的Android NDK本地函数调用问题 [英] Android NDK Native Function Call Issue
问题描述
我试图写一个Android应用程序，它使用了GNU科学图书馆（GSL）的。特别是，我为它的BLAS执行兴趣libgslcblas.so。我决定了Android NDK的优势，并写加载库中的Java程序，并进行适当的函数调用。
I am attempting to write an Android application that makes use of the GNU Scientific Library (GSL). In particular, I am interested in 'libgslcblas.so' for its BLAS implementation. I decided to take advantage of the Android NDK and write a Java program that loads the library and makes the appropriate function calls.
要测试这是如何工作的，我试图写一个简单的程序，将加载libm.so，并进行任意函数调用。这似乎是关于一个简单的用例，因为我能想到的NDK，但我遇到了以下问题：
To test how this would work, I attempted to write a simple program that would load 'libm.so' and make an arbitrary function call. This seemed about a trivial a use-case of the NDK as I could think of, yet I ran into the following issue:
07-05 18:11:07.264: I/System.out(1298): debugger has settled (1464)
07-05 18:11:07.583: D/dalvikvm(1298): No JNI_OnLoad found in /system/lib/libm.so 0x41345988, skipping init
07-05 18:11:07.903: W/dalvikvm(1298): No implementation found for native Lissm/libctest/LibCtest;.sqrt (D)D
我的code是如下：
My code is as follows:
package issm.libctest;
import android.os.Bundle;
import android.app.Activity;
import android.widget.TextView;
public class LibCtest extends Activity
{
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
TextView tv = new TextView(this);
setContentView(R.layout.activity_lib_ctest);
double x = sqrt(4);
tv.setText( "Sine of: " + x);
setContentView(tv);
}
public native double sqrt(double x);
static
{
System.load("/system/lib/libm.so");
}
}
我真的不明白的问题。正如我所说的，这是最简单的使用NDK的我能想到的。有人可以帮我解决这个问题？
I really do not understand the problem. As I said, this is the simplest use of the NDK I can think of. Can someone help me resolve this issue?
谢谢！
推荐答案
阅读上JNI。总之，任意全局C函数不是从Java调用通过NDK。为了能够调用一个函数需要一个非常特殊的签名，大致为：
Read up on JNI. In short, arbitrary global C functions are NOT callable from Java via NDK. In order to be callable, a function needs a very specific signature, along the lines of:
void Java_com_mypackage_MyClass_MyMethod(JNIEnv *jni, jobject thiz, jint arg1);
在Java方面，这将是在包中的类 MyClass的 com.mypackage 的方法：
On the Java side, that would be a method of class MyClass in package com.mypackage:
native void MyMethod(int arg1);
Obviosly，香草开方将不符合要求。所以，你需要提供一个Java友好的包装，每一个你打算从Java调用函数。考虑到这一点，你可能会想提供封装更高层次的概念，仅仅数学元。在Java / C边境混乱不堪;越少，你穿过它，就更好了。
Obviosly, vanilla sqrt won't fit the bill. So you need to provide a Java-friendly wrapper for every function you intend to call from Java. With that in mind, you'll probably want to provide wrappers for higher-level concepts that mere math primitives. The Java/C border is messy; the less you cross it, the better.
此外，库需要从源专为Android的待建。 NDK具有用于该工具。一个现成的二进制为另一个平台（例如，Linux或Windows）将无法使用。
Also, the library needs to be built from sources specifically for Android. NDK has the tools for that. A ready-made binary for another platform (say, Linux or Windows) won't be usable.
编辑：从JDK的 javah的工具，可以采取一个Java类和一帮本地的申报，骨架H / C文件虚拟实现。从本质上讲，翻译从Java方法的原型沿JNI的规则下进行。
The javah tool from JDK can take a Java class with a bunch of native declarations and make skeleton H/C files with dummy implementations. Essentially, translate the method prototypes from Java to C along the JNI rules.
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