pointer and array

[sumitverma1680]

recently i came across this problem in one of C magzines, and it has
kept me wondering for some time.I am posting it here as i myself could
not figure it out ! any comments are most welcome.
I have tried MVC++ for general code test.

the problem is divided in two .c files as follow :

file1.c
#include <stdio.h>
extern void fn();
int arr[10] ;
int main()
{
fn();
}


file2.c
#include <stdio.h>
extern int *arr;
void fn()
{
arr[5]=5;
printf("%d",arr[5]);

}

when the above are compiled and linked there is no error . But during
de-referncing of arr ponter in fn() , memory acesss violation occurs?
can any body explain ? thanks in advance.

--- sumit ---

 

[Ben Pfaff]

file1.c
#include <stdio.h>
extern void fn();

The "extern" keyword is unneeded here.

It would be better to declare the parameters explicitly:

void fn(void);

int arr[10] ;
int main()
{
fn();
}


file2.c
#include <stdio.h>
extern int *arr;

This declaration is incorrect. It should read:
extern int arr[10];
or
extern int arr[];

void fn()
{
arr[5]=5;
printf("%d",arr[5]);

}

when the above are compiled and linked there is no error . But during
de-referncing of arr ponter in fn() , memory acesss violation occurs?
can any body explain ? thanks in advance.

The problem is that you are lying to the compiler: arr is defined
as an array but declared as a pointer. Pointers and arrays are
not the same thing. When you lie to the compiler, it will get
its revenge (as someone smarter than me once said).

 

[Lew Pitcher]

recently i came across this problem in one of C magzines, and it has
kept me wondering for some time.I am posting it here as i myself could
not figure it out ! any comments are most welcome.
I have tried MVC++ for general code test.

the problem is divided in two .c files as follow :

file1.c
#include <stdio.h>
extern void fn();
int arr[10] ;

Here you define arr as an array of 10 integers

int main()
{
fn();

}

file2.c
#include <stdio.h>
extern int *arr;

Here you define arr as a pointer to integers
Let's make sure you understand what that means: You've said that arr
names a space which contains a pointer, which, when dereferenced, will
point to an int.

void fn()
{
arr[5]=5;

Here, you dereference the pointer, add 5 * sizeof(int) to it, and use
that pointer value as the target to store the value (5) to.

The problem is that arr is an array of int, not a pointer. Since arr
is static, it's contents are initialized to zero.

So, your code above dereferences arr, pulling what it thinks is a
pointer out of a space containing 5 integer zero values, adds (5 *
sizeof(int)) to that pointer, and then tries to access the space
pointed to by that value.

printf("%d",arr[5]);

}

when the above are compiled and linked there is no error . But during
de-referncing of arr ponter in fn() , memory acesss violation occurs?

Because *(0 + (5 * sizeof(int))) is not writable by your program.

 

[Keith Thompson]

On Dec 12, 12:45 pm, (e-mail address removed) wrote: [...]

int arr[10] ;

Here you define arr as an array of 10 integers
[...]

extern int *arr;

Here you define arr as a pointer to integers

[...]

Quibble: here he *declares* arr as a pointer to int. (This is a
declaration; it's not a definition.)

 

[Tor Rustad]

The "extern" keyword is unneeded here.

Yes, but fn() do have external linkage.

It would be better to declare the parameters explicitly:

void fn(void);

Agreed, but I see nothing wrong with

extern void fn(void);

which at least, I do use.

 

[Ben Pfaff]

Agreed, but I see nothing wrong with

extern void fn(void);

which at least, I do use.

Why do you prefer to write the extern keyword explicitly?

 

[Tor Rustad]

Why do you prefer to write the extern keyword explicitly?

I have sometimes forgot to use 'static', when using 'extern', I rarely
forget.

 

[Joe Wright]

recently i came across this problem in one of C magzines, and it has
kept me wondering for some time.I am posting it here as i myself could
not figure it out ! any comments are most welcome.
I have tried MVC++ for general code test.

the problem is divided in two .c files as follow :

file1.c
#include <stdio.h>
extern void fn();
int arr[10] ;
int main()
{
fn();
}


file2.c
#include <stdio.h>
extern int *arr;
void fn()
{
arr[5]=5;
printf("%d",arr[5]);

}

when the above are compiled and linked there is no error . But during
de-referncing of arr ponter in fn() , memory acesss violation occurs?
can any body explain ? thanks in advance.

--- sumit ---

Why lie to the compiler? In file1, arr is array 10 of int and indeed has
external linkage. In file2 you declare it a pointer to int. An array is
not a pointer. A pointer is not an array.

 

[Chris Dollin]

Why do you prefer to write the extern keyword explicitly?

I do it so I can grep for it.

 

[Richard Bos]

recently i came across this problem in one of C magzines,

file1.c

int arr[10] ;

file2.c

extern int *arr;

The editors of that "C" magazine would do well to read the C FAQ, and in
particular question 6.1: <http://c-faq.com/aryptr/aryptr1.html>.

Richard

 

[Robert Latest]

I have sometimes forgot to use 'static', when using 'extern', I rarely
forget.

Except that using or not using 'static' makes a difference whereas using or
not using 'extern' doesn't.

robert

 

[Keith Thompson]

recently i came across this problem in one of C magzines,

file1.c

int arr[10] ;

file2.c

extern int *arr;

The editors of that "C" magazine would do well to read the C FAQ, and in
particular question 6.1: <http://c-faq.com/aryptr/aryptr1.html>.

I suspect they're aware of it, or at least of the information that it
presents. The program apparently was presented as a problem; the
reader is being asked to figure out why it doesn't work.