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null pointer

 
 
Peter Nilsson
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      10-16-2011
88888 dihedral <(E-Mail Removed)> wrote:
> I think *C is not good for beginners without HW knowledge ...


I think C is not good for beginners *with* harware knowledge.
By far the biggest mistake most new C programmers make is thinking
of every piece of syntax in terms of cpu instructions. It comes as
a great shock to many when they realise (or are told) that C really
isn't a portable assembler.

I wonder if C had better string handling abilities, would it still
be thought of as such a 'low' high level language?

--
Peter
 
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James Kuyper
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      10-16-2011
On 10/15/2011 11:58 AM, Jean-Christophe wrote:
> On 14 oct, 16:05, John Bode <(E-Mail Removed)> wrote:

....
>> The behavior is "undefined" - any result is possible. ´┐ŻOn platforms
>> such as Windows or *nix, you'll most likely get a segfault.
>> It's definitely a logic error, since NULL represents a well-defined
>> "nowhere"; there shouldn't *be* anything there.

>
> Of course there is something at adress zero :
> on a home-made uP electronic board one can map
> a RAM chip at address zero and read/write here.


Address 0 and a null pointer need not have anything to do with each
other. If it's useful to read or write from address 0, then a null
pointer should not be implemented as a pointer to the memory at address
0, because the C standard prohibits null pointers from comparing equal
to pointers pointing at actual objects or functions.

There's also no inherent reason why address 0 has to even be meaningful.
Valid addresses might be restricted to the range 0x10000000 to 0xFFFFFFFF.

> C compilers prohibits the dereference
> of zero pointer just to check for errors.


No, it prohibits the dereference because it doesn't make logical sense.
Null pointers say "I'm not pointing at anything". Dereferencing a
pointer means "access the object you're pointing at", which is a
ridiculous thing to ask for when it isn't actually pointing at anything.
As a result, on many platforms, attempting to do it can cause your
program to malfunction or abort. This is implemented at the hardware or
operating system level, making it very inefficient for a compiler to
generate the code needed to bypass that behavior. Making the behavior
undefined gives implementors the freedom they need to generate code for
which this is a possible failure mode.
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James Kuyper
 
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