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[glibc] preadv/pwritev question
- From: Yury Norov <ynorov at caviumnetworks dot com>
- To: <cmetcalf at mellanox dot com>
- Cc: <cmetcalf at tilera dot com>, <arnd at arndb dot de>, <catalin dot marinas at arm dot com>, <linux-kernel at vger dot kernel dot org>, <libc-alpha at sourceware dot org>, <pinskia at gmail dot com>
- Date: Tue, 31 May 2016 23:04:54 +0300
- Subject: [glibc] preadv/pwritev question
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Hi Chris,
In path a63c7fa18a (Add sysdeps/unix/sysv/linux/generic/.) you add
this:
+++ b/sysdeps/unix/sysv/linux/generic/wordsize-32/preadv.c
[...]
+static ssize_t
+do_preadv (int fd, const struct iovec *vector, int count, off_t
offset)
+{
+ assert (sizeof (offset) == 4);
+ return INLINE_SYSCALL (preadv, __ALIGNMENT_COUNT (5, 6), fd,
+ vector, count, __ALIGNMENT_ARG
+ __LONG_LONG_PAIR (offset >> 31, offset));
+}
+
And this is the code that is picked up if I choose wordsize-32 for my
AARCH64/ILP32. So I have questions.
1. What is the assert for? We agreed that all new ABIs will be 64-bit
off_t only.
I fixed it internally like this:
+#ifndef __OFF_T_MATCHES_OFF64_T
assert (sizeof (offset) == 4);
+#endif
There is a bunch of similar assertions in glibc.
2. This one looks weird:
__LONG_LONG_PAIR (offset >> 31, offset))
Why 31-bit offset? And why you don't mask 2nd argument?
Later in your patch I see this:
+static ssize_t
+do_preadv64 (int fd, const struct iovec *vector, int count, off64_t
offset)
+{
+ return INLINE_SYSCALL (preadv, __ALIGNMENT_COUNT (5, 6), fd,
+ vector, count, __ALIGNMENT_ARG
+ __LONG_LONG_PAIR ((off_t) (offset >> 32),
+ (off_t) (offset & 0xffffffff)));
+}
And it looks correct to me. If 1st version is correct as well, I think
it should be commented.
Yury.