How do I share variables between C and C++ code? - c++

Question

I'm working on a C++ project that implements C code and I'm stuck on a segmentation fault. The segfault occures when I try to access a global C variable in my C++ code.
Overview of the code:
I have a single c file called video_stage.c with the following code snippet:
#include "video_stage.h"
uint8_t* pixbuf_data = NULL; //pointer to video buffer
vp_os_mutex_t video_update_lock = PTHREAD_MUTEX_INITIALIZER;
C_RESULT output_gtk_stage_transform( void *cfg, vp_api_io_data_t *in, vp_api_io_data_t *out)
{
vp_os_mutex_lock(&video_update_lock);
/* Get a reference to the last decoded picture */
pixbuf_data = (uint8_t*)in->buffers[0];
vp_os_mutex_unlock(&video_update_lock);
return (SUCCESS);
}
This function is periodically called by other C code and updates the pixbuf_data pointer witch points to an RGB videoframe.
It's header file video_stage.h:
#ifndef _IHM_STAGES_O_GTK_H
#define _IHM_STAGES_O_GTK_H
#ifdef __cplusplus
extern "C" {
#endif
#include <config.h>
#include <VP_Api/vp_api_thread_helper.h>
#include <VP_Api/vp_api.h> //hier zit vp_os_mutex in geinclude
PROTO_THREAD_ROUTINE(video_stage, data);
#ifdef __cplusplus
}
#endif
extern uint8_t* pixbuf_data;
extern vp_os_mutex_t video_update_lock;
#endif // _IHM_STAGES_O_GTK_H
The header file contains the extern declaration of the pixbuf_data pointer.
And here the cpp file: device.cc:
#include <iostream>
#include "video_stage.h"
int ardrone_update(ardrone_t *d)
{
uint8_t x;
x = pixbuf_data[0]; //no problem here, this is executed
std::cout << 5 << std::endl; //this is executed too
std::cout << x << std::endl; //segfault occures here
}
When the function in the cpp file is called (by other cpp code), a segfault occures at the cout instruction that prints x.
When I do a printf of the first element of the buffer in the c file, I get what I expect.
I'm sure it has something to do with the mixing of c and c++ code, but according to my research I've done the stuff to make both c and c++ code compatible here.

Answer 1

In C++ code pixbuf_data defined in a C source must be declared with C linkage:
extern "C" uint8_t* pixbuf_data;
Without extern "C" the C++ code must not link, unless there is another (duplicate) definition of pixbuf_data with C++ linkage.

Answer 2

get yourself a debugger and run your program under that. The tracing code does not tell you at all where the segfault appears, IO is slow.
In the code that you show us, you don't seem to allocate memory for pixbuf_data. Anything can happen when you access that without assigning a valid pointer to it.

Answer 3

I am trying to share variable between C project and c++ project. but when i build my solution i got "error LNK2001: unresolved external symbol "struct configuration g_conf ". This what i did:
==== In C project ==
I create a header file and on it i create my struct
typedef struct{ int maxUser; }Configuration;
extern Configuration conf;
==== In C++ project =======
In the main file :
#include "../ProjectC/header.h"
int main(){ conf.maxUser = 10; }