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mechanism behind segmentation violation
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vishal_study@yahoo.com
*nix forums beginner


Joined: 10 Nov 2005
Posts: 1

PostPosted: Thu Nov 10, 2005 8:46 pm    Post subject: mechanism behind segmentation violation Reply with quote
Hi:

When someone tries to dereference a NULL pointer, we get segmentation
violation.
Can someone please let me know the mechanism behind it? I mean how is
this mechanism implemented in kernel.

I have seen a private kernel/OS in which dereferencing a null pointer
doesn't lead to exception. Hence the developers have to be more
careful...

Question (again):
How is this mechanism to generate SEGV on dereferncing a null pointer
implemented? Any pointers would be greatly appreciated.

Thanks,
Vishal.
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Barry Margolin
*nix forums Guru


Joined: 24 Feb 2005
Posts: 323

PostPosted: Fri Nov 11, 2005 1:30 am    Post subject: Re: mechanism behind segmentation violation Reply with quote
In article <1131655600.104821.192150@f14g2000cwb.googlegroups.com>,
"vishal_study@yahoo.com" <vishal_study@yahoo.com> wrote:

Quote:
Hi:

When someone tries to dereference a NULL pointer, we get segmentation
violation.
Can someone please let me know the mechanism behind it? I mean how is
this mechanism implemented in kernel.

It's done by not mapping the zero page into the process's memory, or
turning off both its read and write permissions in the page table entry.
When a user-mode process tries to read anything on this page, it traps
into the kernel in the same way that it would if you used some other
invalid pointer. The kernel processes this trap by setting the
segmentation violation bit in the process's signals.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
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junky_fellow@yahoo.co.in
*nix forums addict


Joined: 19 Jul 2005
Posts: 78

PostPosted: Thu Dec 08, 2005 3:09 pm    Post subject: Re: mechanism behind segmentation violation Reply with quote
vishal_study@yahoo.com wrote:
Quote:
Hi:

When someone tries to dereference a NULL pointer, we get segmentation
violation.
Can someone please let me know the mechanism behind it? I mean how is
this mechanism implemented in kernel.

I have seen a private kernel/OS in which dereferencing a null pointer
doesn't lead to exception. Hence the developers have to be more
careful...

Question (again):
How is this mechanism to generate SEGV on dereferncing a null pointer
implemented? Any pointers would be greatly appreciated.

Any attempt to dereference a null pointer invokes undefined behavior.
However, it's entirely possible (for a given implementation) that
there is an
addressible memory location at address all-bits-zero, and that a C
program is able to
access it by dereferencing a null pointer.

SIGSEGV may be generated on dereferencing the Null pointer, by not
mapping
the NULL pointer address to user address space.
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