C: Realloc twodimensional array created by Malloc

Question

I have a two-dimensional int array allocated as follows:

int **matrix = (int**)malloc(x * sizeof(int));
for (i = 0; i < x; i++)
    matrix[i] = (int*)malloc(y * sizeof(int));

If I do realloc this array by the following code

int **matrix = (int**)realloc(matrix, x * sizeof(int));
for (i = 0; i < x; i++)
    matrix[i] = (int*)malloc((y) * sizeof(int));

will there be any leftover "tails" of second dimension of the array and should I use free() function for them before reallocating, or they will go away themselves?

There is no 2D array, nor something that can be used as one. A pointer is not an array! — too honest for this site
– too honest for this site, Commented Mar 19, 2016 at 21:18
Some of your sizeof expressions are wrong. When allocating/reallocating matrix, you need x*sizeof(int *). — Tom Karzes
– Tom Karzes, Commented Mar 19, 2016 at 21:22
Well I can access elements of this "array" by using indexes as always (array[i][j]) and easily passing it to other functions. That is the only things I need for now. Edit: Thanks Weather Vane — 1valdis
– 1valdis, Commented Mar 19, 2016 at 21:22

Antti Haapala · Accepted Answer · 2016-03-19 21:17:49Z

1

You will completely lose data in your matrix, so this is not really reallocing anything; you could just as well free and malloc the top-level array, getting possibly better performance. Furthermore your code would leak memory, because the 2nd level allocations are not freed.

answered Mar 19, 2016 at 21:17

Antti Haapala

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1valdis Over a year ago

As I suspected. Thanks!

chqrlie · Accepted Answer · 2016-03-20 21:12:18Z

1

Here is how you can resize the 2D matrix while keeping the current values intact, initializing the newly allocated cells to 0 and freeing any extra leftovers:

// matrix was allocated to x and y
// reallocate it to newx, newy
for (i = newx; i < x; i++) { /* free unneeded rows */
    free(matrix[i]);
}
matrix = realloc(matrix, sizeof(*matrix) * newx);
for (i = x; i < newx; i++) { /* initialize the new row pointers */
    matrix[i] = NULL;
}
for (i = 0; i < newx; i++) {
    matrix[i] = realloc(matrix[i], sizeof(*matrix[i]) * newy);
    for (int j = y; j < newy; j++)
        matrix[i][j] = 0;
}

edited Mar 20, 2016 at 21:12

answered Mar 19, 2016 at 22:57

chqrlie

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