pkt_size on UDP output not fixed lenght

[Antonio Jose Tomas Martinez]

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);
>

[Carl Eugen Hoyos]

If you have a patch fixing this issue, please send it - attaching the result of git format-patch - to the FFmpeg development mailing list, patches are ignored on the bug tracker.
To make this a valid bug report, please add the command line you tested together with the complete, uncut console output.

[Antonio Jose Tomas Martinez]

I forked it and patched on ​https://github.com/vidanio/FFmpeg/commit/860098efada8ea55bfb81cf1ac872c639e290580
The commandline I used it, is written above.

ffmpeg -re -i file.ts -c copy -f mpegts udp://127.0.0.1:1234?pkt_size=1316

I will try to submit it to dev mail list.

[Carl Eugen Hoyos]

The only way to get a patch into FFmpeg mainline is to send it - made with git format-patch - to the development mailing list.
If you - instead of sending a patch - want to make this a valid ticket, please add the command line you tested together with the complete, uncut console output.

[Marton Balint]

This is not a bug. If you want fixed packet size, you should use the -flush_packets 0 option.