[FFmpeg-devel] avformat_open_input

[Liangcheng Fu]

i'm in the process of studying the frame work of ffmpeg and i encountered a
problem about understanding the source part which in function
"avformat_open_input" in libavformat:

/* check filename in case an image number is expected */

   if (s->iformat->flags & AVFMT_NEEDNUMBER) {

       if (!av_filename_number_test(filename)) {

           ret = AVERROR(EINVAL);

           goto fail;
       }
what does this part mean and why an image number is expected?
any advice will be appreciated!
Thank you!