Re: Somw questions [ MAYBE OFFTOPIC ]

[ralf@uni-koblenz.de]

On Mon, Apr 05, 1999 at 05:12:50AM -0400, Amol Mohite wrote:

> > A NULL pointer is just yet another invalid address.  There is no special
> > test for a NULL pointer.  Most probably for example (char *)0x12345678
> > will be invalid as a pointer as well and treated the same.  The CPU
> > detects this when the TLB doesn't have a translation valid for the
> > access being attempted.
> 
> Yes but how does it know it is a null pointer ?

Again, it doesn't know that it is a *NULL* pointer.  The kernel just knows
that a user program resulted in the CPU throwing an exception for attempting
an illegal access, that is insufficient permissions for the mapping or
no mapping for the address at all.

> On that note, when c does not allow u to dereference a void pointer , is
> this compiler doing the trick ?

Only ANSI/ISO C doesn't allow to dereference void pointers, GCC allows this
as an extension of the language.  Most machines only have untyped pointers,
for them void * or not would only a difference to the compiler, not the
machine - if the compiler allows it.

> Ok , about the expand down attribute, thats how 32 bit windows does it, so
> i was wondering if linux also does the same.
> 
> 16 bit windows accesses a null pointer with a 0: descriptor.
> 
> Apparently intel allows u to load a 0 but not dererence it.

  Ralf
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