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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 <> wrote:
> On 06/03/2017 01:04 PM, H.J. Lu wrote:
>> On Sat, Jun 3, 2017 at 1:22 AM, Andreas Schwab <> wrote:
>>> On Jun 02 2017, "H.J. Lu" <> 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.  */
+#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?


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