segmentation fault !

wwnn1 · 06-15-2002, 10:54 AM

I have a programme below:

/************* code ****************/
#include<stdio.h>

main()
{
char * ptr;

ptr = (char *)0xb8000000L;
printf("%c\n",ptr[0]);
getch();
}
/**********************************/

this programme can be complied without any error/warning.

but when I run it ( ./a.out ), it display this error message: segmentation fault.

Why?

Mara · 06-15-2002, 01:09 PM

ptr=(char *)0xb8000000L;
It inserts a value to ptr. Then you're using ptr to write the contents of memory in ptr. But you don't know if this address can be used by your program. It can't. You can only use memory the system gives to your program. You should modify your program to allocate memory using malloc, then copy the string to the array.
BTW What would you like to do in this program?

wwnn1 · 06-15-2002, 01:24 PM

Thanks!

Is there some methods to let a process visit the memory that does not allocated to it?

Mara · 06-15-2002, 02:06 PM

AFAIK only when you're root.

herambshembekar · 06-17-2002, 11:40 AM

I first thaught as 0xb8000000L address is of video memory i will not be able to acess within process (user mode).

thinking I will be able to acess it within kernel module, i have written kernel module,

but i was wrong i am geting segmentation fault within kernel module too.

ferrous · 06-18-2002, 04:03 AM

So you wanna be a gangsta and all that shit? Smoke any motherfucker, don't even quit?

There's a concept in operating system design called memory protection. I suggest you read up on the concept before you go mucking about, dropping pointers in random places looking to snoop data. Apart from that, check the man page on mmap.

tundra · 06-18-2002, 04:37 AM

Quote:

So you wanna be a gangsta and all that shit? Smoke any motherfscker, don't even quit?

huh? why the comment?

llama_meme · 06-18-2002, 09:48 AM

No pointer in C or C++ code will point to an actual physical address whatever mode you're running your program in. Remember that the Linux kernel uses paging to create a virtual address space for each process. (Of course this happens in pretty much any multitasking operating system, although paging isn't necessarily the method used).

Alex