Re: [PATCH 1/3] ipc: avoid dereference of null pointer and quiet the GCC warning about uninitialized variable

[Marin Ramesa]

On 18.12.2013 10:46:40, Richard Braun wrote:
> On Wed, Dec 18, 2013 at 10:37:03AM +0100, Marin Ramesa wrote:
> > Compiler needs to check both !a and !b. In order to evaluate !b it
> > must evaluate b. So when the code path is that when entry is a null
> > pointer, the evaluation of b results in a dereference of a null 
> > pointer.
> 
> No, that's wrong. The && and || operators are guaranteed to be
> evaluated left-to-right, and yield if the first operand compares 
> equal to 0. And that's exactly why this check against NULL is done 
> first.

In the expression (!a && !b), if !a equals 0, the compiler must check 
!b == 0 in order to return TRUE. If !a equals 0, that means the entry 
is a null pointer, and evaluation of !b is a dereference of a null 
pointer.