"undefined symbols for architecture armv7" error!

[liyake825]

I want to add a new decoder plugin in vlc for ios.What i have done is addind my interface file av.c in modules/codec/ and the library libav.a in extras/contrib/hosts/arm-apple-darwin10 . Also configure.ac and module.am have been modified.
Then , after i have passed configuration and make, It failed as follows:

Undefined symbols for architecture armv7:
"_sva_avs_videc", referenced from:
_OpenDecoder in libMobileVLCKit.a(liblibavs_plugin_la-libavs.o)
_CloseDecoder in libMobileVLCKit.a(liblibavs_plugin_la-libavs.o)
_DecodeBlock in libMobileVLCKit.a(liblibavs_plugin_la-libavs.o)
ld: symbol(s) not found for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)

It seems that i have some link error about my static library libav.a .
Who can help?Thanks.