[avr-gcc-list] Re: zero length array in nocommon

[David Brown]

Ruud Vlaming wrote:
> On Thursday 24 September 2009 10:51, David Brown wrote:
>
> > As shown by the link you gave, zero-length arrays are only really valid 
> > when they are the last element in a struct (they can be very useful there).
> I cannot read any restrictions on the use of zero-length arrays. It states
> "Zero-length arrays are allowed in GNU C" Period. And then an example 
> is discussed about how it can be usefull.
>  

You are right - it is only flexible arrays (no length at all) that need 
to be at the end of a struct.  However, zero-length arrays are close to 
the same thing as far as the compiler is concerned - they are only 
actually /useful/ at the end of a struct.

> > The problem boils down to that C requires every declared object to have 
> > a unique address.  Theoretically, the compiler/linker could do this by 
> > aliasing the zero-length array into the middle of something else that is 
> > otherwise not addressed by the program.  But in practice, it is done by 
> > allocating at least 1 byte to zero-length data.
> Maybe true, but contraditory to my findings with gcc. When i keep it
> in the common (as is done automatically) gcc has no problems
> whatsoever with the zero length array, and indeed it address may 
> overlap with other variables. It is up to the programmer i guess not
> to misuse this 'feature'.
>
> My question was, why is there a difference in behaviour, and i
> hoped somebody could shine some light on this. But maybe 
> it is like J"org says, just a bug.
>
> R.

I think it is more likely that the bug is that the zero-length arrays 
take no space when allocated in "common", rather than that they /do/ 
take space when allocated elsewhere.  So perhaps you should not protest 
too loudly :-)

I suppose that since the standards don't allow zero-length arrays, I 
their semantics are not very well defined.