Re: lacking an unsigned MAX macro

[Philipp Janda <siffiejoe@...>]

Am 15.05.19 um 04:02 schröbte Coda Highland:
> On Tue, May 14, 2019 at 8:47 PM Jim <djvaios@gmail.com> wrote:
>
> > > it seems that a MAX macro for LUA_UNSIGNED is not #defined in luaconf.h
> > > so that i had to use ULLONG_MAX from <limits.h> directly instead.
> >
> > you could also use
> >
> > lua_Unsigned umax = ~ 0 ;
> >
> > to determine this upper maximum.
> > this has the advantage that you do not need to know what C integer type
> > is actually used as Lua integer type.
>
> Doesn't that need to be:
>
> lua_Unsigned umax = ~(lua_Unsigned)0;
>
> to cover all of the cases that the C standard supports? It's been a while
> since I've had my head that deep in this, but I know that 0 is of type
> signed int. What I don't remember is if sign extension applies BEFORE or
> AFTER converting to unsigned; that expression might end up being
> 0x00000000FFFFFFFF if it converts to unsigned before doing sign extension.


    lua_Unsigned umax = ~(lua_Unsigned)0;
    lua_Unsigned umax = -1;

are both fine.

    lua_Unsigned umax = ~0;

works for two's complement machines because it is equivalent to the `-1` 
case. In the worst case it might be undefined behavior (on one's 
complement machines which cannot represent negative zeros).

The issue you mentioned above applies to

    lua_Unsigned umax = ~0u;


> /s/ Adam

Philipp