[FFmpeg-devel] debuggin ffmpeg

[Maxim Polijakowski]

Am 18.02.2014 14:05, schrieb Wolfgang Haupt:
>
> On 02/18/2014 01:37 PM, Maxim Polijakowski wrote:
>> Am 18.02.2014 11:41, schrieb Wolfgang Haupt:
>>> Hi ffmpeg-devs,
>>>
>>> I'm quite new to ffmpeg just trying to figure out how the mpegts 
>>> demuxer works in detail.
>>> Therefore I started debugging into ffprobe (-show_frames).
>>> However I wonder how you guys do debugging because every second 
>>> variable is optimized out.
>>> I guess optimizing is rather important in ffmpeg, so I'd like to ask 
>>> if it's safe to disable optimizing for debug builds?
>>
>> I debug the ffmpeg executable quite often. Below the description of 
>> how I do that:
>>
>> 1) obtain the debug build by running
>>
>> cd [path/to/your/ffmpeg/local/clone]
>> ./configure --enable-debug=3 --disable-optimizations 
>> --disable-inline-asm
>> make
>>
>> 2) fire up GDB and type in the following commands:
>>
>> gdb [path/to/ffmpeg/executable]
>> set args -i [path/to/input/file] test.avi [or some other output file 
>> or null]
>> break libavformat/mpegts.c:135 [or some other line]
>> run
>>
>> Surely, you can do the same with ffprobe and use another debugger...
>>
>> Best regards
>> Maxim
>> _______________________________________________
>> ffmpeg-devel mailing list
>> ffmpeg-devel at ffmpeg.org
>> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>>
>>
>
> Thx all for your hints, I use gdb in exact the way you stated.
> It's just the configure parameters I wasn't sure about, because I once 
> tried debugging on an ARM device with compiled NEON and a dev told me 
> to better not disable optimizations there.
> I guess ARM NEON is a very special case and I'm working on x64_64 
> right now.
> That's basically why I wanted to ask here, how ffmpeg devs do debugging.

Yes, I understand your interest. Debugging multimedia applications is 
not a trivial task due to huge amount of data being transfered back and 
forth. Therefore, one need to exactly know what to search for before 
starting to debug.

I use several debugging approaches when dealing with codecs or formats:
--> write special purpose verbose functions using "av_log", 
"av_hex_dump", "av_pkt_dump" etc. in order to print out relevant information
--> utilize various hash and checksum algorithms for comparing big data 
chunks
--> write special purpose GDB scripts for dumping internals of binary 
executables if their source code isn't available
--> use scriptable emulators when working with executables for platforms 
other than x86/x64_64

A common example of such an approach would be debugging a particular 
video frame. Neither ffmpeg nor GDB will stop automatically anywhere you 
want. You need to either
rewind to the desired frame manually or do smth like that:

if (avctx->frame_number == 1333)
     av_log(avctx, AV_LOG_ERROR, "Stopping at frame: %d\n", 
avctx->frame_number);

Add this to the "xxx_decode_frame" method of your demuxer or decoder, 
recompile the project, fire up your debugger, set the break point at the 
2nd line of the above mentioned code and you will fall into the debugger 
as far the desired frame has been reached...

Best regards
maxim