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

[David Brown]

Ruud Vlaming wrote:
> On Wednesday 23 September 2009 21:14, Joerg Wunsch wrote:
> > Ruud Vlaming <address@hidden> wrote:
> >
> > > I noticed a strange behaviour when you place a zero length array in
> > > 'nocommon'.
> > No idea why the behaviour changes, but as a note of warning: a
> > zero-length array is not allowed by the C standard.  
> Well, mayby not in ISO C, but certainly in GNU C:
>   http://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> thus at least the gcc compiler should be able to handle it :-)
>
> > No idea what you are trying to achieve here though.
> Although the link gives an example how this could be usefull,
> i found a different application. I configure my projects using
> preprop defines, and if a don't need an array internally, the size
> calculation simply produces a zero. That way no space is taken, 
> and it is much easier than having if-defs around all arrays to see
> if they accedently have size zero. But it seems it does not work
> when they are not in the common space. 
>
> This 'quirk' of gcc does not seem to be specifically avr related 
> but probably we are the only one's left to care about the 
> spoilled byte.
>
> Ruud.

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).

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.

The same issue occurs with C++ classes with no non-static data, which 
can sometimes be useful.