Error building app: Undefined symbols for architecture armv7 - iphone

Question

I am having an issue trying to build my program, which is using a C++ class to dela with FFmpeg objects:
Undefined symbols for architecture armv7:
"__Z13avcodec_open2P14AVCodecContextP7AVCodecPP12AVDictionary", referenced from:
__Z10openStreamPKcb in RTPReader.o
"__Z21avcodec_decode_audio4P14AVCodecContextP7AVFramePiP8AVPacket", referenced from:
__Z13getStreamDataRPhRi in RTPReader.o
"__Z20avformat_close_inputPP15AVFormatContext", referenced from:
__Z11closeStreamv in RTPReader.o
"__Z21avformat_network_initv", referenced from:
__Z10openStreamPKcb in RTPReader.o
"__Z26avcodec_get_frame_defaultsP7AVFrame", referenced from:
__Z13getStreamDataRPhRi in RTPReader.o
"__Z15av_register_allv", referenced from:
__Z10openStreamPKcb in RTPReader.o
"__Z19av_find_best_streamP15AVFormatContext11AVMediaTypeiiPP7AVCodeci", referenced from:
__Z10openStreamPKcb in RTPReader.o
"__Z14av_free_packetP8AVPacket", referenced from:
__Z13getStreamDataRPhRi in RTPReader.o
__Z11closeStreamv in RTPReader.o
"__Z6av_logPviPKcz", referenced from:
__Z10openStreamPKcb in RTPReader.o
__Z13getStreamDataRPhRi in RTPReader.o
"__Z19avformat_open_inputPP15AVFormatContextPKcP13AVInputFormatPP12AVDictionary", referenced from:
__Z10openStreamPKcb in RTPReader.o
"__Z26av_samples_get_buffer_sizePiii14AVSampleFormati", referenced from:
__Z13getStreamDataRPhRi in RTPReader.o
"__Z19avcodec_alloc_framev", referenced from:
__Z13getStreamDataRPhRi in RTPReader.o
"__Z25avformat_find_stream_infoP15AVFormatContextPP12AVDictionary", referenced from:
__Z10openStreamPKcb in RTPReader.o
"__Z13av_read_frameP15AVFormatContextP8AVPacket", referenced from:
__Z13getStreamDataRPhRi in RTPReader.o
ld: symbol(s) not found for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Somehow it is adding a prefix _Z## to all my C++ methods, and I don't know why.
What could be the issue?

Answer 1

Al the __Z## stuff you see, is normal. It is called 'name mangling' in C++, see http://en.wikipedia.org/wiki/Name_mangling for a good explanation.
I have to make a couple of guesses here, lacking more detailed info (sorry, I cannot comment yet, only answer). Looking at the ffmpeg api docs, it seems that FFmpeg has a C API, not C++. If that is indeed the case, your solution may be to wrap all #includes of FFmpeg files with extern "C", for example (see http://developers.sun.com/solaris/articles/mixing.html for explanation):
extern "C" {
#include <someffmpegheader.h>
}
As for the reason the link fails, are you linking against a pre-built ffmpeg library, or are you including the ffmpeg source files in your project? If you're linking to a library, it may not have the correct architectures. If you run the following command in a terminal window, it will tell you what architectures are available in the library:
file libffmpeg.a

Answer 2

It seems to me that this library was linked with extern "C", but you try to link without this clause. Maybe You take .H files from one version, and .LIB files from completly another.
Try use extern "C" clause on imported functions - in C++ when you try access C functions from C++ code You should use this clause