is

luaL_requiref(L, "_G", luaopen_base, 1);
luaL_requiref(L, "package", luaopen_package, 1);
lua_pop(L, 2);

This is how the luaL_openlibs function in linit.c works, except that it loads everything.

Answer 3

I think that post answers your question, but a little more info on the subject. Be aware you can also just 'nil' entire tables.

So in your example the "os" lib you can do a "os = nil" then poof!, the "os" lib/table is gone. Doing a "os.time()" after this example would return an error for a now nonexistent lib/table. Furthermore if you just wanted to nix the "time" method alone in "os" you could just do "os.time = nil".