Re: [Xen-devel] [PATCH 1/6] xen/build: Allow the use of C freestanding headers

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/06/16 14:12, Tim Deegan wrote:
> At 12:24 +0100 on 22 Jun (1466598248), Andrew Cooper wrote:
>> The C standard defines two types of conforming implementation; hosted and
>> freestanding.  A subset of the standard headers are the freestanding headers,
>> requiring no library support at all to use, and therefore usable by Xen.
>>
>> Unfortunately, -nostdinc is an overly large switch, and there is no
>> alternative to only permit inclusion of the freestanding headers.  Removing 
>> it
>> is unfortunate, as we lose the protection it offers, but anyone who does try
>> to use other parts of the standard library will still fail to link.
> I'm afraid I don't think this is a good idea:
>  - Leaving the standard include path around in the Xen build means
>    that the build may differ based on what (unrelated) libraries are
>    installed on the build machines.
>  - There are plenty of ways for an unexpected header to break things
>    that don't fail at link time, e.g. macros and inlines.
>  - "Freestanding" headers can bring in quite a lot of unrelated cruft.
>    See Jan's email about linux/glibc, and I remember seeing similar
>    things on solaris and *BSD when I tidied up stdarg.h. E.g. looking
>    at two machines I'm working on today, on one of them,
>    #include <limits.h> defines __packed, and on the other it does not.
>
> Since what we have already works fine for all the compilers we
> support, I think it ain't broke and we shouldn't fix it.

Except it is broken.

We cannot expect to use -Wformat and not the compiler provided
inttypes.h.  The sizes of constructs like "long long" are implementation
defined, not spec defined.  The compiler is perfectly free to choose
something which doesn't match our inttypes.h, and we would be in the
wrong when it fails to compile.

~Andrew

>
> Nacked-by: Tim Deegan <tim@xxxxxxx>
>
> OTOH, I am in favour of s/bool_t/bool/g, at least wherever field size
> is not an issue, and I can see an argument for moving the type and
> limits definitions into files called stdint.h and limits.h.
>
> Cheers,
>
> Tim.
>
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