man-pages/man2/read.2

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.\"                               1993 Michael Haardt, Ian Jackson.
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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
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.\"
.TH READ 2 2013-02-12 "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,
of 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 file systems, reading small amounts of data will only update the
timestamp 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 "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)