[systemd-devel] compile with clang broken

[Daniele Nicolodi]

On 15/08/2014 16:30, David Herrmann wrote:
> Ok, took me a while, but I now figured out how to cause compilation to
> fail even in expressions that initialize types (_Static_assert is not
> allowed there):
>   #define assert_const(expr)
> ((void)(__builtin_types_compatible_p(int[(expr) ? 1 : -1], int[1])))
> 
> Btw., I like that more than our current assert_cc() fallback. But I
> leave it up to you to decide.
> 
> Anyhow, I found a way to make CONST_MAX work:
> #define CONST_MAX(_A, _B)
> (__builtin_choose_expr(__builtin_constant_p(_A) &&
> __builtin_constant_p(_B), ((_A) > (_B)) ? (_A) : (_B), (void)0))
> 
> This will return (void) in case _A or _B is not constant. Works fine
> on LLVM, I now have to test it on gcc. If it works, I will commit it
> and fix resolvd.

Hello,

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.

In the specific case, the problematic line could then be written as:

uint8_t buffer[CMSG_SPACE(MAXSIZE(struct in_pktinfo,
                                  struct in6_pktinfo)) \
               + EXTRA_CMSG_SPACE];

which IMHO reads a bit better.

Cheers,
Daniele