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 _______________________________________________ systemd-devel mailing list systemd-devel@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/systemd-devel