Question

I am trying to run a simple factorial program written in C++, and link it to java UI.

This is my Java code

package com.lan.factorial;

public class FacLib
{

    public native static long fac(long n);
    static 
    {
        System.loadLibrary("FacLib");
    }
    public static long facI(long n)
    {
        return fac(n);
    }
}

`

This is my C++ code

#include <studio.h>
#include <jni.h>
#include "com_lan_factorial_FacLib.h"
JNIEXPORT jlong JNICALL Java_com_lan_factorial_FacLib_fac(JNIEnv *env, jclass, jlong)
{

    jlong f = 1;
    jlong i;
    for(i = 1; i <= n; i++)
    {
         f *= i;
    }
    return f;
}

this is my make file

LOCAL_PATH:= $(call my-dir)

include $(CLEAR_VARS)

LOCAL_SRC_FILES := com_lan_factorial_main.cpp

LOCAL_MODULE := com_lan_factorial_FacLib
include $(BUILD_SHARED_LIBRARY)

and I keep getting this error when I run ndk-build:

C:\Users\Lan\workspace\Factorial>"..\..\temp\android\android-ndk-r8b\ndk-build"
make: *** No rule to make target `jni/com_lan_factorial_main.cpp', needed by `obj/local/armeabi/objs/com_lan_factorial_FacLib/com_lan_factorial_main.o'.  Stop.

Yes, my NDK in a different folder so I have to use ..\..\, but I do not think it is the problem. and I did run javah

Answer 1

As @nneonneo pointed out correctly, the name of your .cpp file name was not correctly listed in Android.mk.

Now you have a compilation problem in line 4:

JNIEXPORT jlong JNICALL Java_com_lan_factorial_FacLib_fac(JNIEnv *env, jclass, jlong)
                                                                                    ^

You lost the parameter name n, but you use it in line 9:

    for(i = 1; i <= n; i++)

Note that it’s OK to skip the name of the second parameter of type jclass which you never use.

I am not sure your compiler found the com_lan_factorial_FacLib.h file, but that’s not a problem: you can remove this #include statement altogether, your code does not use it.