移植ffmpeg到android的一个编译错误

我要做的事情是把ffmpeg库移植到android下，首先交叉编译ffmpeg，这在ffmpeg的configure里已经支持移植的功能，我选择的是编译成静态库，这样以后在此基础上做一个jni的共享库，就可以大功告成了。

编译完了之后，加上由原先的ffmpeg.c等代码重构后得到的一个类库，就可以封装生成jni的接口，供java层调用。

于是我写了一个Android.mk文件，目的是生成一个jni共享库，但是在链接ffmpeg静态库的时候报以下错误，让我摸不着头脑：

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(sdp.o):
in function ff_sdp_write_media:libavformat/sdp.c:283: error: undefined reference to 'avpriv_split_xiph_headers'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(sdp.o): in function ff_sdp_write_media:libavformat/sdp.c:283:
error: undefined reference to 'avpriv_split_xiph_headers'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(sdp.o): in function ff_sdp_write_media:libavformat/sdp.c:174:
error: undefined reference to 'avpriv_mpeg4audio_sample_rates'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(utils.o): in function try_decode_frame:libavformat/utils.c:961:
error: undefined reference to 'avpriv_h264_has_num_reorder_frames'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(utils.o): in function update_initial_timestamps:libavformat/utils.c:961:
error: undefined reference to 'avpriv_h264_has_num_reorder_frames'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(utils.o): in function compute_pkt_fields.part.14:libavformat/utils.c:961:
error: undefined reference to 'avpriv_h264_has_num_reorder_frames'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(utils.o): in function avformat_find_stream_info:libavformat/utils.c:3057:
error: undefined reference to 'avcodec_pix_fmt_to_codec_tag'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(utils.o): in function avformat_find_stream_info:libavformat/utils.c:3058:
error: undefined reference to 'avpriv_find_pix_fmt'

/home/daniel/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/daniel/transcoder/jni/libtranscoder/lib/libavformat.a(utils.o): in function avformat_find_stream_info:libavformat/utils.c:2701:
error: undefined reference to 'ff_raw_pix_fmt_tags'

collect2: ld returned 1 exit status

看错误的原因我以为是ffmpeg的libavcodec.a里没有把xiph.o编进去，于是grep avpriv_split_xiph_headers -r libavcodec.a里面有找到符号。

后来我怀疑是不是链接静态库的顺序不对，果然是这个原因，我把库链接的顺序调整了就不一样的了。

最终在LOCAL_LDFLAGS += \

.. \

$(LOCAL_PATH)/lib/libavformat.a \

$(LOCAL_PATH)/lib/libavcodec.a \

..

libavformat.a要放在libavcodec.a之前，就能成功编译。