Re: Type safety on wikipedia

[Dave Thompson <david.thompson1@worldnet.att.net>]

On 26 Jan 2006 16:38:29 -0800, "jimmaureenrogers@worldnet.att.net"
<jimmaureenrogers@worldnet.att.net> wrote:
<snip>
> The problem is one of attempting to dereference a null access object.
> The above program compiles without error.
> 
The actual problem is a dangling access (pointer) not a null one, as
already noted elsethread.

> When run, I get [CONSTRAINT_ERROR]...
> It appears that Ada's runtime checks detect an erroneous problem. ...

Yes, at least by default.

> In C, while it is an error to de-reference a null pointer, the runtime
> system
> does nothing to detect the problem. C programs continue with really
> nasty
> garbage values.
> 
Yes and no. The C standard leaves it up to the implementation.

On some (increasingly many) systems with virtual memory where it is
easy (enough) to leave page (or even segment) 0 unmapped, it will give
a clear error, usually even a recoverable signal (roughly like the
exception in Ada). On some (other) systems, (virtual) 0 is accessible
(and accessed) but is reserved and initialized with "useful" data,
such as 4 bytes of zero which in the most common read cases (as a
string, char, or int) gives a safe and possibly even useful result.

But you can't portably rely on, or enforce, this; and garbage is
certainly a possibility.


- David.Thompson1 at worldnet.att.net