Recall a struct from C++ to C

Question

I'm trying to read a struct of a program i wrote in c++ into a c program. I created a .h with the struct and the extern C for the function. Then i wrote the C++ program that read a csv file and save some elements in a struct. Now i have to recall the struct in a C program and read the values but i'm having a lot of problems. I don't know if i make some mistake in the .cpp file , in the .h file or in the C program. I'm working under linux.

My .h file is :

typedef struct Buses{
 int maggiore;
 int minore;
}BusesStruct;

#ifdef __cplusplus
extern "C" {
#endif

    void get_buses(BusesStruct *);

#ifdef __cplusplus
}
#endif

My cpp is

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <algorithm>
#include "lettura.h"
using namespace std;

void get_buses(BusesStruct * bus_scelti){
//leggo il file csv
    string line,nomefile;
    ifstream myfile;
    cout << "Quale file si vuole aprire? " << endl;
    cin >> nomefile;
    myfile.open(nomefile.c_str());
    int j=0;
    if (myfile.is_open()) {
        int primobus;
        int secondobus;
        cout << "Digitare il numero di uno dei due BUS sul quale si vuole eseguire l'operazione (1-9): " << endl;
        cin >> primobus;
        cout << "Digitare il numero di un secondo BUS sul quale si vuole eseguire l'operazione (1-9): " << endl;
        cin >> secondobus;
        int bus1_scelto;
        int bus2_scelto;
        while (getline(myfile,line)) {
            istringstream myline(line);
            string bus[10];
            for (int i=0; i<9; i++){
                    getline(myline,bus[i+1],';');
                }

            for (int i=0; i<10; i++){
                if(i == primobus){
                    int bus1_scelto_c = atoi(bus[i].c_str());
                    bus1_scelto=bus1_scelto_c;
                }
                if(i == secondobus){
                    int bus2_scelto_c = atoi(bus[i].c_str());
                    bus2_scelto=bus2_scelto_c;
                }
            }
            bus_scelti[j].maggiore=max(bus1_scelto,bus2_scelto);
            bus_scelti[j].minore=min(bus1_scelto,bus2_scelto);
            j=j+1;          
        }   
    }
}

In my C file i wrote #include lettura.h to recall the function :

BusesStruct bus_scelti[10005];
get_buses(bus_scelti);

And to define my vector and recall the function. I got the error :

reference not define to 'get_buses'.

Can you help me? Thanks

Why are you using `extern "C" {` ? The `get_buses()` function is C++. [What is the effect of extern "C" in C++?](//stackoverflow.com/q/1041866) — 001, Apr 25 '18 at 14:12
`extern "C" void get_buses(BusesStruct * bus_scelti){` should be in definition too. — 273K, Apr 25 '18 at 14:14
@JohnnyMopp the `extern "C"` is so the function can be called from C code, which OP is trying to do. It doesn't matter that the function definition is in C++. — Kevin, Apr 25 '18 at 14:21
i use the extern "C" to read the values of the struct in the C file. In the c++ file i read the csv and save the elements in the struct. — Iceknight, Apr 25 '18 at 14:21
extern "C" just makes the function name compatible. It doesn't have any effect on the data being passed as arguments. — stark, Apr 25 '18 at 14:23
Ok. I see. I missed that part. Are you getting the error from the linker? — 001, Apr 25 '18 at 14:26
i'm getting the error when i try to complie the C file where i define the vector and recall the function — Iceknight, Apr 25 '18 at 14:29
g++ -c lettura.cpp for the cpp file. For the c file i use g++ complete.c -o complete . — Iceknight, Apr 25 '18 at 14:41
So you aren't linking the C++ file into the executable? That's the problem. — Kevin, Apr 25 '18 at 14:41
Can you wrote me the command i have to use? I'm new on C and linux — Iceknight, Apr 25 '18 at 14:44
the content of question is quite poor as it appears, because it is a build process problem — Swift - Friday Pie, Apr 25 '18 at 15:16
Possible duplicate of [Compiling and linking C and C++ files](https://stackoverflow.com/questions/22154119/compiling-and-linking-c-and-c-files) — Swift - Friday Pie, Apr 25 '18 at 15:17

Kevin · Answer 1 · 2018-04-25T14:52:22.860

1

According to your comment this is how you're compiling:

g++ -c lettura.cpp for the cpp file. For the c file i use g++ complete.c -o complete

This is compiling the C++ file into an object file and then compiling the C file into an executable. You need to link in the object file as well.

A few options:

g++ -c lettura.cpp                   # compile C++ file to lettura.o
gcc -c complete.c                    # compile C file to complete.o
g++ lettura.o complete.o -o complete # link object files into executable 'complete'

g++ lettura.cpp complete.c -o complete # compile and link in one step

Note that I'm using gcc to compile the C code and g++ to compile the C++ code and to link.

Ideally you should use a Makefile for this, but I think that's beyond the scope of this question/answer.

edited Apr 25 '18 at 14:52

answered Apr 25 '18 at 14:47

Kevin

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Better use `-Wall -Wextra -g` for both `gcc` and `g++` – Basile Starynkevitch Apr 25 '18 at 14:52
ok thanks, i'll try – Iceknight Apr 25 '18 at 15:51
Ok, now they compiles without errors but when i launch ./COMPLETE it stops after a while. Maybe i made an error when i recall the values. I'll search for the errors. Thanks – Iceknight Apr 26 '18 at 15:36
The code now compiles and work correctly. Thanks – Iceknight Apr 27 '18 at 07:18

Recall a struct from C++ to C

1 Answers1