extern array

/*
-std=c99 -Wall -Wextra -Wpedantic -Wshadow
*/

/*
 test_inc.h
------------
*/

#ifndef TEST_INC_H
#define TEST_INC_H

#define ARR_1_SIZE 10
#define ARR_2_SIZE 20

//error: invalid application of ‘sizeof’ to incomplete type ‘int[]’
//extern int arr_1[];

extern int arr_1[ARR_1_SIZE];
extern int arr_2[ARR_2_SIZE];

int testFnc();

#endif

/*
 main.c
--------
*/

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

int arr_1[ARR_1_SIZE];
int arr_2[ARR_2_SIZE];

int main()
{

    printf("1: %zu\n", sizeof arr_1  / sizeof *arr_1 );
    printf("2: %zu\n", sizeof arr_2  / sizeof *arr_2 );

    testFnc();

    return 0;
}

/*
 file1.c
---------
*/

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

int testFnc()
{

    printf("1a: %zu\n", sizeof arr_1  / sizeof *arr_1 );
    printf("2a: %zu\n", sizeof arr_2  / sizeof *arr_2 );

    return 0;
}

/*
output:

1: 10
2: 20
1a: 10
2a: 20

*/