Thanks. It just occurs to me that Linux kernel used to have something
like this in routine BUG():
* ((char *) 0) = 0;
It is called when there is a kernel bug. So address 0 should not be
mapped writable, otherwise all C statements " char * p = NULL; * p =
value; " would be legal.
The book "Unix Internals - A Practical Approach" by S.D. Pate has a figure
showing in ELF format, the stack lies BELOW the code segment and grows
downwards. This might have something to do with code starting from
0x8048000.
-Zhihui
On Thu, 11 Jan 2001, John Polstra wrote:
> In article <Pine.SOL.4.21.0101111008490.22923-100000@jade>, Zhiui
> Zhang <[EMAIL PROTECTED]> wrote:
>
> > Although the 4.4 BSD design and implementation book says the text
> > part of a process starts from 0x0000,0000, it actually starts from
> > some place around 0x800,0000 (or 0x8048000 to be exact). What's in
> > the area between 0 - 0x800,0000? Why do we not use it if it is left
> > empty as shown by /proc/pid/map? How is the magic number 0x8048000
> > determined? Thanks.
>
> Processes used to be mapped at address 0 when we used the a.out object
> file format. We changed the starting address to 0x8048000 when we
> switched to the ELF format. That magic address came from SVR4, the
> first system to use ELF.
>
> I am not 100% sure why the SVR4 developers chose that address. I
> think it may have been so that they could map libc and the dynamic
> linker at the fixed address 0, thereby avoiding the need to do any
> run-time relocations on them.
>
> In any case, all ELF-based systems on the x86 architecture seem to
> use this same address. On other architecutures such as the Alpha
> it is entirely different, of course.
>
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