RE: lua_number2int

["Jerome Vuarand" <jerome.vuarand@...>]

Here I get this:

% gcc --version
gcc (GCC) 3.4.4 (mingw special)
Copyright (C) 2004 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

% cat test.c
#include <stdio.h>

int main()
{
	printf("0x%08x\n", (int)(double)0xffffffff);
	printf("0x%08x\n", (int)(unsigned int)(double)0xffffffff);
	printf("%d\n", (int)(double)-4.0);
	printf("%d\n", (int)(unsigned int)(double)-4.0);
	return 0;
}
% make test
% test
0x7fffffff
0xffffffff
-4
0

As you can see with your additionnal unsigned int cast you lose all negative values. That's because (unsigned int)0xffffffff and (int)0xffffffff have two different representations in double precision floating point format. 

> -----Message d'origine-----
> De : lua-bounces@bazar2.conectiva.com.br 
> [mailto:lua-bounces@bazar2.conectiva.com.br] De la part de 
> askok@dnainternet.net
> Envoyé : 14 novembre 2006 07:21
> À : Lua list
> Objet : Re: lua_number2int
> 
> 
> There may be several abnormal things regarding the normal Lua 
> integer handling.
> 
> What I find most weird (met in making the "integer
> patches") is that floating point values are silently 
> converted into integers by 'lua_tointeger()'.  I would 
> consider this a BUG, since if a C routine specifically 
> requests an integer I think it should be provided one. 
> Anyways, that behaviour remains in the "integer patch" so 
> that outside behaviour remains unaltered.
> 
> Could we have this changed, in say 5.2?
> 
> Also "lua_tounsigned()" would be handy.
> 
> -asko
> 
> 
> On Mon, 13 Nov 2006 21:52:56 -0500
>   "Brian Weed" <bw@imaginengine.com> wrote:
> > Today, we converted a project from Lua v5.02 to v5.1.1, and 
> we started 
> >having a problem when converting double to int.
> > I have narrowed my problem down to code that has nothing to do with 
> >Lua.  But I still wanted to get other people's opinions about it.
> > 
> > double d = 0xFFFFFFFF;
> > int n;
> > n = (int)d;
> > 
> > // n now contains 0x80000000
> > 
> > n = (int)(unsigned int)d;
> > 
> > // n now contains 0xFFFFFFFF
> > 
> > (this happens on both VS2005, and GCC 4.0.1 (Apple Computer, Inc. 
> >build 5363))
> > 
> > Since lua_number2int and lua_number2integer both basically 
> do this:  
> >"n = (int)d;"
> > 
> > Wouldn't it be safer to do this: "n = (int)(unsigned 
> int)d;" so as to 
> >preserve the values that would otherwise be truncated ?
> > 
> > What sort of problems would this cause?  Is my fix unportable?
> > 
> > The initial problem we got stems from doing this in Lua:
> > 
> > d = 0xFFFFFFFF
> >Foo(d)
> > 
> > // Assuming Foo() is something like this...
> > 
> > int bind_Foo(lua_State *L)
> > {
> >    Foo(lua_tointeger(L, 1));   // value passed to Foo() 
> >is now
> > truncated to 0x80000000
> > 
> >    return(0);
> > }
> > 
> > Brian
> > 
> > 
> > 
> > 
> 
>