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Pointers and arrays between files..

Posted on 2003-02-25
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Last Modified: 2010-04-15
Please go thru the fn below and tell me how to explain the reason why the pointer notation does not work with external arrays.

file1.c:

int a[10] = { 1,2,3,4};


file2.c:

extern int *a;
int b[10];
#include <stdio.h>

void Print()
{
  int *c = b;

  printf("%d\n", a[0]); //does not print
  printf("%d\n", (&a)[0]); //prints
  printf("%d\n", c[0]); //prints
}
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Question by:nagubala
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4 Comments
 
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Expert Comment

by:zebada
ID: 8015926
If I recall correctly
int *a;
and
int a[10];
are different variable declarations according to the linker - therefor it will not "link" them as the same variable.

You should use
extern int a[];

Regards
Paul
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LVL 46

Accepted Solution

by:
Kent Olsen earned 80 total points
ID: 8016459

Within file1, the declaration 'int a[10]' reserves ten integers.  The nature of an integer array is such that you actually put a 1 into element 0, 2 into element 1, etc.

Withing file2, the declaration 'extern int *a' doesn't reserved space.  It tells the compiler that the "already defined symbol 'a'" contains a pointer to an integer.  Where you stored a 1 in element 0, this declaration treats the contents of 'a' as a pointer to an integer.  If a pointer and an int are the same size, you're using the value 1 as an address.  It implies that the array is created by:

  int *a = (int *)malloc (10 * sizeof (int));
 

Writing the declaration as 'extern int a[]' tells the compiler that the "already defined symbol 'a'" is an array of consecutive integers.


This is quite subtle -- I missed it the first time that I looked at it.  If this had been posted as a quiz, without stating that the code didn't work, I'd have flunked.  :(

Kdo
0
 

Author Comment

by:nagubala
ID: 8017356
Thanks! Thats a good way to put it.
This also points out that C should never have a relaxed syntax that array name and a pointer are the same. If they have made it mandatory that only by applying the & operator one can get the address of an array, then this problem would never arise and that rule will universally be applicable. That will also invalidate the extern int *a declaration.
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LVL 46

Expert Comment

by:Kent Olsen
ID: 8017634

The standard is much more stingent in C++, which was part of the intent when C++ was developed.

Be aware that as you transition the data type from int to char and finally string (char*) that the behavior may fool you, again.

Strings are already pointers to objects so you'll often see '*' and '[]' used on strings just like you tried to use them on integers.


Kdo
0

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