On Fri, Oct 16, 2015 at 8:44 AM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > Hi, > > On Fri, Oct 16, 2015 at 8:03 AM, Ganesh Ajjanagadde <gajja...@mit.edu> > wrote: > >> On Thu, Oct 15, 2015 at 11:21 PM, Ronald S. Bultje <rsbul...@gmail.com> >> wrote: >> > Hi, >> > >> > On Thu, Oct 15, 2015 at 8:37 PM, Ganesh Ajjanagadde <gajja...@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 > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
