Cannot access memory at address 0x8

Applications Programming
1

Hi All
I have a structure pointer and setting that pointer as NULL. When i tried to access the elements in the structure i am getting the error message.
"Cannot access memory at address 0x8". This i tried in LINUX.
When the same program is tried thro UNIX (HP-UX), i am not getting the message instead the null value "\0" is printed.

Can any of you tell me why it is so and if possible what is the necessary action that needs to be taken in LInux to avoid that error message.
I have the following program

#include<stdio.h>
#include<stdlib.h>
struct IMS
{
char *ptr;
int j;
};
typedef struct IMS IMS_FILE;

int main()
{
IMS_FILE *ims_fp;
ims_fp=(struct IMS *)malloc(sizeof(struct IMS));
free(ims_fp);
ims_fp=NULL;
IMS_feof(ims_fp);
}
int IMS_feof( register IMS_FILE *ims_fp)
{
        printf("HAI\n");
}

REgards
RAJ

2

I tried your code on my linux box , but i didn't get the error message you said above

3

mika is right -- the code you posted runs fine. What did you really do?

Are you really trying to dereference a null pointer?

4

Hi All
I have missed one line.
printf("%d",ims_fp->j);

I tried to acess one of the elements of the structure and then it gave the problem.

You may replace the function IMS_feof with the below

int IMS_feof( register IMS_FILE *ims_fp)
{
printf("%d",ims_fp->j);
printf("HAI\n");
}

REgards
rajrk

5

You are dereferencing a null pointer:

ims_fp=NULL;
...
printf("%d",ims_fp->j);

According to the language definition, the behaviour is undefined if you do this, so that's why you have different behaviour on different platforms.
In order to make your code run correctly, you need to either remove the lines

free(ims_fp);
ims_fp=NULL;

or remove the function call

IMS_feof(ims_fp);
6

I understood what you said, but it works in HP-UX. As the value in the members are filled with 0's, while in Linux (64 bit)

uname -a
Linux optiplex64 2.6.9-55.0.2.ELsmp #1 SMP Tue Jun 26 14:14:47 EDT 2007 x86_64

it is not working.

One more point is that in another linux box (32 bit )
<lnx22:~>$ uname -a
Linux 2.4.21-37.ELsmp #1 SMP Wed Sep 7 13:28:55 EDT 2005 i686 i686 i386 GNU/Linux

it works.

So ... i am looking for a suitable solution where the same code should work in both in LInux and HP-UX.

Regards
rkraj

7

Working/not working depends on implementation of kernel.
Basically how particular kernel deals with memory.
Sometimes it might allow you to read whatever your want, or might not.

For example in case of DOS you call is always valid, becuase iterrupts vector area is located in that area.

At the same time it's big security flaw if any procees is allowed to read any piece of memory. It might be a way to easily obtaint passwords, decription keys, etc. That's one of the reasosn, why you have such exception.

Your program is wrong regardless if it's HP-UX or Linux. It does that you didn't intend to do. You must rewrtie in order to avoid defrefencing of tha null-pointer.

8

AFAIK...the standard says that you cannot de-reference a generic pointer but does not say anything about de-referencing a NULL pointer.

9

I am no language lawyer, but AFAIK, it does.