A 2-D array in C is an array of arrays, stored row by row (row-major) in one contiguous block.

#include <stdio.h>

int main(void) {
    int m[2][3] = {
        {1, 2, 3},
        {4, 5, 6},
    };
    for (int r = 0; r < 2; r++) {
        for (int c = 0; c < 3; c++) printf("%d ", m[r][c]);
        printf("\n");
    }
    printf("sizeof m = %zu, sizeof m[0] = %zu\n", sizeof m, sizeof m[0]);

    int *flat = &m[0][0];               /* row-major: rows are laid end to end */
    for (int i = 0; i < 6; i++) printf("%d ", flat[i]);
    printf("\n");
    return 0;
}

Output

1 2 3
4 5 6
sizeof m = 24, sizeof m[0] = 12
1 2 3 4 5 6

address of m[r][c] = base + (r × COLS + c) × sizeof(element)

Passing a 2-D array to a function

All dimensions except the first must be known to the function, because the compiler needs the row length to compute m[r][c].

#include <stdio.h>

#define COLS 3

int trace_like_sum(int m[][COLS], int rows) {        /* fixed column count */
    int s = 0;
    for (int r = 0; r < rows; r++)
        for (int c = 0; c < COLS; c++) s += m[r][c];
    return s;
}

void print_matrix(int rows, int cols, int m[rows][cols]) {   /* C99 VLA parameter */
    for (int r = 0; r < rows; r++) {
        for (int c = 0; c < cols; c++) printf("%3d", m[r][c]);
        printf("\n");
    }
}

int main(void) {
    int a[2][COLS] = {{1, 2, 3}, {4, 5, 6}};
    printf("sum = %d\n", trace_like_sum(a, 2));
    print_matrix(2, COLS, a);
    return 0;
}

Output

sum = 21
  1  2  3
  4  5  6

Matrix multiplication

#include <stdio.h>

int main(void) {
    int A[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int B[3][2] = {{7, 8}, {9, 10}, {11, 12}};
    int C[2][2] = {{0}};
    for (int i = 0; i < 2; i++)
        for (int j = 0; j < 2; j++)
            for (int k = 0; k < 3; k++)
                C[i][j] += A[i][k] * B[k][j];
    printf("%d %d\n%d %d\n", C[0][0], C[0][1], C[1][0], C[1][1]);
    return 0;
}

Output

58 64
139 154

Arrays of strings

#include <stdio.h>

int main(void) {
    char names[3][8] = {"Ann", "Bob", "Charlie"};    /* 2-D char array: 24 bytes */
    const char *langs[] = {"C", "Go", "Rust"};       /* array of pointers */
    for (int i = 0; i < 3; i++) printf("%s/%s ", names[i], langs[i]);
    printf("\n%zu %zu\n", sizeof names, sizeof langs);
    return 0;
}

Output

Ann/C Bob/Go Charlie/Rust
24 24

`char names[3][8]`	`const char *langs[3]`
3 × 8 bytes, each row a writable copy	3 pointers to string literals (read-only)
Rows have a fixed maximum length	Each string can have any length
Wastes space for short strings	Only pointer overhead

How is int a[3][4] laid out in memory?

Why must a function parameter for a 2-D array specify the column count?