Undefined behavior on multiple execution of the same program

Question

I get a strange behavior when I compile the same program in Release in VS2010. If I run the program from cmd.exe multiple times, the results become gibberish. However, if I run the program in Debug, the result is always the same.

Starting a new command prompt makes the program give out the right outputs again.

Any idea of what might be causing this?

Screenshot of the command prompt

Edit : The code :

int main(int argc, char* argv[])
{
    int* R;
    int capacity;

    if (argc < 2)
    {
        cerr << "Veuillez entrer le chemin du fichier d'entree" << endl;
        return 1;
    }

    vector<BTS> stations;
    LoadFromFile(argv[1], stations, capacity);

    int count = 1;
    if (argc == 3)
    {
        count = atoi(argv[2]);
    }

    clock_t startClock = clock();

    R = new int[capacity+1];
    for(int j = 0; j < count; j++)
    {
        memset(R, 0, capacity + 1);

        for(unsigned int i=0; i < stations.size(); i++)
        {
            for(int j=capacity; j >= 0; j--)
            {
                if(j-stations[i].getNumberOfSubscribers() >= 0)
                {
                    R[j] = max(stations[i].getRevenue() + R[j-stations[i].getNumberOfSubscribers()], R[j]);
                }
            }
        }
    }

    // Print the results
    cout << "Value : " << R[capacity] << endl;
    cout << "Temps total : " << ((float)(clock() - startClock) / (float)CLOCKS_PER_SEC) << " s" << endl;

    delete[] R;

    return 0;
}

void LoadFromFile(std::string filePath, std::vector<BTS>& baseStations, int& capacity)
{
    fstream source(filePath);

    int numberOfStation;
    source >> numberOfStation;
    source >> capacity;

    for (int i = 0; i < numberOfStation; i++)
    {
        int index, revenue, numberOfSuscribers;
        source >> index;
        source >> revenue;
        source >> numberOfSuscribers;

        BTS station = BTS(index, revenue, numberOfSuscribers);

        baseStations.push_back(station);
    }

    source.close();
}

The rest of the code is only getters. Now for the file I give as input :

10 25
    1     7     6
    2     4     4
    3     7     6
    4     2     1
    5     7     8
    6     9    10
    7     4     5
    8    10    10
    9     1     1
   10    10    10

Obviously, some undefined behaviour. The code is required to do any meaningful diagnostics. It could be anything reading from or writing to a memory location you're not supposed to use in that way, or a dozen other things that lead to such memory access being done somewhere else, or something else entirely. — user395760
– user395760, Commented Nov 4, 2011 at 1:19
Will post the code, however I'm flabbergasted as of how can this happen only on the second execution of the program — Gab Royer
– Gab Royer, Commented Nov 4, 2011 at 1:24

Mysticial · Accepted Answer · 2011-11-04 01:28:50Z

2

Your memset isn't correct. You forgot the sizeof():

memset(R, 0, capacity + 1);

should be:

memset(R, 0, (capacity + 1) * sizeof(int));

Since you didn't zero all of R, the upper parts of it are undefined and are giving you junk data.

answered Nov 4, 2011 at 1:28

Mysticial

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1 Comment

Gab Royer Over a year ago

Damn. Hadn't done C++ in a while and now it shows. Thanks a lot!

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