-.\" 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 2018-02-02 "Linux" "Linux Programmer's Manual"
.SH NAME
read \- read from a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
-.sp
+.PP
.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
.fi
.SH DESCRIPTION
into the buffer starting at
.IR buf .
.PP
+On files that support seeking,
+the read operation commences at the file offset,
+and the file offset is incremented by the number of bytes read.
+If the file offset is at or past the end of file,
+no bytes are read, and
+.BR read ()
+returns zero.
+.PP
If
.I count
is zero,
.BR read ()
-returns zero and has no other results.
-If
+.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.
+.PP
+According to POSIX.1, if
.I count
is greater than
.BR SSIZE_MAX ,
-the result is unspecified.
-.SH "RETURN VALUE"
+the result is implementation-defined;
+see NOTES for the upper limit on Linux.
+.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
because we are reading from a pipe, or from a terminal), or because
.BR read ()
was interrupted by a signal.
+See also NOTES.
+.PP
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
refers to a file other than a socket and has been marked nonblocking
.RB ( O_NONBLOCK ),
and the read would block.
+See
+.BR open (2)
+for further details on the
+.BR O_NONBLOCK
+flag.
.TP
.BR EAGAIN " or " EWOULDBLOCK
.\" Actually EAGAIN on Linux
.IR buf ,
the value specified in
.IR count ,
-or the current file offset is not suitably aligned.
+or the file offset is not suitably aligned.
.TP
.B EINVAL
.I fd
is orphaned.
It may also occur when there is a low-level I/O error
while reading from a disk or tape.
+A further possible cause of
+.B EIO
+on networked filesystems is when an advisory lock had been taken
+out on the file descriptor and this lock has been lost.
+See the
+.I "Lost locks"
+section of
+.BR fcntl (2)
+for further details.
.TP
.B EISDIR
.I fd
.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"
+.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.
+The types
+.I size_t
+and
+.I ssize_t
+are, respectively,
+unsigned and signed integer data types specified by POSIX.1.
+.PP
+On Linux,
+.BR read ()
+(and similar system calls) will transfer at most
+0x7ffff000 (2,147,479,552) bytes,
+returning the number of bytes actually transferred.
+.\" commit e28cc71572da38a5a12c1cfe4d7032017adccf69
+(This is true on both 32-bit and 64-bit systems.)
+.PP
+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
+leave
+.I st_atime
+(last file access time)
+updates to the server, and client side reads satisfied from the
+client's cache will not cause
+.I 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"):
+.PP
+.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
.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"
+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),
projects / thirdparty / man-pages.git / blobdiff