Re: luaL_checkudata for invalid index?

It was thus said that the Great Marc Lepage once stated:
> Apologies for another silly little question.
>
> In C code, for 5.1, if I call luaL_checkudata but the index doesn't exist
> (say I was expecting an arg but none was provided), is that OK (and I get
> expected failure) or is that somehow bad or undefined?

luaL_checkudata() will raise an error if the item at the given stack index
does not exist, or isn't a userdata. This error can be caught by pcall()
(in Lua) or lua_pcall() (in C) but I rarely do that in my code.

> In the implementation, I can see it calls:
>
> lua_touserdata: If the value at the given acceptable index is a full
> userdata, returns its block address. If the value is a light userdata,
> returns its pointer. Otherwise, returns NULL.

But it calls luaL_typerror() when lua_touserdata() returns NULL, and it's
luaL_typerror() that causes the error to be raised (even though
luaL_checkudata() looks as if it returns NULL, that statement is never
reached and exists, as the comments there says, to keep compilers from
complaining).

> So I guess my question is whether, if I was expecting an arg and it wasn't
> provided, is that an "acceptable index?" (Or, if it's not acceptable,
> that's the "otherwise" clause?)

I wouldn't worry about it. Here's some C code [1]:

static int tcclua_define(lua_State *const L)
{
tcc_define_symbol(
*(TCCState **)luaL_checkudata(L,1,TCC_TYPE),
luaL_checkstring(L,2),
luaL_checkstring(L,3)
);
return 0;
}

And if that function is called with no parameters:

[spc]lucy:~>lua
Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
> tcc = require "org.conman.tcc"
> cc = tcc.new()
> cc.define()
stdin:1: bad argument #3 to 'define' (string expected, got no value)
stack traceback:
[C]: in function 'define'
stdin:1: in main chunk
[C]: ?
>

Call it correctly though:

> cc:define("FOO","bar")
>

And everything is fine.

-spc (So, why does it trigger on parameter 3 first? Because of the rules
of C when evaluating parameters of a function---they're evaluated
from right to left. Why that way? I'll leave that as an exercise
for the reader ... )

[1] Part of a Lua interface to TCC [2][3]. Code can be views here
https://github.com/spc476/lua-conmanorg/blob/master/src/tcc.c

[2] http://en.wikipedia.org/wiki/Tiny_C_Compiler

[3] I based the code on the following TCC codebase:
http://repo.or.cz/w/tinycc.git