C error: incompatible pointer types issue using typedef for int array

I am trying to implement a simple function that assigns the values of one array to another, but I keep getting an error

program.c:90:14: error: incompatible pointer types passing 'longint_t' (aka 'int [3]') to parameter of type 'longint_t *' (aka 'int (*)[3]'); take the address with & [-Werror,-Wincompatible-pointer-types]

My initial though behind this is that it should take 2 longint_t value, usually one will be empty or all zeroes, but not always, and another longint_t value and assign all the values of the second array to the first.

in the end, the variable vars should be of the form 
[ 
[0,0,0],
[0,0,0],
.
.
. 26 arrays of [0,0,0]
]

here’s my bit of code:

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


#define INTSIZE 3   /* max number of digits per integer value */
#define NVARS   26  /* number of different variables */

typedef int longint_t[INTSIZE];

void do_assign(longint_t *var1, longint_t *var2);
void zero_vars(longint_t vars[]);


/* Main program controls all the action
*/
int
main(int argc, char *argv[]) {
    longint_t vars[NVARS];

    zero_vars(vars);
    for (int i=0; i<NVARS; i++){
        for (int j=0; j<3;j++){
            //printf("%d",vars[i][j]);
        }
    }

    return 0;
}

void
zero_vars(longint_t vars[]) {
    int i;
    longint_t zero = {0};
    for (i=0; i<26; i++) {
      do_assign(vars[i], &zero);
     }

    return;
}

void
do_assign(longint_t *var1, longint_t *var2) {
    for (int i=0; i<=3; i++){
        *var1[i] = *var2[i];
    }
}

any help would be appreciated thanks!

>Solution :

Pointers to arrays is a niche C topic, which is very confusing due to array-to-pointer decay.

Your do_assign function should assign one array to another (but it’s buggy; see below). It receives pointers to the arrays.

Your zero_vars function should initialize a 2-D array (array of arrays). It also receives a pointer to an array (this is what the compiler thinks, due to the decay rules). But it gives this pointer a different meaning — it’s a pointer to the first element of the array. This array has NVARS elements, each one of which is an array of INTSIZE integers. Confusing!

In your zero_vars implementation, vars[i] is an array. However, do_assign receives a pointer to an array! So you should take its address:

  do_assign(&vars[i], &zero);

However, in your do_assign implementation, you should dereference your pointers:

    (*var1)[i] = (*var2)[i];

You dereference the pointer first, and index the result afterwards. Due to operator precedence rules, you should specify the order of these operations, explicitly.