man-pages/man2/read.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"  and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\"  and Copyright (C) 2009-2015 Michael Kerrisk, <mtk.manpages.gmail.com>
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.\" 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 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
read \- read from a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.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 .
.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 ()
.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 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
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.
See also NOTES.
.PP
On error, \-1 is returned, and
.I errno
is set to indicate the error.
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.
See
.BR open (2)
for further details on the
.BR O_NONBLOCK
flag.
.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 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.
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
refers to a directory.
.PP
Other errors may occur, depending on the object connected to
.IR fd .
.SH CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
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
.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
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)