Read access violation 0xCDCDCDCD

Question

I keep getting a read access violation. Here is the code that I have.

class List
{
public:
    List();
    List(const List &copy);
    ~List();

    /*List & operator=(const List &rhs);


    Record * headPtr() const;
    void setheadptr(Record * const newhead);

    bool insertatfront(Record newdata);*/


    void importcourselist();
    void Loadmasterlist();
    void storemasterlist();

    Record *makenode(fstream &file);
    Record *makenode(ifstream &file);

private:
    Record *mHead;
    Record *mEnd;

};
List::List()
{
    mHead = nullptr;
    mEnd = nullptr;

}

void List::storemasterlist()
{
    ofstream outfile;
    outfile.open("masterlist.txt");

    mEnd = mHead;
    while (mEnd->getnext()!= nullptr)
    {
        outfile << mEnd << endl;
        mEnd = mEnd->getnext();

    }
    outfile << mEnd;
}

I'm not exactly sure what is going on but, every time I try to debug it it'll send me to the getter function for my pointer which looks like this:

 Record *Record::getnext()
    {
        return mnext;
    }

I believe that my error lies somewhere in my import function. This is where I'm creating the extra empty node.

void List::importcourselist()
{
    ifstream infile;
    string ptoken;
    infile.open("studentlist.csv");

    getline(infile, ptoken);

    mHead = makenode(infile);

    mEnd = mHead;



    while (!infile.eof())
    {
        mEnd->setnext(makenode(infile));
        mEnd = mEnd->getnext();
    }

    infile.close();
}

@M.M my mEnd is Null, how do I solve that, when I fix the loop to stop it at while(mEnd!=null) i still get the same issue — bigpimpin206, Jul 20 '17 at 05:26
Change your code so that you do not do `->` on a null pointer — M.M, Jul 20 '17 at 05:27
Ok, I think my issue might be that I'm creating one final node at the end of my list. — bigpimpin206, Jul 20 '17 at 05:32
0xCDCDCDCD looks like an uninitialized memory marker. When you see that pattern assuming your target is an uninitialzed pointer is usually pretty safe. — user4581301, Jul 20 '17 at 05:32
[SO: In Visual Studio C++, what are the memory allocation representations?](https://stackoverflow.com/a/127404/7478597) provides an overview with memory patterns which can help for debugging. — Scheff's Cat, Jul 20 '17 at 05:35
You should read [Why is `iostream::eof` inside a loop condition considered wrong?](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong). — molbdnilo, Jul 20 '17 at 06:03
If I had a penny for every linked list implementation problem!!! — R Sahu, Jul 20 '17 at 06:03

CiaPan · Answer 1 · 2017-07-21T09:55:59.487

The constant 0xCD is used by Microsoft C++ debug libraries to fill allocated heap blocks. So you can see the pattern 0xCDCDCDCD when you read data from a dynamic heap object which hasn't been initialized by your code.

You didn't show the Record class definition, but I guess your Record::Record constructor fails to set the next pointer to nullptr. When iteration comes to the end of the list you fetch that magic 0xCDCDCDCD value with getnext(). That is an invalid pointer, hence you fail to access the memory at 0xCDCDCDCD with the next getnext() call.

Sources:

Magic number (programming) at Wikpedia
Win32 Debug CRT Heap Internals by Andrew Birkett at his www.nobugs.org
Magic Numbers in Visual C++ (Aug 2009) at Adam Sawicki's asawicki.info

score 0 · Answer 2 · answered Jul 20 '17 at 05:33

0

This is sort of a guess:

you call while(mEnd->getnext() ...) for the loop
then in the body of the loop you call mEnd->getnext() again, which does not deliver the same result; i.E. it might be past the end already.

answered Jul 20 '17 at 05:33

user23573

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Read access violation 0xCDCDCDCD

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