+++ /dev/null
-.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
-.\" 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.
-.\"
-.\" Permission is granted to copy and distribute modified versions of this
-.\" manual under the conditions for verbatim copying, provided that the
-.\" entire resulting derived work is distributed under the terms of a
-.\" permission notice identical to this one.
-.\"
-.\" Since the Linux kernel and libraries are constantly changing, this
-.\" manual page may be incorrect or out-of-date. The author(s) assume no
-.\" responsibility for errors or omissions, or for damages resulting from
-.\" the use of the information contained herein. The author(s) may not
-.\" have taken the same level of care in the production of this manual,
-.\" which is licensed free of charge, as they might when working
-.\" professionally.
-.\"
-.\" 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
-.\" <michael@cantor.informatik.rwth-aachen.de>
-.\" Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aeb@cwi.nl>
-.\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
-.\" Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esr@thyrsus.com>
-.\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
-.\" <michael@cantor.informatik.rwth-aachen.de>
-.\"
-.TH READ 2 2014-05-04 "Linux" "Linux Programmer's Manual"
-.SH NAME
-read \- read from a file descriptor
-.SH SYNOPSIS
-.nf
-.B #include <unistd.h>
-.sp
-.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
-.fi
-.SH DESCRIPTION
-.BR read ()
-attempts to read up to
-.I count
-bytes from file descriptor
-.I fd
-into the buffer starting at
-.IR buf .
-
-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 ()
-.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
-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
-requested; this may happen for example because fewer bytes are actually
-available right now (maybe because we were close to end-of-file, or
-because we are reading from a pipe, or from a terminal), or because
-.BR read ()
-was interrupted by a signal.
-On error, \-1 is returned, and
-.I errno
-is set appropriately.
-In this case, it is left unspecified whether
-the file position (if any) changes.
-.SH ERRORS
-.TP
-.B EAGAIN
-The file descriptor
-.I fd
-refers to a file other than a socket and has been marked nonblocking
-.RB ( O_NONBLOCK ),
-and the read would block.
-.TP
-.BR EAGAIN " or " EWOULDBLOCK
-.\" Actually EAGAIN on Linux
-The file descriptor
-.I fd
-refers to a socket and has been marked nonblocking
-.RB ( O_NONBLOCK ),
-and the read would block.
-POSIX.1-2001 allows either error to be returned for this case,
-and does not require these constants to have the same value,
-so a portable application should check for both possibilities.
-.TP
-.B EBADF
-.I fd
-is not a valid file descriptor or is not open for reading.
-.TP
-.B EFAULT
-.I buf
-is outside your accessible address space.
-.TP
-.B EINTR
-The call was interrupted by a signal before any data was read; see
-.BR signal (7).
-.TP
-.B EINVAL
-.I fd
-is attached to an object which is unsuitable for reading;
-or the file was opened with the
-.B O_DIRECT
-flag, and either the address specified in
-.IR buf ,
-the value specified in
-.IR count ,
-or the current file offset is not suitably aligned.
-.TP
-.B EINVAL
-.I fd
-was created via a call to
-.BR timerfd_create (2)
-and the wrong size buffer was given to
-.BR read ();
-see
-.BR timerfd_create (2)
-for further information.
-.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 terminal,
-and either it is ignoring or blocking
-.B SIGTTIN
-or its process group
-is orphaned.
-It may also occur when there is a low-level I/O error
-while reading from a disk or tape.
-.TP
-.B EISDIR
-.I fd
-refers to a directory.
-.PP
-Other errors may occur, depending on the object connected to
-.IR fd .
-POSIX allows a
-.BR read ()
-that is interrupted after reading some data
-to return \-1 (with
-.I errno
-set to
-.BR EINTR )
-or to return the number of bytes already read.
-.SH CONFORMING TO
-SVr4, 4.3BSD, POSIX.1-2001.
-.SH NOTES
-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
-side attribute caching, but in most situations this will substantially
-increase server load and decrease performance.
-.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 lseek (2),
-.BR open (2),
-.BR pread (2),
-.BR readdir (2),
-.BR readlink (2),
-.BR readv (2),
-.BR select (2),
-.BR write (2),
-.BR fread (3)
-.SH COLOPHON
-This page is part of release 3.79 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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