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open.2: add some comments on O_SYNC and friends 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Christoph Hellwig 2009-09-20 07:41:16 +02:00 committed by Michael Kerrisk
parent aec4704f6d
commit 015221eff8
1 changed files with 31 additions and 9 deletions
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man2/open.2
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@ -47,7 +47,7 @@
.\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and
.\" O_TTYINIT. Eventually these may need to be documented. --mtk
.\"
.TH OPEN 2 2008-12-03 "Linux" "Linux Programmer's Manual"
.TH OPEN 2 2009-09-20 "Linux" "Linux Programmer's Manual"
.SH NAME
open, creat \- open and possibly create a file or device
.SH SYNOPSIS
@ -277,11 +277,16 @@ Try to minimize cache effects of the I/O to and from this file.
In general this will degrade performance, but it is useful in
special situations, such as when applications do their own caching.
File I/O is done directly to/from user space buffers.
The I/O is synchronous, that is, at the completion of a
.BR read (2)
or
.BR write (2),
data is guaranteed to have been transferred.
The
.B O_DIRECT
flag on its own makes at an effort to transfer data synchronously,
but does not give the guarantees of the
.B O_SYNC
that data and necessary metadata are transferred.
To guarantee synchronous I/O the
.B O_SYNC
must be used in addition to
.BR O_DIRECT .
See
.B NOTES
below for further discussion.
@ -660,9 +665,26 @@ amongst others
.BR O_SYNC " and " O_NDELAY .
POSIX provides for three different variants of synchronized I/O,
corresponding to the flags \fBO_SYNC\fP, \fBO_DSYNC\fP and
\fBO_RSYNC\fP.
Currently (2.1.130) these are all synonymous under Linux.
corresponding to the flags
.BR O_SYNC ,
.BR O_DSYNC ,
and
.BR O_RSYNC .
Currently (2.6.31), Linux only implements
.BR O_SYNC ,
but glibc maps
.B O_DSYNC
and
.B O_RSYNC
to the same numerical value as
.BR O_SYNC
Most Linux filesystems don't actually implement the POSIX
.B O_SYNC
semantics, which require all metadata updates of a write
to be on disk on returning to userspace, but only the
.B O_DSYNC
semantics, which require only actual file data and metadata necessary
to retrieve it to be on disk by the time the system call returns.
Note that
.BR open ()