共享库G ++错误分段错误

Question

I am trying to learn shared library (data structures) using c++; every time I run my program, it gives this error:

sachin@sachin-desktop:~$ cd /home/sachin/sDL
sachin@sachin-desktop:~/sDL$ g++ -fPIC -shared myclass.cc -o myclass.so
sachin@sachin-desktop:~/sDL$ g++ class_user.cc -ldl -o class_user
sachin@sachin-desktop:~/sDL$ ./class_user
0
Unable to load Sachin's library 
Segmentation fault (core dumped)
sachin@sachin-desktop:~/sDL$ 

shared.h file

typedef struct node 
{
    struct node *prev;
    int data;
    struct node *next;
}NODE,*PNODE;


class SinglyCLL
{
    private :
    PNODE head;
    PNODE tail;
    public :
    SinglyCLL();
    ~SinglyCLL();
    virtual void InsertFirst(int);
    virtual void InsertLast(int);
    virtual void InsertAtPosition(int,int);
    virtual void DeleteFirst();
    virtual void DeleteLast();
    virtual void DeleteAtPosition(int);
    virtual int Count();
    virtual void Display();

};

myclass.cc file

#include"sharedfile.h"
#include<iostream>
using namespace std;

SinglyCLL::SinglyCLL()
{
    head=NULL;
    tail=NULL;
}

void SinglyCLL::InsertFirst(int ino)
{
    PNODE temp=head;
    PNODE newN=NULL;

    newN=new NODE;
    newN->next=NULL;
    newN->data=ino;
    newN->prev=NULL;

    if((head==NULL)&&(tail==NULL))
    {
        head=newN;
        tail=newN;
        tail->next=head;
        head->prev=tail;
    }

    newN->next=head;
    head->prev=newN;
    head=newN;
    tail->next=head;
    head->prev=tail;

}

void SinglyCLL::Display()
{
    PNODE temp=head;

    if((head==NULL)&&(tail==NULL))
    {
        return;
    }

    do
    {
        cout<<temp->data<<endl;
        temp=temp->next;

    }while(tail->next!=temp);
}

SinglyCLL::~SinglyCLL()
{
    delete head;
    delete tail;
}

extern "C"
{
    SinglyCLL *create()
    {
        return new SinglyCLL;
    }
    void destroy (SinglyCLL* p)
    {
        delete p;
    }
}

class_user.cc file

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <iostream>
#include "sharedfile.h"
using namespace std;

int main()
{
    void *p=NULL;
    SinglyCLL *ptr=NULL;
    SinglyCLL* (*fp1)()=NULL;
    void (*fp2)(SinglyCLL*)=NULL;

    p=dlopen("/home/sachin/sDL/myclass.so",RTLD_LAZY);
    if(!p)
    {
        cout<<p<<endl;
        printf("Unable to load Sachin's library \n");
    }

    fp1=(SinglyCLL*(*)())dlsym(p,"create");
    fp2=(void(*)(SinglyCLL*))dlsym(p,"destroy");

    ptr=fp1();

    ptr->InsertFirst(10);
    /*ptr->InsertAtPosition(70,1);
    ptr->InsertAtPosition(80,2);*/
    ptr->Display();

    fp2(ptr);
    dlclose(p);
    return 0;
}

Every time I run this program, a segmentation fault occurs. I am using singly circular list to create a linked-list to store data. I have gone through the code and failed to locate the error. I am unable to understand why it's not running as I tried to run singly linked-list with shared library which is running fine without any error. I have checked the permissions to access the files, which is fine.

As I am learning to create data structures in a shared library just for educational purposes, there might be chances of improvement or something to change in this code outside the part which causing the problem.

Answer 1

EDIT: After tinkering with your code, I'm changing my answer!

@Zsigmond is correct in pointing out that you need to define the methods in the header file; however, that doesn't completely solve the problem.

I compiled your source and recreated the problem you saw (the segmentation fault on load). I resolved that by adding implementations for the other functions in shared.h

After that, I got this error:

*** Error in ./class_user: double free or corruption (fasttop): 0x0000000002478660 ***

I was able to narrow this down to the fp2(ptr) line; commenting this line out removed the error.

Updated code is below.

Updated shared.h:

// Header Guard (prevents duplicate symbols)
#ifndef __SHARED_H
#define __SHARED_H

// C++ syntax for a struct doesn't need typedef
struct node
{
    node* prev;
    int data;
    node* next;
};


class SinglyCLL
{
private :
    node* head;
    node* tail;
public :
    SinglyCLL();
    ~SinglyCLL();
    virtual void InsertFirst(int);
    virtual void InsertLast(int);
    virtual void InsertAtPosition(int,int);
    virtual void DeleteFirst();
    virtual void DeleteLast();
    virtual void DeleteAtPosition(int);
    virtual int Count();
    virtual void Display();

};

#endif // __SHARED_H

Updated myclass.cc:

#include"shared.h"
#include<iostream>
using namespace std;

SinglyCLL::SinglyCLL()
{
    head=NULL;
    tail=NULL;
}

void SinglyCLL::InsertFirst(int ino)
{
    node* temp=head;
    node* newN=NULL;

    newN=new node;
    newN->next=NULL;
    newN->data=ino;
    newN->prev=NULL;

    if((head==NULL)&&(tail==NULL))
    {
        head=newN;
        tail=newN;
        tail->next=head;
        head->prev=tail;
    }

    newN->next=head;
    head->prev=newN;
    head=newN;
    tail->next=head;
    head->prev=tail;

}

void SinglyCLL::Display()
{
    node* temp=head;

    if((head==NULL)&&(tail==NULL))
    {
        return;
    }

    do
    {
        cout<<temp->data<<endl;
        temp=temp->next;

    }while(tail->next!=temp);
}

SinglyCLL::~SinglyCLL()
{
    delete head;
    delete tail;
}

extern "C"
{
    SinglyCLL *create()
    {
        return new SinglyCLL;
    }
    void destroy (SinglyCLL* p)
    {
        delete p;
    }
}

// Empty implementations for the other functions
void SinglyCLL::InsertLast(int)
{

}

void SinglyCLL::InsertAtPosition(int,int)
{

}

void SinglyCLL::DeleteFirst()
{

}

void SinglyCLL::DeleteLast()
{

}

void SinglyCLL::DeleteAtPosition(int)
{

}

int SinglyCLL::Count()
{

}

Updated class_user.cc:

#include <stdlib.h>
#include <dlfcn.h>
#include <iostream>
#include "shared.h"
using namespace std;

int main()
{
    void *p=NULL;
    SinglyCLL *ptr=NULL;
    SinglyCLL* (*fp1)()=NULL;
    void (*fp2)(SinglyCLL*)=NULL;

    p=dlopen("/home/mike/Projects/C++/myclass.so",RTLD_LAZY);
    if(!p)
    {
        cout<<p<<endl;
        cout << "Unable to load Sachin's library" << endl;
    }

    fp1=(SinglyCLL*(*)())dlsym(p,"create");
    fp2=(void(*)(SinglyCLL*))dlsym(p,"destroy");

    ptr=fp1();

    ptr->InsertFirst(10);
    ptr->InsertAtPosition(70,1);
    ptr->InsertAtPosition(80,2);
    ptr->Display();

    // Un-comment next line to force the 'double free' error.
    //fp2(ptr);
    dlclose(p);
    return 0;
}

Answer 2

Edited on Mike P's suggestion:

You not only have to implement every virtual method you declare in the header; you have to fix your destructor too. It would be something like this:

SinglyCLL::~SinglyCLL()
{
    node *q, *next;

    if (head) {
        next= head->next;
        delete head;

        while ((q= next) != head) {
            next=  q->next;
            delete q;
        }
    }
    head= tail= NULL;
}

Note: actually, as it is a circular list, 'tail' pointer could be removed.