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Array pointer deletion problem, Heap Corruption

Posted on 2007-04-02
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Last Modified: 2012-05-05
Hello,

i have a problem with deleting an array pointer:

char* compareBuffer = new char[patternSize];

This is how i create the array, patternSize is an int.
When i try to do delete[] compareBuffer; later on in the program, i get the following error reported:
-------------------------------------------
Debug Error!
.....
HEAP CORRUPTION DETECTED: after Normal block (#196) at 0x003694C8. CRT detected that the application wrote to memory after end of heap buffer.
--------------------------------------

Can someone explain what this means and how i can fix it? (If it is important, I'm using Visual Studio 2005)

Thank you.
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Question by:b3n_
9 Comments
 
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Expert Comment

by:Darrylsh
ID: 18840908
Can we see some code, especially anything after the delete.  If I had to guess you are using your pointer after you delete it and it might not be obvious such as

delete[] compareBuffer;
*compareBuffer = 0; \\error
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Author Comment

by:b3n_
ID: 18840933
This is the code:

bool findPattern()
      {
            delete[] matchOffsets;
            matchOffsets = new int[256];
                  
            char readChar;

            std::fstream fileReader;
            fileReader.open(fileName, std::ios::in | std::ios::binary);
            if(fileReader.is_open()){
                  while(!fileReader.eof()){
                        fileReader.read(&readChar, sizeof readChar);

                        if((unsigned char)readChar == searchPattern[0]){

                              bool match = true;
                              char* compareBuffer = new char[patternSize];

                              fileReader.seekg((int)fileReader.tellg()-1);                        
                              fileReader.read(compareBuffer, sizeof compareBuffer);
                              for(int i = 0; i < patternSize; i++){
                                    match = (unsigned char)compareBuffer[i] == searchPattern[i];
                                    if(!match){
                                          break;
                                    }
                              }
                              if(!match){
                                    fileReader.seekg((int)fileReader.tellg() - patternSize + 1);
                              } else {
                                    int fileOffset = (int)fileReader.tellg() - patternSize;
                                    matchOffsets[matchCounter++] = fileOffset;
                              }
                              //Here comes the delete
                              delete[] compareBuffer;
                        }
                  }
            } else {
                  std::cout << "Error opening file";
                  return false;
            }
            fileReader.close();

            return true;
      }
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LVL 30

Expert Comment

by:Axter
ID: 18841320
Is matchOffsets being initialized to NULL?

If it's not, this can give you a runtime error, since it's content's can be pointing to anything.

I recommend you use std::vector<char> instead of the above code.
std::vector is a safer way to create a dynamic array, and you don't have to worry about clean up via delete, because the array cleans up after itself.
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LVL 30

Accepted Solution

by:
Axter earned 50 total points
ID: 18841330
>> fileReader.read(compareBuffer, sizeof compareBuffer);

The above line of code is not correct.  the sizeof compareBuffer, will be the size of the pointer, and not the size of the allocated  buffer.
Should be the following:
fileReader.read(compareBuffer, patternSize);

>>matchOffsets = new int[256];
You should avoid using a dynamic array, if you need a small fixed size array.
Since the size of the above array is known at compile time, you can just declare a concrete variable.

int matchOffsets[256] = {0};

This is safer, and more efficient.
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Author Comment

by:b3n_
ID: 18841379
its not about the matchOffsets its about the compare buffer. The rest of the code works fine, i just get problems when i try to delete the compareBuffer after i used it. And in the case of the compareBuffer i dont know how big it will be so i need to use dynamic arrays.
0
 

Author Comment

by:b3n_
ID: 18841517
i found the problem with this code snippet. the problem lies in this line:  fileReader.read(compareBuffer, sizeof compareBuffer); i shouldnt use sizeof comparebuffer, because it returns 4 while the actual buffer size is just 3.

thanks all for commenting on this question.
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LVL 39

Expert Comment

by:itsmeandnobodyelse
ID: 18841731
>>>> HEAP CORRUPTION DETECTED:
That is cause you have written beyond the allocated boundaries of the char array. If you allocate a char array in debug mode it is allocated some more space for the debugger. The debugger writes a signature at end which was checked at deletion. If you have written beyond boundaries the signature most likely was destroyed, hence the debug assertion.

Regards, Alex
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Expert Comment

by:Axter
ID: 18842367
>>i found the problem with this code snippet. the problem lies in this line:  fileReader.read(compareBuffer, sizeof compareBuffer); i shouldnt use sizeof
>>comparebuffer, because it returns 4 while the actual buffer size is just 3.

That's what I posted in my second comment.
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