C and C++ --- undefined reference to - c++

Question

i need help!
I've implemented a .cpp program in the next path: home/virginia/android/vlc/src/input/virtual.cpp
/*
* virtual.cpp
*/
#include <stdlib.h>
#include <jni.h>
#include <math.h>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/imgproc/types_c.h>
#include <opencv2/highgui/highgui_c.h>
#include <opencv2/photo/photo.hpp>
#include <android/log.h>
extern "C" {
#include "virtual.h"
}
#define LOG_TAG "VLC - Imagen 3D - JNI"
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)
using namespace cv;
using namespace std;
extern "C" int camino(void){... }
This programs calls to the opencv functions.
virtual.h ( home/virginia/android/vlc/src/input/virtual.h)
/*
* virtual.h
*/
#ifndef __VIRTUAL_H
#define __VIRTUAL_H
int camino(void);
#endif /*__VIRTUAL_H*/
But the problem is that i need to call to "camino" function from decoder.c (( home/virginia/android/vlc/src/input/decoder.c):
#include "virtual.h"
#ifdef HAVE_CONFIG_H
# include "config.h"
#endif
#include <assert.h>
...
static void DecoderPlayVideo( decoder_t *p_dec, picture_t *p_picture,
int *pi_played_sum, int *pi_lost_sum ){
...
int res = camino();
msg_Warn( p_dec, "Llamada a virtualJNI devuelve %d", &res );
vout_PutPicture( p_vout, p_picture );
...
}
...
I'm getting the next error:
/home/virginia/android-ndk-r8c/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: ../vlc/android/src/.libs/libvlccore.a(decoder.o): in function DecoderDecodeVideo:../../src/input/decoder.c:1501: error: undefined reference to 'camino'
collect2: ld returned 1 exit status
make[1]: *** [obj/local/armeabi-v7a/libvlcjni.so] Error 1
make[1]: Leaving directory `/home/virginia/android/vlc-android'
make: *** [vlc-android/obj/local/armeabi-v7a/libvlcjni.so] Error 2
What am i doing wrong? Thanks so much

Answer 1

Regarding the extern "C" stuff, it's not recommended to do this:
extern "C" {
#include "virtual.h"
}
Instead, you should put it in the header:
/*
* virtual.h
*/
#ifndef __VIRTUAL_H
#define __VIRTUAL_H
#ifdef __cplusplus
extern "C" {
#endif
int camino();
#ifdef __cplusplus
}
#endif
#endif /*__VIRTUAL_H*/
You'll see this in almost all headers that are meant to be shared between C and C++.
As for your real problem, you'll have to show us the actual linker command, not just the error, but it looks like virtual.o isn't being linked into your program, and possibly isn't being built.