Question

Hi all it’s been sometime since I’ve touched C so I’m really rusty on it. I wrote a small program to create a matrix using two dynamic arrays. However, I’m getting this warning and I don’t understand why? I guess I’m not quite sure about pointers to pointers. Can someone help me point out where my problem is? Thanks.

sm.c: In function ‘main’:
sm.c:11:13: warning: initialisation from incompatible pointer type [enabled by default]
sm.c: In function ‘makeMatrix’:
sm.c:27:3: warning: return from incompatible pointer type [enabled by default]


#include <stdio.h>
#include <stdlib.h>

typedef int (**intptr) (); 
intptr makeMatrix(int n); 

int main(int argc, char *argv[]) {

  int n = 2;  

  int **A = makeMatrix(n);
  if(A) { 
    printf("A\n");
  }
  else printf("ERROR");
}

intptr makeMatrix(int size) {
  int **a = malloc(sizeof *a * size);
  if (a) 
  {
    for (int i = 0; i < size; i++)
    {   
      a[i] = malloc(sizeof *a[i] * size);
    }   
  }
  return a;
}

Answer 1

You’ve got some problems here:

typedef int (**intptr) (); 
intptr makeMatrix(int n); 

...

int **A = makeMatrix(n);

The intptr typedef declares a pointer to a pointer to a function that takes an indeterminate number of arguments and returns an int. A is not an int.

You need to write:

int **makeMatrix(int n);


int **A = makeMatrix(n);

Using a typedef won’t help much here.

typedef int **(*intptr)();

That declares a pointer to a function that returns a pointer to a pointer to an int. But writing

intptr makeMatrix(int n);

would declare that makeMatrix() returns a pointer to a function, not an int **.