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Clarify initial description of O_EXCL. 

Clarify description of behavious of O_CREAT | O_EXCL for symbolic links.
Clarify text describing use of lockfiles without O_EXCL.
O_EXCL is supported on NFSv3 and later, with Linux 2.6 and later.
This commit is contained in:
Michael Kerrisk 2008-01-03 08:12:02 +00:00
parent ce0cabd474
commit f4b9d6a511
1 changed files with 36 additions and 12 deletions
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48
man2/open.2
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@ -36,8 +36,11 @@
.\" 2004-12-08, mtk, reordered flags list alphabetically
.\" 2004-12-08, Martin Pool <mbp@sourcefrog.net> (& mtk), added O_NOATIME
.\" 2007-09-18, mtk, Added description of O_CLOEXEC + other minor edits
.\" 2008-01-03, mtk, with input from Trond Myklebust
.\" <trond.myklebust@fys.uio.no> and Timo Sirainen <tss@iki.fi>
.\" Rewrite description of O_EXCL.
.\"
.TH OPEN 2 2007-09-18 "Linux" "Linux Programmer's Manual"
.TH OPEN 2 2008-01-03 "Linux" "Linux Programmer's Manual"
.SH NAME
open, creat \- open and possibly create a file or device
.SH SYNOPSIS
@ -222,25 +225,46 @@ implementation of
.BR opendir (3).
.TP
.B O_EXCL
When used with
Ensure that this call creates the file:
if this flag is specified in conjunction with
.BR O_CREAT ,
if the file already exists it is an error and the
and
.I pathname
already exists, then
.BR open ()
will fail.
In this context, a symbolic link exists, regardless
of where it points to.
The behavior of
.B O_EXCL
is broken on NFS file systems; programs which rely on it for performing
locking tasks will contain a race condition.
The solution for performing
atomic file locking using a lockfile is to create a unique file on
the same file system (e.g., incorporating hostname and pid), use
is undefined if
.B O_CREAT
is not specified.
When these two flags are specified, symbolic links are not followed:
.\" POSIX.1-2001 explicitly requires this behavior.
if
.I pathname
is a symbolic link, then
.BR open ()
fails regardless of where the symbolic link points to.
.B O_EXCL
is not supported on NFS file systems before NFSv3
or on Linux before kernel 2.6;
it is supported on Linux 2.6 and later, with NFSv3 or later.
In environments where NFS
.B O_EXCL
support is not provided, programs that rely on it
for performing locking tasks will contain a race condition.
Portable programs that want to perform atomic file locking using a lockfile,
and need to avoid reliance on NFS support for
.BR O_EXCL ,
can create a unique file on
the same file system (e.g., incorporating hostname and PID), and use
.BR link (2)
to make a link to the lockfile.
If
.BR link (2)
returns 0, the lock is
successful.
returns 0, the lock is successful.
Otherwise, use
.BR stat (2)
on the unique file to check if its link count has increased to 2,