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syscall.2: Add endian details with 64-bit splitting 

Architectures that split 64-bit values across register pairs
usually do so according to their C ABI calling convention (which
means endianness).  Add some notes to that effect, and change the
readahead example to show a little endian example (since that is
way more common than big endian).

Also start a new list of syscalls that this issue does not apply
to.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Mike Frysinger 2017-03-11 11:54:22 -08:00 committed by Michael Kerrisk
parent 4f9d01a93e
commit efa75963cc
1 changed files with 20 additions and 2 deletions
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22
man2/syscall.2
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@ -102,13 +102,14 @@ Thus, using
instead of the wrapper provided by glibc,
the
.BR readahead ()
system call would be invoked as follows on the ARM architecture with the EABI:
system call would be invoked as follows on the ARM architecture with the EABI
in little endian mode:
.in +4n
.nf
syscall(SYS_readahead, fd, 0,
(unsigned int) (offset >> 32),
(unsigned int) (offset & 0xFFFFFFFF),
(unsigned int) (offset >> 32),
count);
.fi
.in
@ -124,6 +125,8 @@ register pair.
That means inserting a dummy value into
.I r1
(the second argument of 0).
Care also must be taken so that the split follows endian conventions
(according to the C ABI for the platform).
Similar issues can occur on MIPS with the O32 ABI,
on PowerPC with the 32-bit ABI, and on Xtensa.
@ -140,6 +143,21 @@ The affected system calls are
.BR sync_file_range (2),
and
.BR truncate64 (2).
.\" You need to look up the syscalls directly in the kernel source to see if
.\" they should be in this list. For example, look at fs/read_write.c and
.\" the function signatures that do:
.\" ..., unsigned long, pos_l, unsigned long, pos_h, ...
.\" If they use off_t, then they most likely do not belong in this list.
This does not affect syscalls that manually split and assemble 64-bit values
such as
.BR _llseek (2),
.BR preadv (2),
.BR preadv2 (2),
.BR pwritev (2).
and
.BR pwritev2 (2).
Welcome to the wonderful world of historical baggage.
.SS Architecture calling conventions
Every architecture has its own way of invoking and passing arguments to the
kernel.