Question

Watching some code found on internet I came across this piece of code:

int (*p)[3];

p = (int(*)[3]) calloc(5*3,sizeof(int));

is it just a way to alloc a matrix?

I understand that int (*p)[3] creates a pointer to an array of 3 int, but I’m not sure about the calloc call: I know that calloc allocate and initialize a number of int (in this case) equal to the first parameter (5*3).
Thus, if I assign it to p it should mean that the pointer now points to the just allocated memory location.

So, since I know my reasoning is wrong, may somebody correct me?

Answer 1

int (*p)[3];

You’re true that this is a pointer to an array of three ints, but this pointer could also point to the beginning of an array of arrays of three ints.

p = (int(*)[3]) calloc(5*3,sizeof(int));

Here, you allocate 5 * 3 ints to this pointer. As int[3] has 3 ints, you just allocated 5 such int[3] arrays.

You may then refer to these int[3] arrays as

p[0] … p[4]

so you get a two-dimensional array ranging

p[0][0] … p[0][2]

p[4][0] … p[4][2]