gcc-3.4.4-20050211: maybe a danger behaviour

[James E Wilson]

Denis Zaitsev wrote:
> a) Formally, the code is correct.  As p->what can never be < 0
> according to its type.

I am assuming this is C code.  C and C++ have different rules for enums.

This isn't strictly true.  The C standard does not say that enums are 
unsigned.  It only says that they are equivalent to an integral type 
with a few restrictions, and an implementation can choose to make it 
signed or unsigned when all enum values are non-negative.  So depending 
on the compiler, it could be possible for p->what to be negative.

> b) GCC _silently_ allows the {-1} initialization for that type, even
> with -Wall.

This is because there is no such thing as an enum type in C, enums are 
just an alias for an integral type, and it is never wrong to store -1 in 
an integral type, even if it is unsigned.  (C++ on the other hand will 
complain here because C++ has actual enum types distinct from integral 
types.)

> Uncommenting the c= -1 member of enum, or explicit casting p->what to
> int solves the problem, of course.  But maybe some warning would be
> appropriate in such a situation?

You will get a warning if you compile with -W.
tmp.c:22: warning: comparison of unsigned expression >= 0 is always true
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com