FFMPEG: undefined reference to `avcodec_register_all' does not link - c++

Question

So I have a very sample code for trying to decode a FFMPEG video stream.
My problem is avcodec does not want to link, to do so I made a clean installation of Ubuntu 13.04. I have build ffmpeg from source following the guide here: https://trac.ffmpeg.org/wiki/CompilationGuide/Ubuntu
I just want to compile my file. Note that my ubuntu does not have any implementations or header files for avcodec. The command line I use is:
gcc -I/home/USER/ffmpeg_build/include -L/home/USER/ffmpeg_build/lib -lavcodec -o test.exe Downloads/auv/src/dronerosvideo/src/ar2.cpp
/tmp/ccKTprFq.o: In function `fetch_and_decode(int, int, bool)':
ar2.cpp:(.text+0x36e):
undefined reference to `avcodec_register_all'
ar2.cpp:(.text+0x378):
undefined reference to `av_log_set_level'
ar2.cpp:(.text+0x382):
undefined reference to `avcodec_find_decoder'
ar2.cpp:(.text+0x3b1):
undefined reference to `avcodec_alloc_context3'
ar2.cpp:(.text+0x3d6):
undefined reference to `avcodec_open2'
ar2.cpp:(.text+0x46d):
undefined reference to `av_init_packet'
ar2.cpp:(.text+0x50a):
undefined reference to `avcodec_decode_video2'
ar2.cpp:(.text+0x534):
undefined reference to `av_free_packet'
/tmp/ccKTprFq.o:(.eh_frame+0x13): undefined reference to
`__gxx_personality_v0'
collect2: error: ld returned 1 exit status
Just for a sane test if I remove the -L argument compiler says:
/usr/bin/ld: cannot find -lavcodec
Which means that the linker finds the library in /home/USER/ffmpeg_build/lib. Also if we check the library for implementation it exists:
nm ffmpeg_build/lib/libavcodec.a | grep "register_all"
0000000000000000 T avcodec_register_all
Also as advised since it is C++ I have exten "C" around the include of the library.
At this point I'm falling out of any ideas at all, why exactly compilation fails?

Answer 1

First, it is C++, so you'll need to use g++ not gcc so that the C++ standard library gets linked. This should get rid of undefined reference to '__gxx_personality_v0'.
Then, the order of libaries to link is actually important.
You'll need to specify a library after the object (or source or other library) using it.
Putting it together, a command line like this works (in my tests):
g++ -o test.exe -I$HOME/ffmpeg/include test.cc -L$HOME/ffmpeg/lib -lavcodec
(Actually, depending on how ffmpeg was built, you might need other libraries as well, like pthreads or libx264)
If you got pkg-config installed, it might be possible to just ask that it for proper clags:
# Since you didn't install ffmpeg to a known location, tell pkg-config about that location.
export PKG_CONFIG_PATH=$HOME/ffmpeg/lib/pkgconfig
g++ -o test.exe $(pkg-config -clags libavcodec) test.cc $(pkg-config -libs libavcodec)