Dynamic 2D array and memory free - c

Question

I'm having a big troubles figuring out how to rightfully create a 2D dynamic array, how to assert the memory and how to free it in the end.
I'll show you parts of my code, and please tell me what I'm doing wrong.
I declare on the dynamic array in main function and send it to BuildMatrix function that is supposed to assert the needed memory to the array and fill it.
that's how I declared on the array and send it to the function Build:
int row, column, i, j;
int **matrix;
BuildMatrix(&matrix, row, column);
now thats BuildMatrix decleration:
void BuildMatrix(int*** matrix, int row, int column);
And that's how I assert the memory (row and column have values that the user chose)
matrix =malloc(row * sizeof(int *));
for (i = 0; i < row; i++)
matrix[i] =malloc(column * sizeof(int));
Now so far everything works just fine, but when I try to free the memory, I get break point error
That's the function I used for freeing the memory:
void ExitAndFree(int** matrix, int row) {
int i;
for (i = 0; i < row; i++) {
free(matrix[i]);
}
free(matrix);
}
The debugger show me that the error is on the first free (if I remove the first one, the second gives the error)
There is also another problem, but I think its to much for now, I'll ask later... :P
Thanks for your help!!
P.S: If you got any good tutorials about pointers and dynamic arrays (I'd rather 2D+ arrays) I'd appreciate it a lot.

Answer 1

I do not think you need a *** pointer here, instead BuildMatrix could just return ** to main(). This design change would make your program easier, as working with *** can be a pain sometimes.
You also are not checking the return values of malloc() and scanf(), which can lead to future problems, and it's just safer to check those first. I also suggest you Don't cast result of malloc(), as this is not really needed in C.
With your errors of free() from the code you posted on #flintlock's answer, There is an inconsistency in your code:
You have declared:
void ExitAndFree(int** matrix, int row)
When this should be instead:
void ExitAndFree(int*** matrix, int row)
This change is needed in your code because you are calling &matrix in main for ExitAndFree(), so having **matrix in this function is not good enough. Again, this is because the code is using ***, which makes life harder.
Your code seems to work here with this change.
With these recommendations, you can also implement your program like this:
#include <stdio.h>
#include <stdlib.h>
int **BuildMatrix(int row, int column);
void PrintAndFree(int **matrix, int row, int column);
int main(void) {
int **matrix, row, column;
printf("\nPlease enter number of rows:\n");
if (scanf("%d", &row) != 1) {
printf("Invalid rows.\n");
exit(EXIT_FAILURE);
}
printf("\nPlease enter number of columns:\n");
if (scanf("%d", &column) != 1) {
printf("Invalid columns.\n");
exit(EXIT_FAILURE);
}
matrix = BuildMatrix(row, column);
PrintAndFree(matrix, row, column);
return 0;
}
int **BuildMatrix(int row, int column) {
int **matrix, rows, cols;
matrix = malloc(row * sizeof(*matrix));
if (matrix == NULL) {
printf("Cannot allocate %d rows for matrix.\n", row);
exit(EXIT_FAILURE);
}
for (rows = 0; rows < row; rows++) {
matrix[rows] = malloc(column * sizeof(*(matrix[rows])));
if (matrix[rows] == NULL) {
printf("Cannot allocate %d columns for row.\n", column);
exit(EXIT_FAILURE);
}
}
printf("\nPlease enter values to the matrix:\n");
for (rows = 0; rows < row; rows++) {
for (cols = 0; cols < column; cols++) {
if (scanf("%d", &matrix[rows][cols]) != 1) {
printf("Invalid value entered.\n");
exit(EXIT_FAILURE);
}
}
}
return matrix;
}
void PrintAndFree(int **matrix, int row, int column) {
int rows, cols;
printf("\nYour matrix:\n");
for (rows = 0; rows < row; rows++) {
for (cols = 0; cols < column; cols++) {
printf("%d ", matrix[rows][cols]);
}
free(matrix[rows]);
matrix[rows] = NULL;
printf("\n");
}
free(matrix);
matrix = NULL;
}

Answer 2

Here a copy&paste ready example...
#include <stdlib.h>
void BuildMatrix(int*** matrix, int row, int column) {
*matrix = malloc(row * sizeof(int *));
int i;
for (i = 0; i < row; i++)
(*matrix)[i] = malloc(column * sizeof(int));
}
void ExitAndFree(int** matrix, int row) {
int i;
for (i = 0; i < row; i++) {
free(matrix[i]);
}
free(matrix);
}
int main()
{
int row = 10, column = 10;
int **matrix;
BuildMatrix(&matrix, row, column);
ExitAndFree(matrix, row);
return 0;
}
However I would strongly recommend against working with raw pointers and separate variables for matrix directly. A better solution Software Engineering wise would be to either use an existing matrix library, or writer your own matrix library in C with struct...