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write.2: BUGS: detail nonatomicity bug with respect to file offset updates 

This bug was fixed in Linux 3.14, with commit
9c225f2655e36a470c4f58dbbc99244c5fc7f2d4
See also http://thread.gmane.org/gmane.linux.kernel/1649458

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-05-04 10:25:48 +02:00
parent c0f72eccff
commit 2afe936518
1 changed files with 40 additions and 1 deletions
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man2/write.2
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@ -36,7 +36,7 @@
.\" gave some examples of why this might occur.
.\" Noted what happens if write() is interrupted by a signal.
.\"
.TH WRITE 2 2013-01-27 "Linux" "Linux Programmer's Manual"
.TH WRITE 2 2014-05-04 "Linux" "Linux Programmer's Manual"
.SH NAME
write \- write to a file descriptor
.SH SYNOPSIS
@ -208,6 +208,45 @@ then the call fails with the error
.BR EINTR ;
if it is interrupted after at least one byte has been written,
the call succeeds, and returns the number of bytes written.
.SH BUGS
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
("Thread Interactions with Regular File Operations"):
.RS 4
All of the following functions shall be atomic with respect to
each other in the effects specified in POSIX.1-2008 when they
operate on regular files or symbolic links: ...
.RE
Among the APIs subsequently listed are
.BR write ()
and
.BR writev (2).
And among the effects that should be atomic across threads (and processes)
are updates of the file offset.
However, on Linux before version 3.14,
this was not the case: if two processes that share
an open file description (see
.BR open (2))
perform a
.BR write ()
(or
.BR writev (2))
at the same time, then the I/O operations were not atomic
with respect updating the file offset,
with the result that the blocks of data output by the two processes
might (incorrectly) overlap.
This problem was fixed in Linux 3.14.
.\" http://thread.gmane.org/gmane.linux.kernel/1649458
.\" From: Michael Kerrisk (man-pages <mtk.manpages <at> gmail.com>
.\" Subject: Update of file offset on write() etc. is non-atomic with I/O
.\" Date: 2014-02-17 15:41:37 GMT
.\" Newsgroups: gmane.linux.kernel, gmane.linux.file-systems
.\" commit 9c225f2655e36a470c4f58dbbc99244c5fc7f2d4
.\" Author: Linus Torvalds <torvalds@linux-foundation.org>
.\" Date: Mon Mar 3 09:36:58 2014 -0800
.\"
.\" vfs: atomic f_pos accesses as per POSIX
.SH SEE ALSO
.BR close (2),
.BR fcntl (2),