Re: Secure Dereference of NULL-Pointer when using list.h

["Jinesh K J" <jineshkj.newsletters@xxxxxxxxx>]

On 10/4/06, Hendrik Post <hendrik@xxxxxxxxxxxx> wrote:
> Hi Ricard,
>
> Thank you for your answer. My problem was my incorrect (?) understanding
> of "s->foo". I thought it being equivalent to "(*s).foo" rather than a
> direct offset calculation.

They are equivalent. The rule is simple, no value is read unless a
read operation is performed. Also, no value is written unless a write
operation is performed. When you write

a = b;

read operation is required on 'b', and a write on 'a'. But when you say

a;  // no operation is specified here

or

;  //  an empty statement

neither read nor a write is asked to perform by us. The compiler will
thus silently ignore such a statement since we have not asked for any
particular operation over 'a'. A read or write operation has two
steps:

Step 1: Get the address of the variable upon which to operate on
Step 2: Read/Write to that address

Now, when you write

a = &b;

both the above steps are performed over 'a' whereas only step 1 is
performed over 'b'. It's because we've not asked the compiler to do
that. Therefore when you say

a = &(s->foo);

the possible steps are:

Step 1: Get the address of s
Step 2: Read the value(address) contained in s
Step 3: Get the address of member foo inside s
Step 4: Read the value of foo

Step 4 is not performed because we've already obtained our required
result in Step 3 or in other words - we've not asked the compiler to
read the value of foo; only the address of it.

I think I've explained too much,

Best of Luck,

Jinesh.

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