Passing array of pointers as argument to function in C - c

Question

I've written a piece of code but I'm not sure about how it works.
I want to create an array of pointers and pass it as argument to a function, like the following:
int main()
{
int *array[10] = {0};
for(int i = 0; i < 3; i++)
{
array[i] = (int *)malloc(3*sizeof(int *));
}
testFunction(array);
for(int i = 0; i < 3; i++)
{
free(array[i]);
}
return 0;
}
void testFunction(int *array[3])
{
//do something
return;
}
What I don't understand is the following. I declare array as an array of pointers, allocate memory to it by using malloc and then proceed to call testFunction. I want to pass the array by reference, and I understand that when I call the function by using testFunction(array), the array decays to a pointer to its first element (which will be a pointer also). But why in the parameters list I have to write (int *array[3]) with * and not just (int array[3])?

Answer 1

A parameter of type * can accept an argument of type [], but not anything in type.
If you write void testFunction(int arg[3]) it's fine, but you won't be able to access array[1] and array[2] and so on, only the first 3 elements of where array[0] points to. Also a comversion is required (call with testFunction((int*)array);.
As a good practice, it's necessary to make the function parametera consistent with what's passed as arguments. So int *array[10] can be passed to f(int **arg) or f(int *arg[]), but neither f(int *arg) nor f(int arg[]).

Answer 2

void testFunction(int **array, int int_arr_size, int size_of_array_of_pointers)
{
for(int j = 0; j < size_of_array_of_pointers; j++)
{
int *arrptr = array[j]; // this pointer only to understand the idea.
for(int i = 0; i < int_arr_size; i++)
{
arrptr[i] = i + 1;
}
}
}
and
int main()
{
int *array[10];
for(int i = 0; i < sizeof(array) / sizeof(int *); i++)
{
array[i] = malloc(3*sizeof(int));
}
testFunction(array, 3, sizeof(array) / sizeof(int *));
for(int i = 0; i < sizeof(array) / sizeof(int *); i++)
{
free(array[i]);
}
return 0;
}

Answer 3

Evering depends on what // do something means in your case.
Let's start from simple : perhaps, you need just array of integers
If your function change only values in array but does not change size, you can pass it as int *array or int array[3].
int *array[3] allows to work only with arrays of size 3, but if you can works with any arrays of int option int *array require additional argument int size:
void testFunction(int *array, int arr_size)
{
int i;
for(i = 0; i < arr_size; i++)
{
array[i] = i + 1;
}
return;
}
Next : if array of pointers are needed
Argument should be int *array[3] or better int **array (pointer to pointer).
Looking at the initialization loop (I changed sizeof(int *) to sizeof(int))
for(int i = 0; i < 3; i++)
{
array[i] = (int *)malloc(3*sizeof(int));
}
I suppose you need 2-dimension array, so you can pass int **array but with sizes of two dimensions or one size for case of square matrix (height equal to width):
void testFunction(int **array, int wSize, int hSize)
{
int row, col;
for(row = 0; row < hSize; row++)
{
for(col = 0; col < wSize; col++)
{
array[row][col] = row * col;
}
}
}
And finally : memory allocation for 2D-array
Consider the following variant of your main:
int main()
{
int **array;
// allocate memory for 3 pointers int*
array = (int *)malloc(3*sizeof(int *));
if(array == NULL)
return 1; // stop the program
// then init these 3 pointers with addreses for 3 int
for(int i = 0; i < 3; i++)
{
array[i] = (int *)malloc(3*sizeof(int));
if(array[i] == NULL) return 1;
}
testFunction(array, 3, 3);
// First, free memory allocated for int
for(int i = 0; i < 3; i++)
{
free(array[i]);
}
// then free memory allocated for pointers
free(array);
return 0;
}
Pay attention, that value returned by malloc should be checked before usage (NULL means memory was not allocated).
For the same reasons check can be added inside function:
void testFunction(int **array, int wSize, int hSize)
{
int row, col;
if(array == NULL) // check here
return;
for(row = 0; row < hSize; row++)
{
if(array[row] == NULL) // and here
return;
for(col = 0; col < wSize; col++)
{
array[row][col] = row * col;
}
}
}