Re: [Qemu-devel] CMSG_SPACE() causing compile time error on Mac OS X

[Peter Maydell <peter.maydell@linaro.org>]

On 18 January 2016 at 17:45, Eric Blake <eblake@redhat.com> wrote:
> On 01/17/2016 04:45 PM, Programmingkid wrote:
>>> What's the definition of the CMSG_SPACE macro under OS X?
>>>
>>> Paolo
>>
>> #define       CMSG_SPACE(l)           (__DARWIN_ALIGN32(sizeof(struct cmsghdr)) + __DARWIN_ALIGN32(l))
>
> And the definition of __DARWIN_ALIGN32()?
>
> It looks like the definition is not properly resulting in a compile-time
> constant, and therefore the warning about the initializer is resulting
> because you can't initialize a dynamically-sized array.  But you still
> haven't shown us why the headers are resulting in a non-constant size.

FWIW, on my OSX system (OSX 10.10.5, Xcode 7.1), compiling the test
program with

  gcc -Wall -E  -o /tmp/varray.S /tmp/varray.c

shows that the array definition expands to

    char control[(((__darwin_size_t)((char
*)(__darwin_size_t)(sizeof(struct cmsghdr)) + (sizeof(__uint32_t) -
1)) &~ (sizeof(__uint32_t) - 1)) + ((__darwin_size_t)((char
*)(__darwin_size_t)(sizeof(int) * 16) + (sizeof(__uint32_t) - 1)) &~
(sizeof(__uint32_t) - 1)))] = { 0 };

the relevant macro definitions being
#define __DARWIN_ALIGNBYTES     (sizeof(__darwin_size_t) - 1)
#define       __DARWIN_ALIGN32(p)       ((__darwin_size_t)((char
*)(__darwin_size_t)(p) + __DARWIN_ALIGNBYTES32) &~
__DARWIN_ALIGNBYTES32)


this is with a 'gcc' that gcc --version reports as
Configured with:
--prefix=/Applications/Xcode.app/Contents/Developer/usr
--with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.0 (clang-700.1.76)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

and which claims itself to be 4.2.1 by the printout from the test
program (ie it's clang under the hood).

I don't get any compiler warnings from
  gcc -Wall -O2  -o /tmp/varray /tmp/varray.c

I don't know what the difference between my setup and John's is
(likely an older clang version).

thanks
-- PMM