> From: avih [mailto:avih...@yahoo.com] > Sent: Wednesday, April 24, 2019 8:16 PM > To: Guo, Yejun <yejun....@intel.com>; FFmpeg development discussions and > patches <ffmpeg-devel@ffmpeg.org> > Subject: Re: [FFmpeg-devel] [PATCH V4 2/2] configure: replace 'pr' with printf > since busybox does not support pr > > > how about add the keyword local which makes the variables not global. > > No. It's not portable and not standard, and will break with a lot of shells > which ffmpeg currently supports. > > Different shells have different ways to limit a variable scope to the current > function, some don't have a way at all, and some limits some variables > automatically (like the value variable in a `for` command). > > There are only two standard ways for limited-scope values: > > - A function's positional parameters are local to each instance (including > each instance in a recursion). > > - A subshell prevents all changes to variables outside of it, with two issues: > 1. Code can't actually modify any variables outside of its subshell... > 2. It can be very expensive where/if (a big if) performance matters. > > There are ways to implement locality of variables while staying portable. In > ffmpeg's configure `pushvar` and `popvar` were used for such functionality > (they still exist in configure but currently unused). Their usage was > replaced with a method based on storing prior values in positional parameter > (in a recursive function) and restoring them later. > > There are also more generic solutions, with varying degrees of complexity and > performance. I wrote such solution myself, and some day I might send a patch > to ffmpeg which incorporates it. > > Till then, there's no generic way to set local variables in `configure` > except the two methods mentioned above, or using a unique enough name. >
thanks a lot for your nice explanation, I'll use the unique enough name. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
