Hi Mauro,
Mauro Carvalho Chehab wrote:
Em 02-05-2012 21:42, Andy Walls escreveu:
On Wed, 2012-05-02 at 19:17 -0300, Mauro Carvalho Chehab wrote:
We can speed-up the conversions, with something like:
enum foo {
BAR
};
if (sizeof(foo) != sizeof(u32))
call_compat_logic().
I suspect that sizeof() won't work inside a macro.
sizeof() is evaluated at compile time, after preprocessing.
It should work inside of a macro.
I tried to compile this small piece of code:
enum foo { BAR };
#if sizeof(foo) != sizeof(int)
void main(void) { printf("different sizes\n"); }
#else
void main(void) { printf("same size\n"); }
#endif
It gives an error:
/tmp/foo.c:2:11: error: missing binary operator before token "("
So, either this doesn't work, because sizeof() is evaluated too late,
or some trick is needed.
Weird enough, cpp generates the error, but the expression is well-evaluated:
$ cpp /tmp/foo.c
# 1 "/tmp/foo.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/tmp/foo.c"
/tmp/foo.c:2:11: error: missing binary operator before token "("
enum foo { BAR };
sizeof() is processed by C compiler while #if is preprocessor directive,
and its arguments have to be evaluable by the preprocessor, which is the
problem here.
The C compiler can also optimise away things like that but it's more
difficult to see whether that takes place or not; one would need to look
at the resulting assembly code.
Regards,
--
Sakari Ailus
sakari.ailus@xxxxxx
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