C: In the file F2.C, why sizeof doesn't work on the array a[ ]?
I have an array declared in file 'F1.C' as,
int a[ ] = { 1, 2, 3, 4, 5, 6 } ;
and used in the file 'F2.C' as,
extern int a[ ] ;
In the file F2.C, why sizeof doesn't work on the array a[ ]?
An extern array of unspecified size is an incomplete type. You cannot apply sizeof to it, because sizeof operates during compile time and it is unable to learn the size of an array that is defined in another file. You have three ways to resolve this problem:
1. In file 'F1.C' define as,
int a[ ] = { 1, 2, 3, 4, 5, 6 } ;
int size_a = sizeof ( a ) ;
and in file F2.C declare as,
extern int a[ ] ;
extern int size_a ;2. In file 'F1.H' define,
#define ARR_SIZ 6 In file F1.C declare as, #include "F1.H" int a[ ARR_SIZ ] ; and in file F2.C declare as, #include "F1.H" extern int a[ ARR_SIZ ] ;
3. In file 'F1.C' define as,
int a[ ] = { 1, 2, 3, 4, 5, 6, -1 } ;
and in file 'F2.C' declare as,
extern int a[ ] ;
Here the element -1 is used as a sentinel value, so the code can understand the end without any explicit size.
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