-.\" Hey Emacs! This file is -*- nroff -*- source.
-.\"
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
-.\" 1993 Michael Haardt, Ian Jackson.
+.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
.\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
.\" <michael@cantor.informatik.rwth-aachen.de>
.\"
-.TH READ 2 2009-02-23 "Linux" "Linux Programmer's Manual"
+.TH READ 2 2014-05-04 "Linux" "Linux Programmer's Manual"
.SH NAME
read \- read from a file descriptor
.SH SYNOPSIS
.I fd
into the buffer starting at
.IR buf .
-.PP
+
+On files that support seeking,
+the read operation commences at the current file offset,
+and the file offset is incremented by the number of bytes read.
+If the current file offset is at or past the end of file,
+no bytes are read, and
+.BR read ()
+returns zero.
+
If
.I count
is zero,
.BR read ()
-returns zero and has no other results.
+.I may
+detect the errors described below.
+In the absence of any errors,
+or if
+.BR read ()
+does not check for errors, a
+.BR read ()
+with a
+.I count
+of 0 returns zero and has no other effects.
+
If
.I count
is greater than
.BR SSIZE_MAX ,
the result is unspecified.
-.SH "RETURN VALUE"
+.SH RETURN VALUE
On success, the number of bytes read is returned (zero indicates end of
file), and the file position is advanced by this number.
It is not an error if this number is smaller than the number of bytes
On error, \-1 is returned, and
.I errno
is set appropriately.
-In this case it is left unspecified whether
+In this case, it is left unspecified whether
the file position (if any) changes.
.SH ERRORS
.TP
.B EIO
I/O error.
This will happen for example when the process is in a
-background process group, tries to read from its controlling tty,
+background process group, tries to read from its controlling terminal,
and either it is ignoring or blocking
.B SIGTTIN
or its process group
set to
.BR EINTR )
or to return the number of bytes already read.
-.SH "CONFORMING TO"
+.SH CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
-On NFS file systems, reading small amounts of data will only update the
-timestamp the first time, subsequent calls may not do so.
+On NFS filesystems, reading small amounts of data will update the
+timestamp only the first time, subsequent calls may not do so.
This is caused
by client side attribute caching, because most if not all NFS clients
leave st_atime (last file access time)
updates to the server and client side reads satisfied from the
client's cache will not cause st_atime updates on the server as there are no
server side reads.
-Unix semantics can be obtained by disabling client
+UNIX semantics can be obtained by disabling client
side attribute caching, but in most situations this will substantially
increase server load and decrease performance.
-.PP
-Many file systems and disks were considered to be fast enough that the
-implementation of
-.B O_NONBLOCK
-was deemed unnecessary.
-So,
-.B O_NONBLOCK
-may not be available on files
-and/or disks.
-.SH "SEE ALSO"
+.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 read ()
+and
+.BR readv (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 read ()
+(or
+.BR readv (2))
+at the same time, then the I/O operations were not atomic
+with respect updating the file offset,
+with the result that the reads in the two processes
+might (incorrectly) overlap in the blocks of data that they obtained.
+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),
.BR ioctl (2),
.BR select (2),
.BR write (2),
.BR fread (3)
+.SH COLOPHON
+This page is part of release 3.68 of the Linux
+.I man-pages
+project.
+A description of the project,
+information about reporting bugs,
+and the latest version of this page,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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