yet another wrong warning

[Phil Edwards]

On Wed, Nov 01, 2000 at 11:04:28AM -0800, Ulrich Drepper wrote:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> $ cat v.c
> #define SWAP(x, y) ({ typeof(x) _tmp = x; x = y; y = _tmp; })
> int a;
> int b;
> void
> foo (void)
> {
>   SWAP (a, b);
> }
> $ gcc -c -Wall v.c
> v.c: In function `foo':
> v.c:9: warning: value computed is not used
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Which value isn't used?

The result of the expression "y = _tmp;" I think.  The ({...}) is one of
those expression compound statement GCC extension things, right?  The last
statement in the list is evaluated as the value of the whole thing; that
value never gets used.

If you write "int c = SWAP (a, b);" -- yes, it's silly -- I would bet the
warning goes away.  (Probably to be replaced by something about c never
being used.)


Phil

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