Section (2) read
read — read from a file descriptor
read() attempts to read up
count bytes from
into the buffer starting at
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
read() returns zero.
count is zero,
may detect the errors
described below. In the absence of any errors, or if
read() does not check for
read() with a
count of 0 returns
zero and has no other effects.
According to POSIX.1, if
count is greater than
SSIZE_MAX, the result is
implementation-defined; see NOTES for the upper limit on
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
read() was interrupted by a
signal. See also NOTES.
On error, −1 is returned, and
errno is set appropriately. In this case, it
is left unspecified whether the file position (if any)
The file descriptor
fdrefers to a file other than a socket and has been marked nonblocking (
O_NONBLOCK), and the read would block. See open(2) for further details on the
- EAGAIN or EWOULDBLOCK
The file descriptor
fdrefers to a socket and has been marked nonblocking (
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.
fdis not a valid file descriptor or is not open for reading.
bufis outside your accessible address space.
The call was interrupted by a signal before any data was read; see signal(7).
fdis attached to an object which is unsuitable for reading; or the file was opened with the
O_DIRECTflag, and either the address specified in
buf, the value specified in
count, or the file offset is not suitably aligned.
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
SIGTTINor 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. A further possible cause of 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 Lost locks section of fcntl(2) for further details.
fdrefers to a directory.
Other errors may occur, depending on the object connected
The types size_t and ssize_t are, respectively, unsigned and signed integer data types specified by POSIX.1.
similar system calls) will transfer at most 0x7ffff000
(2,147,479,552) bytes, returning the number of bytes actually
transferred. (This is true on both 32-bit and 64-bit
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_zsingle_quotesz_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.
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 (Thread Interactions with Regular File Operations):
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: ...
Among the APIs subsequently listed are
read() and 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 open(2)) perform a
read() (or 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.
This page is part of release 4.16 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
This manpage is Copyright (C) 1992 Drew Eckhardt;
and Copyright (C) 1993 Michael Haardt, Ian Jackson.
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
Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faithcs.unc.edu>
Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aebcwi.nl>
Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aebcwi.nl>
Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esrthyrsus.com>
Modified Sat Jul 12 20:45:39 1997 by Michael Haardt