incorrect checksum for freed object

Question

I am getting this error (memory location varies between runs):

q2(4910,0x7fff7a1d4300) malloc: *** error for object 0x7fdf79c04bd8: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug
Abort trap: 6

This is the function that crashes:

public:
// construct a 'rows X cols' matrix.
SMatrix(int rows, int cols) {
    if (rows<1 || cols<1) {
        cout<<"Invalid row/col value(s).";
        exit(-1);
    }
    this->_rows = rows;
    this->_cols = cols;
    this->_vertical = new simpleNode [rows];
    this->_horizontal = new simpleNode [cols];
    if (this->_vertical == NULL || this->_horizontal==NULL) {
        cout<<"Exiting";
        exit(-1);
    }
    initArrays();
}

It crashes on this particular line:

  this->_horizontal = new simpleNode [cols];

The function that calls:

int main() {
      SMatrix bigM(500,500);
      bigM.setElement(10,20,17);
      cout <<" bigM - total size in bytes: (implementation depended): "
       << bigM.sizeInBytes() << endl << endl; 

      SMatrix m1(7,10),m2(7,10),m4(10,2),m5(7,2); //Crashes on m4(10,2)
}

Other functions that could be relevant:

struct simpleNode {
    Node* _next;
};
int _rows; //Number of rows in this SMatrix
int _cols; //Number of columns in this SMatrix
simpleNode * _vertical; //array (simpleNode)
simpleNode * _horizontal;  //array (simpleNode)
/*Initiate the horizontal/vertical arrays to point to null*/
void initArrays() {
    int i;
    for (i=0; i<this->_rows; i++)
        this->_horizontal[i]._next = NULL;
    for (i=0; i<this->_cols; i++)
        this->_vertical[i]._next = NULL;
}

I am on OSX. I compiled with -g and ran it with GDB but Program exited normally. How can I debug this if I don't use XCode? Also a hint on how to fix the problem would be very helpful.

Edit: I'm running the output file and sometimes it runs while others it gives me the error. Seems to be at a random order. Also, the program never fails when I run it on gdb it always exits correctly. Why is this happening?

Answer 1

Your limits are reversed in your initialization code. You create your arrays like this:

this->_vertical = new simpleNode [rows];   // <== uses rows for sizing vertical
this->_horizontal = new simpleNode [cols]; // <== uses cols for sizing horizontal

But your initialization does this:

for (i=0; i<this->_rows; i++) // <== limit is rows, but you walking horizontal
    this->_horizontal[i]._next = NULL;
for (i=0; i<this->_cols; i++) // <== limit is cols, but you walking vertical
    this->_vertical[i]._next = NULL;

Unless rows and cols are the same value, this code invokes undefined behavior. Fix this by using the same values as you sized your allocation with

for (i=0; i<this->_rows; i++)
    this->_vertical[i]._next = NULL;
for (i=0; i<this->_cols; i++)
    this->_horizontal[i]._next = NULL;

Honestly a much better approach would use RAII containers such as std::vector<>, but I leave that as an exercise for you.

Best of luck, and hope it helps.