CMake undefined reference to 问题

Simples/app/src/main/cpp/../../../../ffmpeg-3.3.3/build/android/arm64-v8a/lib/libavcodec.a(utils.o): In function `avcodec_string':

Simples/ffmpeg-3.3.3/libavcodec/utils.c:3239: undefined reference to`av_get_media_type_string’

Simples/ffmpeg-3.3.3/libavcodec/utils.c:3260: undefined reference to `av_fourcc_make_string'
Simples/app/src/main/cpp/../../../../ffmpeg-3.3.3/build/android/arm64-v8a/lib/libavformat.a(apngenc.o): In function `apng_write_chunk':
Simples/ffmpeg-3.3.3/libavformat/apngenc.c:64: undefined reference to `av_crc_get_table'
Simples/ffmpeg-3.3.3/libavformat/apngenc.c:72: undefined reference to `av_crc'
Simples/ffmpeg-3.3.3/libavformat/apngenc.c:75: undefined reference to `av_crc'
Simples/app/src/main/cpp/../../../../ffmpeg-3.3.3/build/android/arm64-v8a/lib/libavformat.a(apngenc.o): In function `flush_packet':
Simples/ffmpeg-3.3.3/libavformat/apngenc.c:209: undefined reference to `av_crc_get_table'
Simples/ffmpeg-3.3.3/libavformat/apngenc.c:209: undefined reference to `av_crc'

出现该问题很可能是target_link_librarie()中依赖库的顺序不对，需要调整target_link_librarie() 中依赖库的顺序。

比如这里 `av_get_media_type_string’ 是 avutil中一个函数，所以，可以看出 libavcodec 是依赖 libavutil的 ，则avcodec必须写在avutil的前面。

切记：是 avcodec 写在 avutil 的前面，而不是后面。

target_link_libraries( # Specifies the target library.

                       ffmpeg-lib

                       # Links the target library to the log library

                       # included in the NDK.

                       ${log-lib}

                       ${android-lib}

                       swscale

                       avformat

                       avcodec

                       swresample

                       avutil

                       avfilter

                       ${z-lib}

                       ${m-lib}

                       )