Re: multiple global contexts

[Peter Shook <pshook@...>]

Dylan Cuthbert wrote:
> I may have read the documentation wrong, but I don't like the way lua lets
> all lua co-routines/functions have access to the global scope.
>
> Is there a way to have multiple global scopes, so I don't have to worry
> about script writers breaking other people's data or accessing stuff they
> shouldn't?  I'd like to keep these "global" scopes local to each object (and
> have multiple coroutines running within the same "space" on an object).
>
> Also, if this is possible, what kind of overhead (memory footprint/creation
> time) does the lua_state structure have? (I presume I'll need multiples of
> these)

Hi Dylan,

The lua_State structure is 96 bytes when compiled with gcc on a PC.

You can use lua_replace(L, LUA_GLOBALSINDEX);
to change the globals for a thread.
Here is an example:

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

static int report (lua_State *L, int status) {
  const char *msg;
  if (status) {
    msg = lua_tostring(L, -1);
    if (msg == NULL) msg = "(error with no message)";
    fprintf(stderr, "%s\n", msg);
    lua_pop(L, 1);
  }
  return status;
}

static int fixglobals (lua_State *L) {
  lua_newtable(L); /* new table for globals */
  lua_newtable(L); /* metatable for new globals */
  lua_pushliteral(L, "__index");
  lua_pushvalue(L, LUA_GLOBALSINDEX);  /* __index tries old common 
globals */
  lua_settable(L, -3);
  lua_setmetatable(L, -2);
  lua_replace(L, LUA_GLOBALSINDEX);
}

int main(int argc, char *argv[])
{
  lua_State *L = lua_open();

  luaopen_base(L);
  luaopen_table(L);
  luaopen_io(L);
  luaopen_string(L);
  luaopen_math(L);
  luaopen_debug(L);

  if(argc>2) {
    int status, i;
    lua_State *C1 = lua_newthread(L);
    lua_State *C2 = lua_newthread(L);

    fixglobals(C1);
    lua_pushliteral(C1, "c1");
    lua_setglobal(C1, "name");
    lua_pushnumber(C1, 2);
    lua_setglobal(C1, "N");

    fixglobals(C2);
    lua_pushliteral(C2, "c2");
    lua_setglobal(C2, "name");
    lua_pushnumber(C2, 3);
    lua_setglobal(C2, "N");

    printf("loading %s\n", argv[1]);
    if (report(C1, luaL_loadfile(C1, argv[1]) )) exit(1);
    printf("loading %s\n", argv[2]);
    if (report(C2, luaL_loadfile(C2, argv[2]) )) exit(2);

    printf("start looping\n");
    for (i=0; i<10; i++) {
      if (report(C1, lua_resume(C1, 0) )) exit(3);
      if (report(C2, lua_resume(C2, 0) )) exit(4);
      printf("i = %d\n", i);
    }
  }

  lua_setgcthreshold(L, 0);  /* collected garbage */
  lua_close(L);
  return 0;
}

$ gcc -g -o test coroutine.c  -Llib -llua -llualib

$ cat c2.lua

print('env', name, getfenv())

not_dead_yet = true

while not_dead_yet do
  for i = 1,N do
    print(name, i)
    coroutine.yield(i)
  end
end

$ ./test c2.lua c2.lua
loading c2.lua
loading c2.lua
start looping
env     c1      table: 0xa045980
c1      1
env     c2      table: 0xa045a60
c2      1
i = 0
c1      2
c2      2
i = 1
c1      1
c2      3
i = 2
c1      2
c2      1
i = 3
c1      1
c2      2
i = 4
c1      2
c2      3
i = 5
c1      1
c2      1
i = 6
c1      2
c2      2
i = 7
c1      1
c2      3
i = 8
c1      2
c2      1
i = 9