[FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Andreas Rheinhardt]

Martin Storsjö:
> On Thu, 21 Mar 2024, Andreas Rheinhardt wrote:
> 
>> Andreas Rheinhardt:
>>> C11 provides static assertions via _Static_assert and
>>> provides static_assert as a convenience define for this
>>> in assert.h. MSVC 19.27 declares support for C11, but does
>>> not support _Static_assert, but somehow supports
>>> static_assert. That's therefore what we use.
>>>
>>> But apparently there are some old GCC toolchains where
>>> _Static_assert is supported, but assert.h does not provide
>>> the fallback define. Some fate boxes are affected by this
>>> [1].
>>>
>>> This commit therefore checks whether static_assert works
>>> with assert.h included; if not, it errors out. Users like
>>> the above can still add -Dstatic_assert=_Static_assert
>>> to cflags as a workaround.
>>>
>>> [1]:
>>> https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7
>>>
>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>>> ---
>>> This is what a test without fallback looks like.
>>> Posted to gather opinions on what people prefer.
>>>
>>>  configure | 13 +++++++++++++
>>>  1 file changed, 13 insertions(+)
>>>
>>> diff --git a/configure b/configure
>>> index 6d7b33b0ff..c2d2c70c20 100755
>>> --- a/configure
>>> +++ b/configure
>>> @@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h
>>> "__cplusplus >= 201103L" ||
>>>  check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
>>>      { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L"
>>> && stdc="c11" || die "Compiler lacks C11 support"; }
>>>
>>> +test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
>>> +#include <assert.h>
>>> +#include <stddef.h>
>>> +struct Foo {
>>> +    int a;
>>> +    void *ptr;
>>> +} obj;
>>> +static_assert(offsetof(struct Foo, a) == 0,
>>> +              "First element of struct does not have offset 0");
>>> +static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) +
>>> sizeof(obj.a),
>>> +              "elements not properly ordered in struct");
>>> +EOF
>>> +
>>>  check_cppflags -D_FILE_OFFSET_BITS=64
>>>  check_cppflags -D_LARGEFILE_SOURCE
>>>
>>
>> Jan has tested old toolchains and found out that his GCC 4.7 has proper
>> C11 headers; so this seems to be unique to Michael's setup. This makes
>> me prefer this patch instead of the version with the fallback. (Michael
>> can simply add -Dstatic_assert=_Static_assert to his cflags.)
>> Of course others are still invited to share their opinions.
> 
> Both patches seem to work fine with MSVC 19.27 - I vaguely prefer the v2
> version, which is simpler.
> 
> 
> But to me, we could also just revert the change to
> libavcodec/ccaption_dec.c, and declare that we require MSVC 19.28
> instead. MSVC 19.27, when executed with -std:c11 without -nologo, it
> prints this:
> 
>     /std:c11 is a preview implementation of the ISO C11 standard, and
>     we're eager to hear about bugs and suggestions for improvements.
>     However, note that these features are provided as-is without support.
> 
> And I don't have any specific reasons for wanting to use this compiler -
> I just tested the lowest version that was supposed to be supported
> earlier and noted that it had broken recently. So to me, reverting to
> requiring _Static_assert would be quite ok as well.
> 

We can actually do both: Test for static_assert and for _Static_assert
(to exclude MSVC 19.27; is 19.28 still supposed to be a preview
implementation?).
The reason I prefer static_assert in the codebase is that _Static_assert
is actually deprecated with C23 (although I don't think it will be
removed any time).

- Andreas