Re: somebody dropped a (warning) bomb

Linus Torvalds
2007-02-09T00:42:57Z

On Thu, 8 Feb 2007, David Rientjes wrote:
>
> Your struct:
>
> struct dummy {
> int flag:1;
> } a_variable;
>
> should expect a_varible.flag to be signed, that's what the int says.

No it's not.

You just don't understand the C language.

And if you don't understand the C language, you can't say "that's what the
int says". It says no such thing.

The C language clearly says that bitfields have implementation-defined
types. So when you see

struct dummy {
int flag:1;
} a_variable;

if you don't read that as "oh, the sign of 'flag' is implementation-
defined", then you simply aren't reading it right.

Is it "intuitive"? Nobody ever really called C an _intuitive_ language. C
has a lot of rules that you simply have to know. The bitfield sign rule is
just one such rule.

> There is no special case here with regard to type.

Sure there is. Read the spec.

I don't understand why you are arguing. YOU ARE WRONG.

Bitfields simply have implementation-defined signedness. As do enums. As
does "char". It really is that simple.

The *real* special case is actually "int" and "long". In many ways, the
fact that those *do* have a well-specified signedness is actually the
exception rather than the rule.

Most C types don't, and some you can't even tell (do pointers generate
"signed" or "unsigned" comparisons? I'll argue that a compiler that
generates signed comparisons for them is broken, but it tends to be
something you can only see with a standards- conforming proghram if you
can allocate memory across the sign boundary, which may or may not be
true..)

Linus