java - jni can't find method from native lib?

in Java ,the native method :

public native String hello();

package name:  pack

class name: Hello

C++: method name:

jstring
Java_pack_Hello_hello
(
    JNIEnv* env,
    jobject this
)

I use ndk compile this file ,but app crash ;

after i add this line : extern "C" into my hello.cpp
the error is gone ,every thing is ok.

so why ?