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

[Andreas Rheinhardt]

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.

- Andreas