Function to initialize two dimensional array - c

Question

I have a c program in which I want to initialize a 2 dimensional array.
So I made this function :
void initLayer(int **layer, int *dimensions) {
printf("initLayer\n");
layer = malloc(sizeof(int*) * dimensions[0]);
for (int i = 0; i < dimensions[0]; i++) {
layer[i] = malloc(sizeof(int) * dimensions[1]);
}
}
When I use this function there is no problem, but when I try to read the 2D array later I always get a segmentation fault.
I think it may be because the initialization made in the function are not saved when its finished.
Do you know how I could correct my function ? Thank you in advance.

Answer 1

To passing pointer to function you need one more pointer.
int **matrix; is an array of arrays, so to fill it you need to pass it as a pointer, which is int ***layer. but it is weird.
also for changing data by pointer you need to add a star * before it. *layer = ...
#include <stdlib.h>
void initLayer(int ***layer, int *dimensions)
{
*layer = malloc(sizeof(int *) * dimensions[0]);
for (int i = 0; i < dimensions[0]; i++)
{
*(*layer + i) = malloc(sizeof(int) * dimensions[1]);
}
}
int main()
{
int **matrix;
int dimensions[2] = { 4, 6 };
initLayer(&matrix, dimensions);
// then do whatever you want
for (int i = 0; i < dimensions[0]; i++)
{
for (int j = 0; j < dimensions[1]; j++)
{
matrix[i][j] = i * j;
}
}
}
as for me, better to use typedef to make code more readable:
#include <stdlib.h>
typedef int * Array;
typedef int ** Matrix;
void initLayer(Matrix *layer, Array dimensions)
{
*layer = malloc(sizeof(Array) * dimensions[0]);
for (int i = 0; i < dimensions[0]; i++)
{
(*layer)[i] = malloc(sizeof(int) * dimensions[1]);
}
}
int main()
{
Matrix matrix;
int dimensions[2] = { 4, 6 };
initLayer(&matrix, dimensions);
// then do whatever you want
for (int i = 0; i < dimensions[0]; i++)
{
for (int j = 0; j < dimensions[1]; j++)
{
matrix[i][j] = i * j;
}
}
}

Answer 2

When you call the function, the int **layer pointer is copied. So, when you do layer = malloc(...) what actually happens is the function sets its local copy to the malloc result. What you want is to mutate the variable which you called the function with. You can do this by taking a int ***layer and passing in &layer when calling initLayer. Note that you must then use *layer instead of layer in your code.

Answer 3

You have two approaches here:
to pass a reference to the double pointer (***int in this case)
or to return the allocated pointer as the result of your function:
in the first case:
void initLayer(int ***layer, int *dimensions) {
printf("initLayer\n");
*layer = malloc(sizeof(int*) * dimensions[0]);
for (int i = 0; i < dimensions[0]; i++) {
layer[i] = malloc(sizeof(int) * dimensions[1]);
}
}
you pass a reference to a pointer, instead of passing the (uninitialized) pointer. Remember, in C, all parameters are passed by value. In this case, you can call your function as:
...
int**vector;
...
initLayer(&vector, dims); /* you pass the address of your double pointer */
In the second case:
int** initLayer(int *dimensions) {
printf("initLayer\n");
int **layer = malloc(sizeof(int*) * dimensions[0]);
for (int i = 0; i < dimensions[0]; i++) {
layer[i] = malloc(sizeof(int) * dimensions[1]);
}
return layer;
}
in this case, you call it as:
...
int**vector;
...
vector = initLayer(dims); /* you receive your double pointer as a return value */