Re: [Xen-devel] [PATCH v2] tools/libxl: libxl_get_scheduler should return an int

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 03/21/2014 12:43 PM, Ian Campbell wrote:
> On Fri, 2014-03-21 at 12:25 +0000, Julien Grall wrote:
>> libxl_get_scheduler returns either a valid value in enum range or ERROR_FAIL.
>>
>> As function return type is an enum, chekcing if the value is negative will
> 
> "checking"

I will update the commit message.

>> be always false. Therefore both GCC and clang will never go to the error
>> case.
> 
> Are there any API compatibility concerns here? I think there isn't
> because
>       enum foo bar = get_foo()
> and
>       int bar = get_foo()
> are both always valid whether get_foo() returns an enum foo or an int.
> 
> So I think we can make this change without a LIBXL_HAVE define. Would be
> good to confirm and include the rationale in the commit log.

C99, 6.7.2.2p4

Each enumerated type shall be compatible with char, a signed integer
type, or an unsigned integer type. The choice of type is
implementation-defined,108) but shall be capable of representing the
values of all the members of the enumeration. [...]

I gave a try with a simple test file:
typedef enum libxl_sched
{
  SCHED_SCHED,
} libxl_sched;

int f(void);

int main(void)
{
    libxl_sched sched = f();

    return 0;
}

I compiled twice, the second time I have changed f return type.

The result is the same with gcc 4.7 on x86. For ARM with gcc 4.6.3 there
is an extra move on the first version. IHMO it's not harmfull.

Regards

-- 
Julien Grall

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