Increasing the Size of a Dynamically Allocated 2D Array in C

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Lesson 68Mixed lesson

Increasing the Size of a Dynamically Allocated 2D Array in C

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Lesson format7 sections1 video2 code blocks1 practice itemUpdated Aug 27, 2026

Resizing a 2D array is harder than resizing a 1D array because the row-pointer structure has two levels of indirection. Adding rows means reallocating the pointer array and allocating new row buffers. Adding columns means reallocating every existing row. Both operations must preserve existing data. Both have failure paths that require cleanup of partially completed work. This is the kind of code where a single missed free leaks memory and a single missed NULL check crashes the program.

Increase The Size Of A Dynamically Allocated 2D Array

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

typedef struct {
    int **rows;
    int num_rows;
    int num_cols;
} Grid;

int grid_init(Grid *g, int rows, int cols) {
    g->rows = malloc(rows * sizeof(int *));
    if (!g->rows) return -1;
    for (int i = 0; i < rows; i++) {
        g->rows[i] = calloc(cols, sizeof(int));
        if (!g->rows[i]) {
            for (int j = 0; j < i; j++) free(g->rows[j]);
            free(g->rows);
            return -1;
        }
    }
    g->num_rows = rows;
    g->num_cols = cols;
    return 0;
}

int grid_add_row(Grid *g) {
    int **tmp = realloc(g->rows, (g->num_rows + 1) * sizeof(int *));
    if (!tmp) return -1;
    g->rows = tmp;

    g->rows[g->num_rows] = calloc(g->num_cols, sizeof(int));
    if (!g->rows[g->num_rows]) return -1;

    g->num_rows++;
    return 0;
}

void grid_free(Grid *g) {
    for (int i = 0; i < g->num_rows; i++) free(g->rows[i]);
    free(g->rows);
    g->rows = NULL;
    g->num_rows = 0;
    g->num_cols = 0;
}

int main(void) {
    Grid g;
    grid_init(&g, 2, 3);

    g.rows[0][0] = 1; g.rows[0][1] = 2; g.rows[0][2] = 3;
    g.rows[1][0] = 4; g.rows[1][1] = 5; g.rows[1][2] = 6;

    printf("Before (%dx%d):\n", g.num_rows, g.num_cols);
    for (int i = 0; i < g.num_rows; i++) {
        for (int j = 0; j < g.num_cols; j++)
            printf("%d ", g.rows[i][j]);
        printf("\n");
    }

    grid_add_row(&g);
    g.rows[2][0] = 7; g.rows[2][1] = 8; g.rows[2][2] = 9;

    printf("\nAfter (%dx%d):\n", g.num_rows, g.num_cols);
    for (int i = 0; i < g.num_rows; i++) {
        for (int j = 0; j < g.num_cols; j++)
            printf("%d ", g.rows[i][j]);
        printf("\n");
    }

    grid_free(&g);
    return 0;
}

Run grid_resize

root@vulnarex:~#gcc -Wall -fsanitize=address grid_resize.c -o grid_resize && ./grid_resize

The existing rows are untouched. The pointer array is extended by one slot. The new row is allocated and zero-initialized with calloc. The cleanup in grid_free walks all rows, then frees the pointer array. Every allocation has a corresponding free. Every failure path cleans up what was allocated so far. This discipline is not optional. It is the price of manual memory management.

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quiz BLOCK (★ 40 XP)

When adding a row to a row-pointer 2D array, what must be reallocated?

Select your proof vectors above

Dynamic 2D arrays are the gateway to matrices, image buffers, and grid-based simulations. The two-level allocation pattern is fiddly but mechanical. Get the cleanup right and the rest is bookkeeping. Pointers and memory module complete. Next: strings, where we spend 18 lessons mastering the standard library's string functions and their security implications.

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Laboratory sanity code

Isolate active probes on matched virtual networks and keep execution streams sandboxed.

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

typedef struct {

int **rows;

int num_rows;

int num_cols;

} Grid;

int grid_init(Grid *g, int rows, int cols) {

g->rows = malloc(rows * sizeof(int *));

if (!g->rows) return -1;

for (int i = 0; i < rows; i++) {

g->rows[i] = calloc(cols, sizeof(int));

if (!g->rows[i]) {

for (int j = 0; j < i; j++) free(g->rows[j]);

free(g->rows);

return -1;

}

g->num_rows = rows;

g->num_cols = cols;

return 0;

}

int grid_add_row(Grid *g) {

int **tmp = realloc(g->rows, (g->num_rows + 1) * sizeof(int *));

if (!tmp) return -1;

g->rows = tmp;

g->rows[g->num_rows] = calloc(g->num_cols, sizeof(int));

if (!g->rows[g->num_rows]) return -1;

g->num_rows++;

return 0;

}

void grid_free(Grid *g) {

for (int i = 0; i < g->num_rows; i++) free(g->rows[i]);

free(g->rows);

g->rows = NULL;

g->num_rows = 0;

g->num_cols = 0;

}

int main(void) {

Grid g;

grid_init(&g, 2, 3);

g.rows[0][0] = 1; g.rows[0][1] = 2; g.rows[0][2] = 3;

g.rows[1][0] = 4; g.rows[1][1] = 5; g.rows[1][2] = 6;

printf("Before (%dx%d):\n", g.num_rows, g.num_cols);

for (int i = 0; i < g.num_rows; i++) {

for (int j = 0; j < g.num_cols; j++)

printf("%d ", g.rows[i][j]);

printf("\n");

}

grid_add_row(&g);

g.rows[2][0] = 7; g.rows[2][1] = 8; g.rows[2][2] = 9;

printf("\nAfter (%dx%d):\n", g.num_rows, g.num_cols);

for (int i = 0; i < g.num_rows; i++) {

for (int j = 0; j < g.num_cols; j++)

printf("%d ", g.rows[i][j]);

printf("\n");

}

grid_free(&g);

return 0;

}