mirror of https://github.com/mkerrisk/man-pages
151 lines
5.0 KiB
Groff
151 lines
5.0 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" 1993 Michael Haardt, Ian Jackson.
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faith@cs.unc.edu>
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.\" Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
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.\" <michael@cantor.informatik.rwth-aachen.de>
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.\" Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aeb@cwi.nl>
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.\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
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.\" Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
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.\" <michael@cantor.informatik.rwth-aachen.de>
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.\"
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.TH READ 2 1997-07-12 "Linux 2.0.32" "Linux Programmer's Manual"
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.SH NAME
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read \- read from a file descriptor
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.sp
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.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
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.fi
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.SH DESCRIPTION
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.BR read ()
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attempts to read up to
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.I count
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bytes from file descriptor
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.I fd
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into the buffer starting at
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.IR buf .
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.PP
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If
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.I count
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is zero, \fBread\fP() returns zero and has no other results.
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If
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.I count
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is greater than SSIZE_MAX, the result is unspecified.
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.PP
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.SH "RETURN VALUE"
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On success, the number of bytes read is returned (zero indicates end of
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file), and the file position is advanced by this number.
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It is not an error if this number is smaller than the number of bytes
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requested; this may happen for example because fewer bytes are actually
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available right now (maybe because we were close to end-of-file, or
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because we are reading from a pipe, or from a terminal), or because
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\fBread\fP() was interrupted by a signal.
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On error, \-1 is returned, and
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.I errno
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is set appropriately. In this case it is left unspecified whether
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the file position (if any) changes.
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.SH ERRORS
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.TP
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.B EAGAIN
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Non-blocking I/O has been selected using
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.B O_NONBLOCK
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and no data was immediately available for reading.
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.TP
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.B EBADF
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.I fd
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is not a valid file descriptor or is not open for reading.
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.TP
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.B EFAULT
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.I buf
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is outside your accessible address space.
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.TP
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.B EINTR
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The call was interrupted by a signal before any data was read.
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.TP
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.B EINVAL
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.I fd
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is attached to an object which is unsuitable for reading;
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or the file was opened with the
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.B O_DIRECT
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flag, and either the address specified in
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.IR buf ,
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the value specified in
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.IR count ,
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or the current file offset is not suitably aligned.
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.TP
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.B EIO
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I/O error. This will happen for example when the process is in a
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background process group, tries to read from its controlling tty,
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and either it is ignoring or blocking SIGTTIN or its process group
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is orphaned. It may also occur when there is a low-level I/O error
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while reading from a disk or tape.
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.TP
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.B EISDIR
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.I fd
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refers to a directory.
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.PP
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Other errors may occur, depending on the object connected to
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.IR fd .
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POSIX allows a
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.BR read ()
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that is interrupted after reading some data
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to return \-1 (with
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.I errno
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set to EINTR) or to return the number of bytes already read.
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.SH "CONFORMING TO"
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SVr4, SVID, AT&T, POSIX, X/OPEN, 4.3BSD
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.SH RESTRICTIONS
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On NFS file systems, reading small amounts of data will only update the
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time stamp the first time, subsequent calls may not do so. This is caused
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by client side attribute caching, because most if not all NFS clients
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leave st_atime (last file access time)
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updates to the server and client side reads satisfied from the
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client's cache will not cause st_atime updates on the server as there are no
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server side reads. UNIX semantics can be obtained by disabling client
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side attribute caching, but in most situations this will substantially
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increase server load and decrease performance.
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.PP
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Many filesystems and disks were considered to be fast enough that the
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implementation of
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.B O_NONBLOCK
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was deemed unneccesary. So, O_NONBLOCK may not be available on files
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and/or disks.
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.SH "SEE ALSO"
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.BR close (2),
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.BR fcntl (2),
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.BR ioctl (2),
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.BR lseek (2),
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.BR open (2),
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.BR pread (2),
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.BR readdir (2),
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.BR readlink (2),
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.BR select (2),
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.BR write (2),
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.BR fread (3),
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.BR readv (3)
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