struct + dynamic memory allocation + free ()

[gianx80]

Hi,
I'm studying the basis of C, C++ and Assembly languages at my university (I have two exams about these subjects, for now) ... and I have a problem ^^.
I wrote a program in C (not so optimized, this isn't our present goal for my professors) that recieves in input an integer (matrix size) and a double pointer to a square matrix and print on screen the co-ordinate of each square sub-matrix from size 2 to n - 1 size (where n is the integer that my function recieves in input) that has inside the biggest sum of its own members and the sum itself.
Then I modified the program so I created a struct in which I dynamically allocated two arrays in which i store the co-ordinate that my function finds. So I can allocate them, I can populate themm I can access their elements but I can't free them (I recieve an error regarding the corruption of the heap).
I can't figure out the problem ... I can dynamically allocate the matrix and free it, why I can't free my two arrays?

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

typedef struct cont_
{
	int *datax;
	int *datay;
} cont;

int** mtx_a (int num);
int mtx_i (int num, int **mtx);
void mtx_p (int num, int **mtx);
int mtx_c (int num, int **mtx, cont data);
void mtx_d(int num, int **mtx);

cont mtx_sa (int num);
int mtx_ss (int num, int x, int y, cont data);
void mtx_sd (cont data);

int main ()
{
	// (1) Dichiaro le variabili.
	int num = 0;
	int check = 0;
	
	// (2) Chiedo all'utente di inserire il lato della matrice.
	do
	{
		printf("Inserire la lunghezza del lato della matrice quadrata: ");
		scanf("%d", &num);

		if (num <= 0)
			printf("Il numero deve essere maggiore di 0\n");

	} while (num <= 0);

	// (I) Store Allocation
	cont data = mtx_sa (num);

	// (3) Dichiaro dopppio puntatore ed alloco spazio per matrice.
	int **test = mtx_a (num);

	// (4) Inizializzo matrice.
	mtx_i (num, test);

	// (5) Stampo la matrice.
	mtx_p (num, test);

	// (6) Eseguo operazioni su sotto-matrici quadrate.
	mtx_c (num, test, data);

	int i = 0;
	for (i = 2; i < num; i++)
	{
		printf("%d # %d\n", data.datax [i], data.datay [i]);
	}

	// (III) Store 3
	mtx_sd (data);
	
	// (7) Libero lo spazio allocato dinamicamente per la matrice.
	mtx_d (num, test);

	// (9) Metto in pausa l'applicazione per verificare i risultati e ritorno.
	system ("pause");
	return 0;
}

int** mtx_a (int num)
{
	int i  = 0;
	int j = 0;
	
	int **tmp = (int **) calloc (num, sizeof (int *));
	for (i = 0; i < num; i++)
	{
		tmp[i] = (int *) malloc (num * sizeof (int));
	}
	
	return tmp;
}

int mtx_i (int num, int **mtx)
{
	int i = 0;
	int j = 0;
	int val = 0;

	srand ((unsigned) time (NULL));

	for (i = 0; i < num; i++)
	{
		for ( j = 0; j < num; j++)
		{
			mtx [i][j] = val;
			val = (val + j + rand ()) % 9;
		}
	}
	
	return 0;
}

void mtx_p (int num, int **mtx)
{
	int i = 0;
	int j = 0;

	for (i = 0; i < num; i++)
	{
		for (j = 0; j < num; j++)
		{
			printf("%d", mtx [i][j]);
			
			if (j == num - 1)
				printf("\n");
			else
				printf(" ");
		}
	}
	printf("\n");
	
	return;
}

int mtx_c (int num, int **mtx, cont data)
{
	int i = 0;
	int j = 0;
	int z = 0;
	
	int side = num - 1;
	int lap = side * side;
	int sommaT = 0;
	int sommaD = 0;
	int I = 0;
	int J = 0;
	int lato = 2;
	int dis = 1;
	int pos1D = 0;
	int pos2D = 0;

	do
	{	
		for (z = lap; z > 0; z--)
		{
			for (i = I; i <= I + dis; i++)
			{
				for (j = J; j <= J + dis ; j++)
				{
					sommaT = sommaT + mtx [i][j];
				}
			}
			
			if (sommaT > sommaD)
			{
				sommaD = sommaT;
				pos1D = I;
				pos2D = J;
			}

			J++;

			if (J >= side)
			{
				I = I++;
				J = 0;
			}

			sommaT = 0;
		}

		// Store 2
		mtx_ss ((num + 1) - side, pos1D, pos2D, data);

		printf("Lato: %d # Somma sotto-matrice qudarata: %d. Coord. x: %d # y: %d\n\n", ((num + 1) - side), sommaD, pos1D, pos2D);

		side--;
		lap = side * side;
		lato++;
		sommaD = 0;
		I = 0;
		J = 0;
		dis++;
			
	} while (side >= 2);

	return 0;
}

void mtx_d(int num, int **mtx)
{
	int i = 0;

	for (i = 0; i < num; i++)
	{
		free (mtx [i]);
	}
	free (mtx);

	return;
}

cont mtx_sa (int num)
{
	cont data;

	data.datax = (int *) calloc (num + 1, sizeof (int));
	data.datay = (int *) calloc (num + 1, sizeof (int));

	data.datax [0] = 0;
	data.datay [0] = 0;
	data.datax [1] = 0;
	data.datay [1] = 0;
	data.datax [num] = 0;
	data.datay [num] = 0;
	data.datax [num + 1] = 0;
	data.datay [num + 1] = 0;
	
	return data;
}

int mtx_ss (int num, int x, int y, cont data)
{
	data.datax [num] = x;
	data.datay [num] = y;
	
	return 0;
}

void mtx_sd (cont data)
{	
	free (data.datax);
	free (data.datay);

	return;
}

[kbw]

This is a C++ program that uses C constructs.

There's a lot to think about, but to start with:

mtx_sa allocates a 2D array of ints. The line:
cont data = mtx_sa (num);
should read:
int** data = mtx_sa (num);

[gianx80]

But mtx_sa returns a "cont" variable, so i think could be correct to write cont data = mtx_sa (num); (I create a variable of cont type then I initialize it with mtx_sa that returns a cont variable).

P.S.
Is this a C++ program? I think I wrote it in C ... could you explain me the concept?

P.S.
Excuse for my English

[kbw]

You're right. I misread it.

There's a lot of procedural code there. I'll think about it some more.

[kbw]

In cont mtx_sa (int num), you allocate data.datax = (int *) calloc (num + 1, sizeof (int));. Then you do data.datax [num + 1] = 0;. You also do the same for datay.

This is an array bounds error and corrupts the heap.

[gianx80]

Solved, thanks ... it was so obvious but I couldn't see it ... thanks!