+++
title = 'FFmpeg development notes'
+++

## Websites

* [ffmpeg.org](https://ffmpeg.org/)
* [code.ffmpeg.org](https://code.ffmpeg.org/FFmpeg/FFmpeg) - code forge
* [trac.ffmpeg.org](https://trac.ffmpeg.org/query?status=!closed&type=defect&order=id&desc=1) - bug tracker, mostly abandoned in favor of forgejo
* [ffmpeg-devel mailing list](https://lists.ffmpeg.org/mailman3/lists/ffmpeg-devel.ffmpeg.org/) - development mailing list, mostly abandoned in favor of forgejo
* [ffmpeg-devel IRC](https://libera.catirclogs.org/ffmpeg-devel/2026-06) - development chat, very active
* [patchwork.ffmpeg.org](https://patchwork.ffmpeg.org/project/ffmpeg/list/) - archive of patches sent to ffmpeg-devel

## Documentation

* Command-line tools, filters, etc: https://ffmpeg.org/ffmpeg-all.html
* Component libraries: https://ffmpeg.org/doxygen/trunk/index.html

## Building

List all compile options:
```sh
$ ./configure --help | vim -
```

Quick development build; takes about 90 seconds on an M1 MacBook Air:
```sh
$ ./configure --disable-optimizations --disable-asm
$ make -j $(nproc)
$ make -j $(sysctl -n hw.logicalcpu)
```

Troubleshooting build:
```sh
$ ./configure --disable-optimizations --assert-level=2 --toolchain=clang-asan
$ ./configure --disable-optimizations --assert-level=2 --toolchain=clang-asan-ubsan
```

`--enable-memory-poisoning` makes `av_malloc` and friends basically just do `memset(ptr, 0x2a, size)`. Does not add any asan-style bounds checking.

## Regression testing

FATE documentation: https://ffmpeg.org/fate.html, TLDR:
```sh
$ ./configure --disable-optimizations --assert-level=2 --toolchain=clang-asan --samples=$HOME/Proj/FFmpeg-FATE
$ make
$ make fate-rsync
$ make fate
```

## Benchmarking

checkasm docs: https://checkasm.videolan.me/

Build within FFmpeg:
```sh
$ make checkasm
```

List tests:
```sh
$ ./tests/checkasm/checkasm --list-tests | sort | vim -

```

The easiest way to see what test calls what functions is to check the source code:
<br>
[`-> tests/checkasm/checkasm.c`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/branch/master/tests/checkasm/checkasm.c).

Example:
```sh
$ ./tests/checkasm/checkasm --bench --test=vf_bwdif
```
<details>
<summary>Output</summary>

```
checkasm:
 - CPU: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (000906E9)
 - Timing source: x86 (rdtsc)
 - Bench duration: 1000 µs per function (4003730 cycles)
 - Random seed: 2684063072
C:
 - vf_bwdif.bwdif8 [OK]
 - vf_bwdif.bwdif10 [OK]
 - vf_bwdif.bwdif8.line3 [OK]
 - vf_bwdif.bwdif8.edge  [OK]
 - vf_bwdif.bwdif8.intra [OK]
SSE2:
 - vf_bwdif.bwdif8       [OK]
 - vf_bwdif.bwdif10      [OK]
SSSE3:
 - vf_bwdif.bwdif8       [OK]
 - vf_bwdif.bwdif10      [OK]
AVX2:
 - vf_bwdif.bwdif8       [OK]
 - vf_bwdif.bwdif10      [OK]
checkasm: all 6 tests passed
Benchmark results:
  name                       cycles (vs ref)
  bwdif8_c:                 28876.2
  bwdif8_sse2:               1056.1 (25.12x)
  bwdif8_ssse3:               870.5 (22.92x)
  bwdif8_avx2:                550.6 (40.40x)
  bwdif8.edge.s0.p0_c:       1373.2
  bwdif8.edge.s0.p1_c:       1310.2
  bwdif8.edge.s1.p0_c:       5368.5
  bwdif8.edge.s1.p1_c:       5824.8
  bwdif8.intra_c:             584.1
  bwdif8.line3.rnd.p0_c:    40020.9
  bwdif8.line3.rnd.p1_c:    30286.6
  bwdif10_c:                27988.9
  bwdif10_sse2:               935.6 (27.51x)
  bwdif10_ssse3:              864.2 (27.04x)
  bwdif10_avx2:               594.3 (36.53x)
```

</details>

<!-- ## libavformat architecture

Note: This is all based off of FFmpeg 8.1.2 (commit `38b88335f99e76ed89ff3c93f877fdefce736c13`)

Stepping through [`doc/examples/remux.c`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/doc/examples/remux.c):

<style>
.ffstackframe {
	border-left: 2px solid #5555FF;
	border-top: 1px solid #AAAAAA;
	border-bottom: 1px solid #AAAAAA;
	padding-left: 20px;
}
.ffstackframe > * {
	margin-top: 0;
	margin-bottom: 0;
}
</style>

<div class="ffstackframe">

[`doc/examples/remux.c:75`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/doc/examples/remux.c#L75):
`main` calls `avformat_open_input`.


<div class="ffstackframe">

[`libavformat/demux.c:241`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L241):
`avformat_open_input` calls `avformat_alloc_context` to allocate memory.

[`libavformat/demux.c:266`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L266):
`avformat_open_input` calls `init_input`.

<div class="ffstackframe">

[`libavformat/demux.c:185`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L185):
`init_input` calls `av_probe_input_buffer2`, which reads the first 2048 bytes of the input file
and passes it to `av_probe_input_format2`, which calls `avprobe_input_format3`.

<div class="ffstackframe">

[`libavformat/format.c:175`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/format.c#L175):
`av_probe_input_format3` attemptes to locate id3 tags within the buffer

[`libavformat/format.c:191`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/format.c#L191):
`av_probe_input_format3` iterates over each registered `FFInputFormat`, and calls the `probe`
function pointer, if present. `probe` takes the buffer and returns a score that represents how
likely the data matches that format. Some other factors are taken into account, such as the
extension of the file and what id3 tags were found.

The most likely `AVInputFormat *` (composite class in `FFInputFormat`) is returned if found,
otherwise `NULL`.

</div>

[`libavformat/demux.c:305`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L305):
`init_input` allocates any private data required by the `FFInputFormat`.

[`libavformat/demux.c:322`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L322):
`init_input` calls the `read_header` function pointer, if present.

<div class="ffstackframe">

`read_header` will typically call `avformat_new_stream` to allocate streams.

</div>

[`libavformat/demux.c:355`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L355):
`init_input` calls the `update_stream_avctx`, which updates contexts for each stream. I am not sure
why this is needed since no codecs should be open at this point, only demuxing.

<div class="ffstackframe">

[`libavformat/demux.c:194`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/libavformat/demux.c#L194):
`update_stream_avctx` iterates over each detected stream and calls `avcodec_parameters_to_context`
if `FFStream->need_context_update == 1`.

</div>
</div>

To summarize, `avformat_open_input` probes the file to detect the container, initializes the
demuxer instance, and reads the file header.

</div>

[`doc/examples/remux.c:80`](https://code.ffmpeg.org/FFmpeg/FFmpeg/src/commit/38b88335f99e76ed89ff3c93f877fdefce736c13/doc/examples/remux.c#L80):
`main` calls `avformat_find_stream_info`.

</div> -->