[Bug c/59520] a possible inconsistency in error diagnostics with "-pedantic -std=c99"

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59520

--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Fri, 20 Dec 2013, su at cs dot ucdavis.edu wrote:

> In particular, are the following well-defined according the standard or they
> have undefined behavior? 

In both cases, you are accessing uninitialized padding bits.  ISO C allows 
for the possibility of type punning through unions (in a footnote added in 
C99 TC3, not in normative text), but this is still uninitialized data.  
Whether accesses to uninitialized data are completely undefined behavior, 
or only produce unspecified values in some cases, is less clear.  See 
N1747 and the discussion under DR 451 in the draft Chicago minutes N1764.  
(Your examples concern padding bits in objects with static storage 
duration, rather than uninitialized objects with automatic storage 
duration, but the issues are much the same.)  The general desire is to 
allow optimizations that may mean an uninitialized object does not act as 
if it has any consistent value, without making it undefined behavior to 
call memcpy (for example) on objects that may be only partly initialized.