Passing linked list from C to C++ - c++

Question

Are there known issues with passing linked lists from C code to C++?
I have a Qt dialogue that I'm adding to legacy C code. A Qt class (in a C++ library) calls a C library function, which returns a static pointer to a linked list. That list is created in the C library. For example:
C code:
typedef my_struct_s my_struct_t, *my_struct_p;
struct {
int some_data;
double some_more_data;
my_struct_p next;
} my_struct_s;
void create_list()
{
my_struct_p next;
SP_head = (my_struct_p) calloc(1, sizeof(my_struct_t));
next = (my_struct_p) calloc(1, sizeof(my_struct_t));
SP_head->some_data = 1;
next->some_data = 2;
SP_head->next = next;
}
static my_struct_p SP_head=(my_struct_p)0;
extern my_struct_p get_list() {
if(!SP_head)
create_list();
return SP_head;
}
C++ code:
myclass::do_something()
{
my_struct_p list = get_list();
for(my_struct_p ptr = list; ptr; ptr = ptr->next)
{
std::cout << ptr->value;
}
}
SP_head is valid and contains a bunch of entries when I obtain it. In my debugger I can see the next entries are populated and valid on return from the function get_list().
When I try to assign ptr=ptr->next, whether inside a for loop or at the end of a while loop or via a temporary pointer, the value is null. i.e
temp_ptr = ptr->next;
// temp_ptr is null
// but I can see ptr->next looks ok in a debugger
I've now changed the code to copy each element of the list into an array and return that array, and it works fine. Is this something to do with the stack and an incompatibility with C and C++, or just an indication that I have a memory problem somewhere else?

Answer 1

Of course, C++ is prepared to handle C, then you should assume that there are no incompatibility when the proper interface is used.
After all, there are many SW out there implemented in C and interfaced from C++. I'm using, for instance, SWI-Prolog - implemented in C - from Qt.
But to compile your code, I had to do several changes: here the bottom line
file my_list.h
#ifdef __cplusplus
extern "C" {
#endif
struct my_struct_s {
int some_data;
double some_more_data;
struct my_struct_s *next;
};
typedef struct my_struct_s my_struct_t, *my_struct_p;
void create_list();
extern my_struct_p get_list();
#ifdef __cplusplus
}
#endif
file my_list.c
#include "my_list.h"
#include <stdio.h>
#include <stdlib.h>
static my_struct_p SP_head=(my_struct_p)0;
void create_list()
{
my_struct_p next;
SP_head = (my_struct_p) calloc(1, sizeof(my_struct_t));
next = (my_struct_p) calloc(1, sizeof(my_struct_t));
SP_head->some_data = 1;
next->some_data = 2;
SP_head->next = next;
}
my_struct_p get_list() { return SP_head; }
file main.cpp
#include "my_list.h"
#include <iostream>
class myclass {
public:
void do_something()
{
my_struct_p list = get_list();
for(my_struct_p ptr = list; ptr; ptr = ptr->next)
std::cout << ptr->some_data << std::endl;
}
};
int main() {
create_list();
myclass c;
c.do_something();
}
now that compile and run:
g++ my_list.c main.cpp
./a.out
1
2