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Re: [PATCH v3 2/2] posix: Implement preadv2 and pwritev2
On Sat, Jun 3, 2017 at 4:23 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 06/03/2017 01:04 PM, H.J. Lu wrote:
>> On Sat, Jun 3, 2017 at 1:22 AM, Andreas Schwab <schwab@linux-m68k.org> wrote:
>>> On Jun 02 2017, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>>
>>>> The x86-64 LO_HI_LONG can't be used for p{readv,writev}{64}v2. Add a
>>>> new macro, LO_HI_LONG_FLAGS, to pass the off{64}_t and flags arguments.
>>>
>>> Why can't LO_HI_LONG just pass the padding unconditionally on x86_64?
>>>
>>
>> To avoid the unnecessary (long) (((uint64_t) (val)) >> 32).
>
> I think the question is why you can't define it like this:
>
> (val), 0
>
> ? Are you concerned about the additional overhead of passing that
> unnecessary zero at the end of the parameter list for other system
> calls? Or would this result in an observable kernel interface
> difference and break stuff?
My patch has
ndex 7b8bd79..a3fe2fa 100644
--- a/sysdeps/unix/sysv/linux/x86_64/sysdep.h
+++ b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
@@ -389,4 +389,9 @@
#undef LO_HI_LONG
#define LO_HI_LONG(val) (val)
+/* Provide a macro to pass the off{64}_t and flags arguments on
+ p{readv,writev}{64}v2. */
+#undef LO_HI_LONG_FLAGS
+#define LO_HI_LONG_FLAGS(val, flags) (val), 0, (flags)
+
#endif /* linux/x86_64/sysdep.h */
For LO_HI_LONG, it doesn't mater what the second one is. It makes
no difference if -1 is passed. Why bother with 0?
--
H.J.