Question

Problem solved, thank you all for the help

I’ve got a bit of a problem here it’s not something that’s blowing my program up, but it’s just bothering me that I can’t fix it. I have a function reading in some data from a file, at the end of the execution, the stack around variable longGarbage is corrupted. I’ve looked around a bit and found that a possible cause is writing to invalid memory. I cleaned up some memory leaks that I had and the problem still persists. What’s confusing me is that it happens when the function finishes executing, so it appears to be happening when the variable goes out of scope. Here’s the code…

CHCF::CHCF(std::string fileName)
: PAKID("HVST84838672")
{
FILE * archive = fopen(fileName.c_str(), "rb");
std::string strGarbage = "";
unsigned int intGarbage = 0;
unsigned long longGarbage = 0;
unsigned char * data = 0;
char charGarbage = '0';

if (!archive)
{
    fclose (archive);
    return;
}

for (int i = 0; i < 12; i++)
{
    fread(&charGarbage, 1, 1, archive);
    strGarbage += charGarbage;
}


if (strGarbage != PAKID)
{
    fclose(archive);
    throw "Incorrect archive format";
}
strGarbage = "";

fread(&_gameID, sizeof(_gameID),1,archive);
fread(&_fileCount, sizeof(_fileCount),1,archive);

for (int i = 0; i < _fileCount; i++)
{
    fread(&longGarbage, 8,1,archive); //file offset

    fread(&intGarbage, 4, 1, archive);//fileName

    for (int i = 0; i < intGarbage; i++)
    {
        fread(&charGarbage, 1, 1, archive);
        strGarbage += charGarbage;
    }

    fread(&longGarbage, 8, 1, archive); //fileSize

    fread(&intGarbage, 4, 1, archive); //fileType

    data = new unsigned char[longGarbage];

    for (long i = 0; i < longGarbage; i++)
    {
        fread(&charGarbage, 1, 1, archive);
        data[i] = charGarbage;
    }

    switch ((FILETYPES)intGarbage)
    {
    case MAP:
        _maps.append(strGarbage, new CFileData(strGarbage, FILETYPES::MAP, data, longGarbage));
        break;

    default:
        break;
    }

    delete [] data;
    data = 0;
    strGarbage.clear();
    longGarbage = 0;

}
fclose(archive);
} //error happens here

Here is the CFileData constructor:

CFileData::CFileData(std::string fileName, FILETYPES type, unsigned char *data, long fileSize)
{
_fileName = fileName;
_type = type;
_data = new unsigned char[fileSize];

for (int i = 0; i < fileSize; i++)
    _data[i] = data[i];
}

Answer 1

What’s happening is that something is writing to memory around or over the location of longGarbage which is causing the corruption.

You don’t say what development environment you are using. One way to diagnose this would be to set a breakpoint that triggers when a specific memory location changes. Choose a memory location that overlaps the area of corruption and wait for it to trigger unexpectedly.

Another way to diagnose this would be to examine code that changes memory around or over longGarbage. That could be almost anything of course but likely candidates are modifications to ‘data’, modifications to ‘intGarbage’ and modifications to ‘longGarbage’ itself.

We can narrow things down even further because we can (usually) be fairly sure the assignment operator itself is safe. Code like data = new... isn’t likely to be the culprit so really we need to focus on memory changes that involve taking the address of ‘data’, ‘intGarbage’ or ‘longGarbage’. In particular memory changes that change more bytes than they should.

Several others have already pointed out that a long is probably not eight bytes in length. If you pass the wrong length to fread, the extra bytes retrieved have to go somewhere.