[systemd-devel] compile with clang broken

David Herrmann dh.herrmann at gmail.com
Fri Aug 22 05:02:23 PDT 2014


On Sat, Aug 16, 2014 at 1:29 PM, Daniele Nicolodi <daniele at grinta.net> wrote:
> On 16/08/2014 12:35, David Herrmann wrote:
>> On Fri, Aug 15, 2014 at 5:22 PM, Daniele Nicolodi <daniele at grinta.net> wrote:
>>> this may be completely stupid, but if the only use case you have for
>>> CONST_MAX() is for computing the size of a data structure, I find
>>> something like
>>> #define MAXSIZE(A, B) sizeof(union { __typeof(A) a; __typeof(B) b;})
>>> a little more clear and less magic, and I believe it has the same
>>> guarantees that the solution you found.
>> Your MAXSIZE macro might add padding:
>> This union has size 8, not 5 (64bit). But CONST_MAX would return 5.
>> Not sure whether that really matters, though. And we could probably
>> add __packed__ to the definition.
> Indeed it does add padding. Adding the __packed__ attribute solves the
> problem:
> #define MAXSIZE(A, B) sizeof(           \
>    union __attribute__((__packed__)) {  \
>       __typeof(A) a; __typeof(B) b;})
>> However, I noticed that GCC complains about using
>> statement-expressions to initialize static-const structure members,
>> even with my 'const' annotations added to MAX. *sigh*
>> Thus, I think I'll keep CONST_MAX, as we'd require a 3rd macro otherwise.
> My proposal was based on the fact that the only use of CONST_MAX there
> is (was?) in the code was about array size declarations, and I find
> MAXSIZE() much easier to understand.

I've added the macro now.


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