Android Studio cmake和jni的一些坑

---

1.路径

修改前CMakeLists.txt动态库配置部分

add_library( avcodec-57
             SHARED
             IMPORTED )

set_target_properties( avcodec-57
                       PROPERTIES IMPORTED_LOCATION
                       libs/armeabi/libavcodec-57.so )

执行后出错

Error while executing 'D:\Android\Sdk\cmake\3.6.3155560\bin\cmake.exe' with arguments {--build E:\MyApplication2\app\.externalNativeBuild\cmake\debug\armeabi --target native-lib}
ninja: error: 'libs/armeabi/libavcodec-57.so', needed by '../../../../build/intermediates/cmake/debug/obj/armeabi/libnative-lib.so', missing and no known rule to make it

在提示信息可以看出编译时的路径为

E:\MyApplication2\app\.externalNativeBuild\cmake\debug\armeabi

因此在CMakeList.txt中增加一个配置，即CMakeList.txt所在的目录

set(CURRENT_DIR ${CMAKE_SOURCE_DIR})

//在Gradle Console输出信息
//CMAKE_SOURCE_DIR: E:/MyApplication2/app
message("CURRENT_DIR:" ${CMAKE_SOURCE_DIR})

add_library( avcodec-57
             SHARED
             IMPORTED )

set_target_properties( avcodec-57
                       PROPERTIES IMPORTED_LOCATION
                       ${CURRENT_DIR}/libs/armeabi/libavcodec-57.so )

---

2.使用cpp开发时的extern “C”

//file native-lib.cpp
#include 
#include 
//用extern "C"将C库的头文件和代码用｛｝括起来
extern "C"
{
//ffmpeg使用c开发，所以头文件要用extern "C"声明
#include "libavcodec/avcodec.h"
jstring
Java_com_jesure_myapplication_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    char info[10000] = {0};
    sprintf(info, "%s\n", avcodec_configuration());
    //cpp中的JNIEnv是一级指针，在c中是二级指针
    //在c中下面语句写为 return (*env)->NewStringUTF(info);
    return env->NewStringUTF(info);
}

}