Question

I have this code for filling a table but I keep getting a segmentation error. I’m at my wit’s end trying to find what the error could be. My function takes in 2 maps and iterates through them to find the common strings. It takes the int value of these commons strings and puts it in a table to count the number of times there are common strings.

myMap findTable(mapGraph * dbgraph1, mapGraph * dbgraph2)
{
typedef mapGraph::const_iterator iter;
typedef myMap::const_iterator mapiter;

iter it1 = dbgraph1->begin();
iter it2 = dbgraph2->begin();
int count =0;
myMap * newTable = NULL;

//iterating through the 2 samples of dbgraphs
while (it1 != dbgraph1->end() && it2 != dbgraph2->end())
{
    //a match is found for 2 strings
    if (it1->first == it2->first)
    {
        //the component ids of first sample
        int compdb1 = it1->second->comp;

        //the component ids of second sample
        int compdb2 = it2->second->comp;

        //inserting the component ids and counts in the map
        newTable->insert(make_pair(make_pair(compdb1, compdb2), count));
        count++;

        for (mapiter it = newTable->begin(); it != newTable->end(); it++)
        {
            printf("%i %i\t %i\n", it->first.first, it->first.second, it->second);
        }

        it1++;
        it2++;

    }

    //match not found
    else
        it1++;
        it2++;

}
printf("\nCLEAR\n");
return newTable;
}

This is the error:

Address 0x10 is not stack'd, malloc'd or (recently) free'd
Invalid read of size 8
Process terminating with default action of signal 11 (SIGSEGV)
Access not within mapped region at address 0x10

Answer 1

newTable is NULL:

myMap * newTable = NULL;

and is never assigned to a valid object before:

newTable->insert(make_pair(make_pair(compdb1, compdb2), count));

Dereferencing a NULL pointer is undefined behaviour. Either dynamicaly allocate a myMap instance for newTable:

myMap* newTable = new myMap(); // Remember to delete later.

or use a stack allocated instance:

myMap newTable;

This is an error:

//match not found
    else
        it1++;
        it2++;

resulting in it2 being incremented on every iteration of the loop and will result in ++ being called on an end() iterator. Change to:

//match not found
    else
    {
        it1++;
        it2++;
    }

Or to simplify the code just increment the iterators at one place in the loop as they are incremented always (in each branch of the if\else).