Messages in this thread | | | Date | Fri, 10 Oct 2008 14:21:57 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [git pull] x86 updates for v2.6.28, phase #1 |
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Linus Torvalds wrote: > > Yeah. If I recall the namespace rules correctly, you have to have a > underscore followed by another underscore or an upper-case letter. Those > cannot even be used for local variables by a conforming program. > > Single-underscore + lower-case letter is only reserved as an external > identifier. I _think_.
Strictly speaking, they are "reserved for identifiers with file scope in both identifier and tag space" (POSIX.1-2004, xsi § 2.2.2, lines 718-).
> So a conforming program could still do > > int main(int argc, char **argv) > { > int _asm_x86_types_h = 1; > return _asm_x86_types_h; > } > > and I _think_ it should be ok.
Correct.
> So a system macro definition that could mess with something (admittedly > crazy as that) has to be either double underscores or underscore + > uppercase, so as long as the prefix is "_ASM" (or _LINUX), we're ok. > > Double underscores in the _middle_ of the identifiers are a non-issue, and > are just ugly.
Double underscores when used in external identifiers (not an issue here) can conflict with some C++ mangling schemes, in particular the old Cfront scheme (which had to be compatible with a standard C compiler.) gcc seems to use another scheme, but it doesn't either appear to include any non-C characters, which means it's at least theoretically possible to step on its toes.
>> That being said, I don't personally like the double underscores. > > I agree, inside the identifier in particular they do seem pointless. At > the beginning, they are useful for things like __i386__ or __x86, where > you need the double underscore because 'i' is lower-case. > > Or maybe I mis-remember the name space rules.
No, you're right on track.
There is an additional rule, which is _de_facto_ since it is out of scope for the standards: identifiers of the form __foo__ (double underscores at the start *and* end) are often considered reserved for the *compiler*.
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