mirror of https://github.com/mkerrisk/man-pages
150 lines
5.0 KiB
Groff
150 lines
5.0 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
|
|
.\"
|
|
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
|
|
.\" 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.\" manual page may be incorrect or out-of-date.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
|
|
.\" professionally.
|
|
.\"
|
|
.\" 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 <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 1997-07-12 "Linux 2.0.32" "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 .
|
|
.PP
|
|
If
|
|
.I count
|
|
is zero, \fBread\fP() returns zero and has no other results.
|
|
If
|
|
.I count
|
|
is greater than SSIZE_MAX, the result is unspecified.
|
|
.PP
|
|
.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
|
|
\fBread\fP() 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
|
|
Non-blocking I/O has been selected using
|
|
.B O_NONBLOCK
|
|
and no data was immediately available for reading.
|
|
.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.
|
|
.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 EIO
|
|
I/O error. This will happen for example when the process is in a
|
|
background process group, tries to read from its controlling tty,
|
|
and either it is ignoring or blocking 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 EINTR) or to return the number of bytes already read.
|
|
.SH "CONFORMING TO"
|
|
SVr4, SVID, AT&T, POSIX, X/OPEN, 4.3BSD
|
|
.SH RESTRICTIONS
|
|
On NFS file systems, reading small amounts of data will only update the
|
|
time stamp 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.
|
|
.PP
|
|
Many filesystems and disks were considered to be fast enough that the
|
|
implementation of
|
|
.B O_NONBLOCK
|
|
was deemed unneccesary. So, O_NONBLOCK may not be available on files
|
|
and/or disks.
|
|
.SH "SEE ALSO"
|
|
.BR close (2),
|
|
.BR fcntl (2),
|
|
.BR ioctl (2),
|
|
.BR lseek (2),
|
|
.BR open (2),
|
|
.BR readdir (2),
|
|
.BR readlink (2),
|
|
.BR select (2),
|
|
.BR write (2),
|
|
.BR fread (3),
|
|
.BR readv (3)
|