Android NDK: Static library used is different than precompiled library specified

问题

I build a C++ library manually using the Android SDK compiler. The result is libMyUtils.a.

I'm using the following in my Java/JNI test application:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := MyUtils
LOCAL_SRC_FILES := ../../../../libs/libMyUtils.a

include $(PREBUILT_STATIC_LIBRARY)

LOCAL_MODULE    := AndroidTests
LOCAL_STATIC_LIBRARIES    := MyUtils

include $(BUILD_SHARED_LIBRARY)

When I build the console shows the following:

[armeabi] Install        : libMyUtils.so => libs/armeabi/libMyUtils.so

Now for some bizarre reason the library ../../../../libs/libMyUtils.a is a couple megabytes, but the library libs/armeabi/libMyUtils.so is only 5KB. Isn't it supposed to be the same library?

When I run my test application I get UnsatisfiedLinkError. Obviously the native function I'm calling isn't located in the library. What am I doing wrong?

回答1:

A full explanation, as your Android.mk does not make much sense to me: sorry if you already know some of that.

Static libraries should be pure C/C++, and wrapped using the Android NDK in order to be usable from Java.

For example, assuming your static library is built from a simple .c and .h files:

highfive.h:

int giveMeFive();

highfive.c:

#include "highfive.h"

int giveMeFive() {
  return 5;
}

This can be compiled as a static library using the Android NDK compiler, which apparently you already know how to do: this will give us a highfive.a library.

In this form, this library is unusable from Java, but it can be wrapped using the Android NDK. See the Android NDK documentation for naming conventions etc...

highfiveWrapper.c:

#include "highfive.h"

jint
Java_your_package_name_HighFive_giveMeFive(JNIEnv *env, jobject o) {
  return (jint) giveMeFive();
}

and its corresponding Java file:

package your.package.name;

class HighFive {
  static {
    System.loadLibrary("highfive");
  }

  public native int giveMeFive();
}

Now, how do we compile all this to get it to work:

Android.mk:

include $(CLEAR_VARS)
LOCAL_MODULE            := libhighfive-prebuilt
LOCAL_SRC_FILES         := path/to/highfive.a
LOCAL_EXPORT_C_INCLUDES := path/to/highfive.h/folder
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE      := highfive
LOCAL_SRC_FILES   := path/to/highfiveWrapper.c
LOCAL_STATIC_LIBRARIES := libhighfive-prebuilt
include $(BUILD_SHARED_LIBRARY)

And there you should be able to use your native library as you wished to!

Hope this helps!

回答2:

Usually, static libraries contain many objects, which contain many functions, of which many are unused. That's why the linker only pulls the referenced objects from static libraries. So, in the example given by @mbrenon, highfiveWrapper.c defines which components of highfive.a will be linked into highfive.so.

But in your setup, you need whole static library to be loaded. OK, there is a special word for it: LOCAL_WHOLE_STATIC_LIBRARIES.

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := MyUtils
LOCAL_SRC_FILES := ../../../../libs/libMyUtils.a

include $(PREBUILT_STATIC_LIBRARY)

LOCAL_MODULE    := AndroidTests
LOCAL_WHOLE_STATIC_LIBRARIES    := MyUtils

include $(BUILD_SHARED_LIBRARY)

回答3:

In the end the solution was to build the MyUtils library as a prebuilt shared library. This way the linker doesn't strip anything. Then I modified my makefile as follows:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := MyUtils
LOCAL_SRC_FILES := ../../../../libs/libMyUtils.so

include $(PREBUILT_SHARED_LIBRARY)

LOCAL_MODULE    := AndroidTests
LOCAL_SHARED_LIBRARIES    := MyUtils

include $(BUILD_SHARED_LIBRARY)

Notice the .so extention and the PREBUILT_SHARED_LIBRARY and BUILD_SHARED_LIBRARY scripts.

来源：https://stackoverflow.com/questions/20137375/android-ndk-static-library-used-is-different-than-precompiled-library-specified

标签

android

c++

android-ndk

java-native-interface