did some googling and search the LKML archives.
the jist of the story (DISCLAIMER: according to me :))
-the kernel does NOT handle null pointer derefernce,
the compiler does!
-for the C porgramming language NULL is *generally*
(but not strictly!) defined as 0. Therefore it is
the compilers job to see if 0 is being used in a
pointer context, and convert it into a bit pattern
that translates into an invalid addrress for a
particular h/w (the CPU catches invalid mem. addr
exceptions).
-0 on some arch.s *is* a valid address.
-If on your system uses the value of NULL other than 0,
you need to tell the compiler as well.
(i have a feeling i'm going to get flamed for this!)
On Thu, 2004-11-18 at 14:35, Vishal Soni wrote:
> > I remember a big discussion happening on LKML on this
> > topic (i.e., NULL vs 0) a couple of months back. It must
> > be still available in their mail archives. Try googling for
> > "Use NULL instead of integer 0" & you must find it. Must
> > say it was a pretty heated discussion. Dont know what
> > the outcome was. I stopped following it after sometime.
> Tried some thing and pasting a copy here...which tells clearly abt NULL
> pointer and using zero(integer)
> C code -- used
> file name : null.c
> int main()
> {
> int *p= NULL;
> return 0;
> }
>
> file name : zero.c
> int main()
> {
> int p = 0;
> return 0;
> }
>
> Assembly snippet on 64 bit machine for null.c
> main:
> .LFB3:
> pushq %rbp
> .LCFI0:
> movq %rsp, %rbp
> .LCFI1:
> movq $0, -8(%rbp) <------------
> movl $0, %eax
> leave
> ret
>
> Assembly snippet on 64 bit machine for zero.c
> main:
> .LFB3:
> pushq %rbp
> .LCFI0:
> movq %rsp, %rbp
> .LCFI1:
> movl $0, -4(%rbp) <-------------
> movl $0, %eax
> leave
> ret
> Check out the 7th line..... of both the snippets
> If we see here 8 bytes are deducted from base pointer(in 64 bit machine --
> when variable p was NULL pointer)
> and 4 bytes are deducted from rbp when p was integer....(obvious... Right
> !!!!)
> and thus the code differs.......
>
> Whereas in 32 bit machine, assembly snippets for null.c and zero.c are
> similar.
> main:
> pushl %ebp
> movl %esp, %ebp
> subl $8, %esp
> andl $-16, %esp
> movl $0, %eax
> addl $15, %eax
> addl $15, %eax
> shrl $4, %eax
> sall $4, %eax
> subl %eax, %esp
> movl $0, -4(%ebp)
> movl $0, %eax
> leave
> ret
>
> Interesting conclusions and concept to imbibe :)
> Regards,
> Vishal.
>
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