Can't create shared library with static inside

Question

Please help I search throw whole internet but I can't find answer ...

C Layer

I have created simple function int mean(int, int); and place it in calc_mean.h and initialize it in calc_mean.c here are this two files.

calc_mean.c

#include "calc_mean.h"

int mean(int a, int b)  
{  
return (a+b) / 2;  
}  

calc_mean.h

int mean(int, int);

Then I create make file and build archive (.a) file called Test_Archive.a using following make file

makefile

GCC := C:\Tools\ndk-toolchain\ndk-standalone\bin\arm-linux-androideabi-gcc.exe
GPP := C:\Tools\ndk-toolchain\ndk-standalone\bin\arm-linux-androideabi-g++.exe
AR  := C:\Tools\ndk-toolchain\ndk-standalone\bin\arm-linux-androideabi-ar.exe

default: all

all: obj
    $(AR) r Test_Archive.a *.o

obj:
    $(GPP) -c *.c

I get Test_Archive.a archive file. Now I want to add this archive file to JNI and call mean function from my JNI project, for that I have created JNI and Java files which you can see below.

---

JAVA Layer

How you can see here in java layer I have TestJavaClass that have one native method called JMean this method have two int arguments and return also int.

public class TestJavaClass
{
    /** Default Constructor
     * 
     */
    public TestJavaClass( ) {

    }

    /** Test Function. 
     *  
     * @param a
     * @param b
     * @return
     */
    public native int JMean( int a, int b);
}

---

JNI Layer

This is JNI header file

Test_Library.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_Fido_OSTXLib_OSTX */

#ifndef _Included_com_Fido_OSTXLib_OSTX
#define _Included_com_Fido_OSTXLib_OSTX
#ifdef __cplusplus
extern "C" {
#endif

/*
 * Class:     com_Fido_OSTXLib_OSTX
 * Method:    JMean
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_com_Fido_OSTXLib_OSTX_JMean
  (JNIEnv *, jobject, jint, jint);

#ifdef __cplusplus
}
#endif
#endif

Test_Library.c

#include "Test_Library.h"
#include "calc_mean.h"

JNIEXPORT jint JNICALL Java_com_Fido_OSTXLib_OSTX_JMean( JNIEnv* env, jobject thiz, jint a, jint b )
{
    return mean( a, b );
}

I put this two files Test_Library.h, Test_Library.c and calc_mean.h, Test_Archive.a and create make file Android.mk

Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE           := Test_Library
LOCAL_SRC_FILES        := Test_Library.c

LOCAL_LDLIBS := -LTest_Archive.a

include $(BUILD_SHARED_LIBRARY)

but when I want to create .so file, the following error occurs, why ?

$ ../../ndk-build Compile thumb : Test_Library <= Test_Library.c SharedLibrary : libTest_Library.so C:/cygwin/home/android-ndk-r5b/Fido/ProjectOSTX/obj/local/armeabi/objs/OSTX_Library/OSTX_Library.o: In function Java_com_Fido_OSTXLib_OSTX_JMean': C:/cygwin/home/android-ndk-r5b/Fido/ProjectOSTX/jni/OSTX_Library.c:6: undefined reference tomean' collect2: ld returned 1 exit status make: * [/home/android-ndk-r5b/Fido/ProjectOSTX/obj/local/armeabi/libOSTX_Library.so] Error 1

Why undefined reference to `mean' ?

Answer 1

Here is what I did to build the code successfully:

1. In your Eclipse Android Project create the folder jni.

2. Add the files calc_mean.c; calc_mean.h; Test_Library.c and Android.mk to the jni folder

3. Remove the line #include "Test_Library.h"from the Test_Library.c file. Everything else can stay as it is.

4. In the cygwin command line go to the root folder of your Eclipse Android project and run ndk-build

If you want to build your calc_mean also as static library during the ndk-build process then you can change your Android.mk file as follows:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE           := Test_Archive
LOCAL_SRC_FILES        := calc_mean.c

# compiler flags.
LOCAL_LDLIBS   = -lz -lm
LOCAL_CFLAGS   = -Wall -pedantic -std=c99 -g

include $(BUILD_STATIC_LIBRARY)

include $(CLEAR_VARS)

LOCAL_MODULE := Test_Library
LOCAL_STATIC_LIBRARIES := Test_Archive
LOCAL_SRC_FILES := Test_Library.c


include $(BUILD_SHARED_LIBRARY)

Answer 2

As I suppos the main reason was compiler settings, I make some modifications in makefile and my .a archive linked to shared library, here are modifications.

GCC := C:\Tools\ndk-toolchain\ndk-standalone\bin\arm-linux-androideabi-gcc.exe
GPP := C:\Tools\ndk-toolchain\ndk-standalone\bin\arm-linux-androideabi-g++.exe
AR  := C:\Tools\ndk-toolchain\ndk-standalone\bin\arm-linux-androideabi-ar.exe

OPTIONS  :=\
-fpic \
-ffunction-sections \
-funwind-tables  \
-fstack-protector \
-D__ARM_ARCH_5__ \
-D__ARM_ARCH_5T__ \
-D__ARM_ARCH_5E__ \
-D__ARM_ARCH_5TE__ \
-Wno-psabi \
-march=armv5te \
-mtune=xscale \
-msoft-float \
-mthumb \
-Os \
-fomit-frame-pointer \
-fno-strict-aliasing \
-finline-limit=64 \
-DANDROID \
-Wa, \
-O2 \
-DNDEBUG \
-g \

default: all


all: obj
    $(AR) r libtestlibrary.a *.o

obj:
    $(GCC) $(OPTIONS) -c *.c

Answer 3

Simply use C compiler instead!

You compile with C++ compiler (Makefile has "$(GPP) -c *.c") and the function symbol name includes parameter information and is called different: not "mean", but something more complex, similar as Java names symbols internally. You can use "nm" to see what's inside an object file.

For example my g++ generates:

# nm x.o
00000000 T _Z4meanii

So there is no "mean" (but "Z4meanii"). Using a C compiler, and the symbol names are straightforward:

# nm x.o
00000000 T mean

(Please note that the same source code content was compiled)

So change in you Makefile:

obj:
    $(GCC) -c *.c

---

If for some reason a C++ compiler must be used, then extern "C" must be added:

calc_mean.h (simplified, without inclusion guards etc.)

#ifdef __cplusplus
extern "C" {
#endif

int mean(int, int);

#ifdef __cplusplus
} // extern "C"
#endif

In general, if you need C symbol names in C++ files, use extern "C". Test_Library.h uses it correctly and provides a nice example. You can put the same extern "C" logic (best including if #ifdef __cplusplus block) to your header file. Some consider it good practice to always do so.

But note that there are several, sometimes subtle, differences between C and C++, so I recommend to compile C sources with C compiler, although I know that there can be advantages using a C++ compiler.

oki,

Steffen