I used the r20b to compile the code, but got errors.
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Execution failed for task ':app:externalNativeBuildSSDMobilnetDebug'.

Build command failed.
Error while executing process /home/user/Android/Sdk/cmake/3.6.4111459/bin/cmake with arguments {--build /home/user/xitao/Bokeh/NCNNCam-master/app/.externalNativeBuild/cmake/SSDMobilnetDebug/armeabi-v7a --target ncnn_jni}
[1/1] Linking CXX shared library /home/user/xitao/Bokeh/NCNNCam-master/app/build/intermediates/cmake/SSDMobilnet/debug/obj/armeabi-v7a/libncnn_jni.so
FAILED: : && /home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ --target=armv7-none-linux-androideabi21 --gcc-toolchain=/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64 --sysroot=/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/sysroot -fPIC -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -mfpu=vfpv3-d16 -fno-addrsig -march=armv7-a -mthumb -Wa,--noexecstack -Wformat -Werror=format-security -stdlib=libc++ -std=c++11 -fopenmp -mfloat-abi=softfp -mfpu=neon -fopenmp -DSTANDALONE_DEMO_LIB -std=c++11 -fno-exceptions -fno-rtti -O2 -Wno-narrowing -fPIE -O0 -fno-limit-debug-info -Wl,--exclude-libs,libgcc.a -Wl,--exclude-libs,libatomic.a -static-libstdc++ -Wl,--build-id -Wl,--warn-shared-textrel -Wl,--fatal-warnings -Wl,--exclude-libs,libunwind.a -Wl,--no-undefined -Qunused-arguments -Wl,-z,noexecstack -Wl,-z,relro -Wl,-z,now -Wl,--allow-multiple-definition -Wl,--whole-archive -fPIE -v -fopenmp -shared -Wl,-soname,libncnn_jni.so -o /home/user/xitao/Bokeh/NCNNCam-master/app/build/intermediates/cmake/SSDMobilnet/debug/obj/armeabi-v7a/libncnn_jni.so CMakeFiles/ncnn_jni.dir/ncnn/ncnn_jni.cpp.o /home/user/xitao/Bokeh/NCNNCam-master/app/src/main/jni/ncnn/libs/armeabi-v7a/libncnn.a -landroid -llog -ljnigraphics -lm -latomic -lz -latomic -lm && :
Android (5058415 based on r339409) clang version 8.0.2 (https://android.googlesource.com/toolchain/clang 40173bab62ec746213857d083c0e8b0abb568790) (https://android.googlesource.com/toolchain/llvm 7a6618d69e7e8111e1d49dc9e7813767c5ca756a) (based on LLVM 8.0.2svn)
Target: armv7-none-linux-android21