Question

Some one help me on this please.

I am working on a simple app the does factorial on android, using ndk.

I want to have 2 .cpp files, one for class factorial, and one will be the main that invokes method from that class.
I don’t know how to deal with the header problem when it comes to build, please help.

#include "com_lan_factorial_FacLib.h"

JNIEXPORT jlong JNICALL Java_com_lan_factorial_FacLib_fac
  (JNIEnv *env, jclass clazz, jlong n)

{


    jlong result = (jLong) (fac(n));

    return result;
}

This code call fac method. this is Main.cpp

#include <stdio>
#include "com_lan_factorial_FacLib.h"

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

This class create fac method. This is fac.cpp

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_CFLAGS := -Wno-psabi
LOCAL_MODULE := libfac
LOCAL_SRC_FILES := fac.cpp
LOCAL_C_INCLUDES := $(LOCAL_PATH)
include $(BUILD_STATIC_LIBRARY)

include $(CLEAR_VARS)

LOCAL_CFLAGS := -Wno-psabi
LOCAL_MODULE := FacLib
LOCAL_SRC_FILES := Main.cpp 
LOCAL_STATIC_LIBRARIES := libfac
include $(BUILD_SHARED_LIBRARY)

The compile log is:

C:\Users\Lan\workspace\Factorial>..\..\temp\android\android-ndk-r8b\ndk-build
"Compile++ thumb : FacLib <= Main.cpp
jni/Main.cpp: In function 'jlong Java_com_lan_factorial_FacLib_fac(JNIEnv*, jclass, jlong)':
jni/Main.cpp:7:30: error: 'fac' was not declared in this scope
make: *** [obj/local/armeabi/objs/FacLib/Main.o] Error 1

Im not sure why fac is not declare

Answer 1

Is your second file really called com_lan_factorial_fac.cpp.callMain? The problem with it is that does not have a .cpp extension. Its extension is callMain. Rename to something that ends with .cpp, and edit your Anrdoid.mk to list both file names:

LOCAL_SRC_FILES := com_lan_factorial_callMain.cpp some_other_file_name.cpp

You can list all your files in one line, or you can split it up like this:

LOCAL_SRC_FILES := com_lan_factorial_callMain.cpp \
    some_other_file_name.cpp

The other error message is that the file com_lan_factorial_fac.h can’t be found. Does such a file exist?

EDIT: your Android.mk is wrong. You are not building two libraries – you’re building one, from two source files. It should go like this:

LOCAL_PATH:= $(call my-dir) 
include $(CLEAR_VARS) 

LOCAL_CFLAGS := -Wno-psabi 

LOCAL_MODULE := libfac 

LOCAL_SRC_FILES := fac.cpp Main.cpp

include $(BUILD_STATIC_LIBRARY) 

And in your Main.cpp, you should declare that a function called fac() exists, between the #include line and the Java_com_lan_factorial_FacLib_fac function:

extern long fac(long n);

Your compilation error message has to do with the lack of this line.

The LOCAL_C_INCLUDES line is not generally necessary.

By the way, the builtin C datatype long and jlong as defined by Android JNI are not the same. Better rewrite your fac() to work with jlong‘s.