Language in C 面接の質問と回答

Question: If we have declared an array as global in one file and we are using it in another file then why doesn't the sizeof operator works on an extern array?

Answer: An extern array is of incomplete type as it does not contain the size. Hence we cannot use sizeof operator, as it cannot get the size of the array declared in another file. To resolve this use any of one the following two solutions:
1. In the same file declare one more variable that holds the size of array. For example,

array.c

int arr[5] ;
int arrsz = sizeof ( arr ) ;

myprog.c

extern int arr[] ;
extern int arrsz ;
2. Define a macro which can be used in an array
declaration. For example,

myheader.h

#define SZ 5

array.c

#include "myheader.h"
int arr[SZ] ;

myprog.c

#include "myheader.h"
extern int arr[SZ] ;