Exception thrown at 0x00007FF83DE5499E (ucrtbased.dll) in HW5_NEW.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF. Error in my code. What could cause it?

Question

Exception thrown at 0x00007FF83DE5499E (ucrtbased.dll) in HW5_NEW.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF. Error in my code. What could cause it?

Ron Petlyarsky 1

Hello everyone! I am new to coding in C++ and I'm studying 2d dynamic arrays and I am having a problem with handling pointers and dynamic allocations. The program crushes in the third func loop and gives me the error in the title. I know it's related to memory mismanagement but I honestly don't know how to approach it at this point :( Would appreciate if you could point me to the problematic section in my code and explain it. Thank you!

#include <iostream>  
#include <time.h>  
#include <string.h>  
using namespace std;  
const int lowLevel = 1, highLevel = 3, noNames = 0, nullCell = 1, maxLen = 31, maxIncrease = 2;  
const char enter = '\n';  
  
void insertDynamicName(char* p, char* name, int currentLen)  
{  
    char *pTemp = new char[currentLen];  
    if (pTemp != nullptr)  
    {  
        strcpy_s(pTemp, currentLen, name);  
        p = pTemp;  
    }  
      
}  
  
void pCharArrDelete(char** pArr, int rows)  
{  
    for (int i = 0; i < rows; i++)  
    {  
        delete[]pArr[i];  
    }  
    delete[]pArr;  
}  
  
void copyCharPointers(char** source, char** dest, int qty)  
{  
    for (int i = 0; i < qty; i++)  
        dest[i] = source[i];  
}  
  
void addNullPtrs(char** studentNames, int maxSize, int i)  
{  
    while (i < maxSize)  
    {  
        studentNames[++i] = nullptr;  
    }  
}  
  
char** addNames(int* pMaxSize, int* pNumNames)  
{  
    char inputName[maxLen] = { 0 };  
    int currentLen = 0, i;  
    char** studentNames = new char* [*pMaxSize], ** temp = nullptr;  
    if (studentNames == nullptr)  
    {  
        return nullptr;  
    }  
    for (i = 0; *inputName != enter; i++)  
    {  
        cout << "\nPlease enter the name of the student: ";  
        cin >> inputName;  
        currentLen = strlen(inputName) + nullCell;  
        insertDynamicName(studentNames[i], inputName, currentLen);  
        if (studentNames[i] == nullptr)  
        {  
            pCharArrDelete(studentNames, *pNumNames);  
            return nullptr;  
        }  
        if (i == *pMaxSize)  
        {  
            temp = new char* [*pMaxSize];  
            copyCharPointers(studentNames, temp, *pMaxSize);  
            pCharArrDelete(studentNames, *pMaxSize);  
            studentNames = nullptr;  
            *pMaxSize *= maxIncrease;  
            studentNames = new char* [*pMaxSize];  
            if (studentNames == nullptr)  
            {  
                return nullptr;  
            }  
            copyCharPointers(temp, studentNames, i);  
            addNullPtrs(studentNames, *pMaxSize, i);  
            pCharArrDelete(temp, i);  
            temp = nullptr;  
        }  
    }  
    *pNumNames = ++i;  
    return studentNames;  
}  
  
int main()  
{  
    srand(time(NULL));  
    int numNames = 0, maxSize = 1;  
    int* pMaxSize = &maxSize, * pNumNames = &numNames;  
    char** studentNames = addNames(pMaxSize, pNumNames);  
    if (**studentNames == enter)  
    {  
        cout << "No student names were entered. Please try again.";  
        numNames = 0;  
        pCharArrDelete(studentNames, maxSize);  
        studentNames = addNames(pMaxSize, pNumNames); //user will be prompted to try entering the names again, since he didn't enter anything and tried to continue the input  
    }  
    if (studentNames == nullptr) //allocation failed  
    {  
        cout << "Error in memory allocation! Program will exit.\n";  
        return 0;  
    }  
    for (int i = 0; i < numNames; i++)  
        cout << studentNames[i] << endl;  
  
    return 0;  
  
}

RLWA32 49,651 Reputation points

2023-01-02T12:25:14.537+00:00

Access violations typically result from de-referencing a null pointer or a pointer containing an invalid address.

What you should do is use the Visual Studio Debugger to step through your code line by line. While you are doing this you should examine the contents of variables and function arguments when the function is called and when it returns. Pay particular attention to the contents of any pointer variables.

Also, remember that when a variable is passed by value in C/C++ the function receives a copy of the variable. Any changes made by the function to a variable passed by value are made to the copy. The original variable contents remain unchanged.
David Lowndes 4,726 Reputation points

2023-01-02T12:26:23.167+00:00

Your code crashes when it calls pCharArrDelete because the pointer passed to it is pointing nowhere - because insertDynamicName hasn't updated what you perhaps think it should. Presumably the latter function should accept a pointer to a char *. However, the problems don't stop there and time is short.
As you're learning, learn to use the debugger, and do yourself a big favour and don't write code that uses new/delete, malloc, free, pointers, etc. By all means be aware of them, but as you're finding, they're a minefield for mistakes once you start to do anything non-trivial. Use the C++ standard library collection classes, not low-level 'C' code.
Minxin Yu 13,506 Reputation points Microsoft External Staff

2023-01-17T07:44:37.24+00:00

Hi, @Ron Petlyarsky
May I know if your problem is solved?

2 answers

Your answer

RLWA32 49,651 Reputation points

2023-01-02T12:25:14.537+00:00

Access violations typically result from de-referencing a null pointer or a pointer containing an invalid address.

What you should do is use the Visual Studio Debugger to step through your code line by line. While you are doing this you should examine the contents of variables and function arguments when the function is called and when it returns. Pay particular attention to the contents of any pointer variables.

Also, remember that when a variable is passed by value in C/C++ the function receives a copy of the variable. Any changes made by the function to a variable passed by value are made to the copy. The original variable contents remain unchanged.
David Lowndes 4,726 Reputation points

2023-01-02T12:26:23.167+00:00

Your code crashes when it calls pCharArrDelete because the pointer passed to it is pointing nowhere - because insertDynamicName hasn't updated what you perhaps think it should. Presumably the latter function should accept a pointer to a char *. However, the problems don't stop there and time is short.
As you're learning, learn to use the debugger, and do yourself a big favour and don't write code that uses new/delete, malloc, free, pointers, etc. By all means be aware of them, but as you're finding, they're a minefield for mistakes once you start to do anything non-trivial. Use the C++ standard library collection classes, not low-level 'C' code.
Minxin Yu 13,506 Reputation points Microsoft External Staff

2023-01-17T07:44:37.24+00:00

Hi, @Ron Petlyarsky
May I know if your problem is solved?

Answer 1

Minxin Yu 13,506 Microsoft External Staff

Hi, @Ron Petlyarsky

Add & in insertDynamicName();

void insertDynamicName(char* &p, char name[], int currentLen)

In addition, copyCharPointers copies the pointer. But the pointers will be deleted in pCharArrDelete. You need to store the value.

Best regards,

Minxin Yu

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Ron Petlyarsky 1 Reputation point

2023-01-04T15:19:16.75+00:00

Hey there! Thanks for you help.
In regards to copyCharPointers, isn't copying addresses from one char** to another enough?
How can I improve that func?
Minxin Yu 13,506 Reputation points Microsoft External Staff

2023-01-05T02:07:31.663+00:00

You need to allocate memory for dest[i]. For your reference: Copying a char** into another char**

Considering you are using C++ instead of C, it is recommended using std::vector and string instead of char*

Answer 2

While not related to any of the current issues you have with
your program's operation, there is a basic (and somewhat
common) error in the way you are testing for failure
of the new operator. C++ compilers from many years ago
would generate code that would return NULL if a new
or new[] failed (usually because of insufficient heap
memory available to satisfy the request).

However, for many years now that is not what happens.
Instead, a bad_alloc exception will be thrown if
new/new[] fails. So testing for nullptr after each
new/new[] is pointless as the code will never be
executed (and the pointer will not be set to nullptr
or NULL/null/0).

To have new/new[] return a nullptr instead of throwing
an exception you must use the nothrow version:

new(std::nothrow) ...

So if you want your code to actually work as written
in the event of an allocation failure you must change
all of your new statements to use that form.

"If there's insufficient memory for the allocation request,
operator new throws a std::bad_alloc exception. Or, it returns
nullptr if you've used the placement form new(std::nothrow),
or if you've linked in non-throwing operator new support.
For more information, see Allocation failure behavior."

new and delete operators
https://learn.microsoft.com/en-us/cpp/cpp/new-and-delete-operators?view=msvc-170

Also see:

new operator (C++)
https://learn.microsoft.com/en-us/cpp/cpp/new-operator-cpp?view=msvc-170

By way of an example which illustrates both behaviours,
you can build this small program and run it with a
breakpoint set on the return statement. Try it first with
the statement:

cpp = new char*;

Check the output in the console window.

Then rebuild it using the nothrow version:

cpp = new (nothrow) char*;

Check the output in the console window.

#include <iostream>  
using namespace std;  
  
int main()  
{  
    int n;  
    char ** cpp = nullptr;  
    for (n = 0; n < 2000000000; ++n)  
    {  
        try{  
            cpp = new char*; // throws a bad_alloc exception  
            //cpp = new (nothrow) char*; // returns nullptr  
        }  
        catch (std::bad_alloc)  
        {  
            cout << "new failed! (bad_alloc)" << endl;  
            break;  
        }  
          
        if (cpp == nullptr)  
        {  
            cout << "new failed! (nullptr)" << endl;  
            break;  
        }  
    }  
  
    return 0;  
}

Wayne

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Exception thrown at 0x00007FF83DE5499E (ucrtbased.dll) in HW5_NEW.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF. Error in my code. What could cause it?

2 answers

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