[FFmpeg-devel] usage of AV_GLUE?

[Ganesh Ajjanagadde]

On Fri, Oct 16, 2015 at 8:44 AM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> Hi,
>
> On Fri, Oct 16, 2015 at 8:03 AM, Ganesh Ajjanagadde <gajjanag at mit.edu>
> wrote:
>
>> On Thu, Oct 15, 2015 at 11:21 PM, Ronald S. Bultje <rsbultje at gmail.com>
>> wrote:
>> > Hi,
>> >
>> > On Thu, Oct 15, 2015 at 8:37 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
>> > wrote:
>> >
>> >> Hi all,
>> >>
>> >> Examining libavutil/macros, there is AV_GLUE/AV_JOIN (why are there
>> >> two?) defined for preprocessor token pasting. However, it is not being
>> >> used anywhere, instead people have use "##". There are a number of
>> >> options regarding this:
>> >> 1. Start using them, and convert the ## usage to AV_GLUE/AV_JOIN.
>> >> 2. Don't bother at all.
>> >> 3. Deprecate and remove them at some point.
>> >>
>> >
>> >  ../libavcodec/flacdsp_lpc_template.c:#define FUNC(n) AV_JOIN(n ## _,
>> > SAMPLE_SIZE)
>> > ../libavcodec/flacdsp_template.c:#   define FSUF   AV_JOIN(SAMPLE_SIZE,
>> p)
>> > ../libavcodec/flacdsp_template.c:#define FUNC(n) AV_JOIN(n ## _, FSUF)
>> > ../libavcodec/h264_mb_template.c:#   define FUNC(n) AV_JOIN(n ##
>> _simple_,
>> > BITS)
>> >
>> > You have to understand _why_ we use this. The problem is that in some
>> cases
>> > (sometimes in a compiler-dependent way, other times more generally), you
>> > need to expand preprocessor tokens before concatenating them. Older
>> > versions of MSVC were bad at this, but you can see gcc suffers from it
>> also
>> > if you look for uses of cat() in
>> > libavcodec/x86/vp9dsp_init_16bpp_template.c. If you change cat into just
>> > ##, it stops working.
>> >
>> > And yes, cat, should probably be AV_JOIN, I forgot about AV_JOIN.
>>
>> Got it, thanks. I still suppose AV_GLUE is redundant (isn't it the
>> same as AV_JOIN in all these use cases?).
>
>
> To force-expand, you sometimes need to call a macro in a macro. Again, see
> cat() - all uses are themselves in macros. Let's say I wanted to use BPC
> directly in a line of actual code - it would fail. By making a macro call
> another macro (catcat, basically), we force-expand preproc tokens and
> everything works as expected.
>
> So I guess AV_JOIN is public and AV_GLUE is an internal implementation
> detail, but since it's in a public header, we gave it a prefix.

Makes sense. Some day if I am terribly bored and have nothing to do, I
may read the ISO C spec on the preprocessor and understand it at a
deeper level.

>
> Ronald
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