A Lua module loads symbol from global namespace

builtin.c

#define LUA_LIB
#include <lua.h>
#include <lauxlib.h>

#include <string.h>

#define LUA_OFSEP  "_"
#define LUA_IGMARK "-"
#define LUA_POF    "luaopen_"


#ifdef _WIN32

#include <Windows.h>

static void pusherror (lua_State *L) {
    int error = GetLastError();
    char buffer[128];
    if (FormatMessageA(FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_FROM_SYSTEM,
                NULL, error, 0, buffer, sizeof(buffer)/sizeof(char), NULL))
        lua_pushstring(L, buffer);
    else
        lua_pushfstring(L, "system error %d\n", error);
}

static lua_CFunction lsys_sym (lua_State *L, const char *sym) {
    HMODULE lib = GetModuleHandle(NULL);
    lua_CFunction f = (lua_CFunction)GetProcAddress(lib, sym);
    if (f == NULL) pusherror(L);
    return f;
}

#else

#include <dlfcn.h>

static lua_CFunction lsys_sym (lua_State *L, const char *sym) {
    void *lib = dlopen(NULL, RTLD_NOW|RTLD_GLOBAL);
    lua_CFunction f = (lua_CFunction)dlsym(lib, sym);
    dlclose(lib);
    if (f == NULL) lua_pushstring(L, dlerror());
    return f;
}

#endif

static int lookforfunc (lua_State *L, const char *sym) {
    lua_CFunction f = lsys_sym(L, sym);
    if (f == NULL) return 0;
    lua_pushcfunction(L, f);
    return 1;
}

static int loadfunc (lua_State *L, const char *modname) {
    const char *openfunc;
    const char *mark;
    modname = luaL_gsub(L, modname, ".", LUA_OFSEP);
    mark = strchr(modname, *LUA_IGMARK);
    if (mark) {
        int stat;
        openfunc = lua_pushlstring(L, modname, mark - modname);
        openfunc = lua_pushfstring(L, LUA_POF"%s", openfunc);
        stat = lookforfunc(L, openfunc);
        if (!stat) return stat;
        modname = mark + 1;  /* else go ahead and try old-style name */
    }
    openfunc = lua_pushfstring(L, LUA_POF"%s", modname);
    return lookforfunc(L, openfunc);
}

static int searcher_Cbuiltin (lua_State *L) {
    const char *name = luaL_checkstring(L, 1);
    if (loadfunc(L, name)) {
        lua_pushstring(L, "(builtin)");
        return 2;
    }
    lua_pushfstring(L, "\n\tno module '%s' in (builtin)",
            lua_tostring(L, 1));
    return 1;
}

LUALIB_API int luaopen_builtin(lua_State *L) {
    lua_getglobal(L, "package");
    if (lua_isnil(L, -1))
        return luaL_error(L, "can not find global 'package' module");
    lua_getfield(L, -1, "searchers");
    if (lua_isnil(L, -1)) {
        lua_getfield(L, -1, "loaders");
        if (lua_isnil(L, -1))
            return luaL_error(L, "can not find 'searchers' or 'loaders' in 'package'");
    }
    lua_pushcfunction(L, searcher_Cbuiltin);
    lua_rawseti(L, -2, lua_rawlen(L, -2) + 1);
    return 0;
}

/* win32cc: flags+='-s -O3 -mdll -DLUA_BUILD_AS_DLL'
 * win32cc: output='builtin.dll' libs+='-llua53'
 * unixcc: flags+='-s -O3 -shared -fPIC' output='builtin.so' */