Android NDK / JNI - undefined reference to function defined in custom header file

Question

Using JNI, I am trying to write a native C++ method for the Android NDK that makes a call to a C function defined in a custom header file. However, I am getting an undefined reference error for my C function call.

Here is my C++ code that makes a call to the C function and returns its result to Java as a jstring:

#include <jni.h>  #include "gesture_detector.h"  JNIEXPORT jstring JNICALL Java_com_example_bmtitest_JavaAbstractionLayer_callGestureAnalysis(JNIEnv *env, jobject obj, jfloat previousX, jfloat previousY, jfloat currentX, jfloat currentY) {     return env->NewStringUTF(gestureAnalysis(previousX, previousY, currentX, currentY)); } 

Here is my C function:

#include <stdio.h>  #include "gesture_detector.h"  //implemented from gesture_detector.h const char* gestureAnalysis(float previousX, float previousY, float currentX, float currentY) {     float xOffset = currentX - previousX;     float yOffset = currentY - previousY;      if(xOffset == 0 && yOffset == 0)     {         return "TAP";     }     return "0"; } 

Here is my Android.mk code:

LOCAL_PATH := $(call my-dir)  include $(CLEAR_VARS)  LOCAL_MODULE    := gestureDetector LOCAL_SRC_FILES := gestureDetector.c NativeAbstractionLayer.cpp LOCAL_LDLIBS    := -landroid  include $(BUILD_SHARED_LIBRARY) 

Apparently, it seems the function definition defined in the custom header file (gesture_detector.h) isn't being found. I think it may be a problem in my Android.mk file.

Could anyone tell me what am I doing wrong here?

Answer 1

An "undefined reference" error comes from the linker. Your header file only satisfied the compiler.

However, since you are mixing C and C++ your problem is likely name mangling. Basically, you need to tell the C++ compiler that the function you are trying to call was created by a C compiler rather than a C++ one, and so does not have argument type codes grafted onto its name. Right now it doesn't know that, so is trying to call the function by a C++ style decorated name which differs from the plain C name of the function the linker actually has available.

Add this at the beginning of your gesture_detector.h file:

#ifdef __cplusplus extern "C" { #endif 

And this at the end

#ifdef __cplusplus } #endif 

And do a clean rebuild.

If your real jni glue logic is as trivial as the version presented here, switching to a C version of that could also be an option - but beware that jni syntax is different in C and C++, so you can't just change the file extension.