[FFmpeg-trac] #7710(avformat:new): pkt_size on UDP output not fixed lenght

[FFmpeg]

#7710: pkt_size on UDP output not fixed lenght
-------------------------------------+-------------------------------------
             Reporter:  DVBInside    |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avformat     |                  Version:  git-
             Keywords:  pkt_size     |  master
  fixed length                       |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 When you want to output a fixed lenght of UDP packets at say 1316 bytes
 (7*TS) using the UDP option pkt_size, the output is limited to 1316 bytes
 but not fixed to that size.
 How to reproduce:
 {{{
 % ffmpeg -re -i file.ts -c copy -f mpegts
 udp://127.0.0.1:1234?pkt_size=1316
 ffmpeg version N-93057-g860098efad Copyright (c) 2000-2019 the FFmpeg
 developers
   built with gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
   configuration: --prefix=/usr
   libavutil      56. 26.100 / 56. 26.100
   libavcodec     58. 46.100 / 58. 46.100
   libavformat    58. 26.100 / 58. 26.100
   libavdevice    58.  6.101 / 58.  6.101
   libavfilter     7. 48.100 /  7. 48.100
   libswscale      5.  4.100 /  5.  4.100
   libswresample   3.  4.100 /  3.  4.100}}}

 tcpdump will show this:
 tcpdump -i lo0 udp port 1234
 >
 14:09:49.247075 IP 192.168.1.64.57226 > 127.0.0.1.1234: UDP, length 1316
 14:09:49.247156 IP 192.168.1.64.57226 > 127.0.0.1.1234: UDP, length 1316
 14:09:49.247314 IP 192.168.1.64.57226 > 127.0.0.1.1234: UDP, length 564
 14:09:49.247490 IP 192.168.1.64.57226 > 127.0.0.1.1234: UDP, length 1316
 14:09:49.247680 IP 192.168.1.64.57226 > 127.0.0.1.1234: UDP, length 1316
 14:09:49.247915 IP 192.168.1.64.57226 > 127.0.0.1.1234: UDP, length 752
 >

 I guess this can be fixed stably just changing:
 libavformat/aviobuf.c#241
 >
 - flush_buffer(s);
 + if (!s->max_packet_size || s->buf_ptr - s->buffer >= s-max_packet_size)
 flush_buffer(s);
 >

--
Ticket URL: <https://trac.ffmpeg.org/ticket/7710>
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