How to find the number of rows and columns of a dynamically created matrix?

Question

Based on my knowledge, I created a function that returns an initialized matrix:

int** initMatriz(int rows, int cols){
    int i,j;
    int** matriz;

    //aloca memoria para todas as linhas
    matriz = (int**) malloc(rows * sizeof(int*));
    for(i=0; i < rows ; i++){
        //para cada linha eu aloco o nmr de colunas
        matriz[i] = (int*) malloc(cols * sizeof(int));
        //inicializa 
        for(j=0 ; j < cols ; j++){
            matriz[i][j] = 0;
        }
    }

    return matriz;
}

At the time of initializing:

int** matriz = initMatriz(rows, cols);

From that point on, how do I find out the number of rows and the number of columns?

Answer 1

int** matriz = initMatriz(rows, cols);

From that point on, how do I find out the number of rows and the number of columns?

Well, the answer is right there. Just look at the parameters rows and cols and they will give you the answer.

Okay, maybe you want to know how to do this if you no longer have access to the values of rows and cols. And the answer is that you can’t do that, at least not in a portable way. The value of the pointer is a number that tells on which memory address begins the block of memory that has been allocated, but says nothing about its size.

If you know the deep details of your implementation of malloc, maybe you can find this information in the memory allocation tables that are created or some other type of internal information maintained by malloc. However, a solution based on this is inherently nonportable.

The solution adopted in almost all cases includes you encapsulating the pointer to the array and the size of the array allocated together in the same structure for this. This way, you eliminate the fundamental problem that is not having the size of the area allocated along with that area itself. For example:

typedef struct {
    int rows;
    int cols;
    int** pointer;
} Matriz;

Matriz* initMatriz(int rows, int cols) {
    int i, j;

    // Aloca a memória para todas as linhas.
    int **matriz = (int**) malloc(rows * sizeof(int*));
    for (i = 0; i < rows; i++) {
        // Para cada linha, eu aloco o número de colunas.
        matriz[i] = (int*) malloc(cols * sizeof(int));
        // Inicializa.
        for (j = 0; j < cols; j++) {
            matriz[i][j] = 0;
        }
    }
    Matriz *resultado = (Matriz *) malloc(sizeof(Matriz));
    resultado->rows = rows;
    resultado->cols = cols;
    resultado->pointer = matriz;

    return resultado;
}

To destroy an array in order to free up memory:

void freeMatriz(Matriz *matriz) {
    for (int i = 0; i < matriz->rows; i++) {
        free(matriz->pointer[i]);
    }
    free(matriz->pointer);
    free(matriz);
}