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[linuxjm/jm.git] / manual / LDP_man-pages / draft / man2 / read.2

.\" 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.\" manual page may be incorrect or out-of-date.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>
.\"
.\" Japanese Version Copyright (c) 1997 HANATAKA Shinya
.\"         all rights reserved.
.\" Translated 1997-02-23, HANATAKA Shinya <hanataka@abyss.rim.or.jp>
.\" Modified 1997-09-28, HANATAKA Shinya <hanataka@abyss.rim.or.jp>
.\" Updated & Modified 2005-02-24, Yuichi SATO <ysato444@yahoo.co.jp>
.\" Updated 2005-09-06, Akihiro MOTOKI <amotoki@dd.iij4u.or.jp>
.\" Updated 2005-10-14, Akihiro MOTOKI
.\" Updated 2008-02-10, Akihiro MOTOKI, LDP v2.77
.\" Updated 2009-04-13, Akihiro MOTOKI, LDP v3.20
.\"
.\"WORD:        descriptor              ¥Ç¥£¥¹¥¯¥ê¥×¥¿¡¼
.\"WORD:        buffer                  ¥Ð¥Ã¥Õ¥¡¡¼
.\"WORD:        signal                  ¥·¥°¥Ê¥ë
.\"WORD:        process group           ¥×¥í¥»¥¹¡¦¥°¥ë¡¼¥×
.\"WORD:        backgraound             ¥Ð¥Ã¥¯¥°¥é¥ó¥É
.\"WORD:        orphan                  ¸ÉΩ
.\"WORD:        open                    ¥ª¡¼¥×¥ó
.\"WORD:        directory               ¥Ç¥£¥ì¥¯¥È¥ê
.\"WORD:        object                  ¥ª¥Ö¥¸¥§¥¯¥È
.\"
.TH READ 2 2009-02-23 "Linux" "Linux Programmer's Manual"
.\"O .SH NAME
.SH ̾Á°
.\"O read \- read from a file descriptor
read \- ¥Õ¥¡¥¤¥ë¡¦¥Ç¥£¥¹¥¯¥ê¥×¥¿¡¼¤«¤éÆɤ߹þ¤à
.\"O .SH SYNOPSIS
.SH ½ñ¼°
.nf
.B #include <unistd.h>
.sp
.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
.fi
.\"O .SH DESCRIPTION
.SH ÀâÌÀ
.\"O .BR read ()
.\"O attempts to read up to
.\"O .I count
.\"O bytes from file descriptor
.\"O .I fd
.\"O into the buffer starting at
.\"O .IR buf .
.BR read ()
¤Ï¥Õ¥¡¥¤¥ë¡¦¥Ç¥£¥¹¥¯¥ê¥×¥¿¡¼ (file descriptor)
.I fd
¤«¤éºÇÂç
.I count
¥Ð¥¤¥È¤ò
.I buf
¤Ç»Ï¤Þ¤ë¥Ð¥Ã¥Õ¥¡¡¼¤ØÆɤ߹þ¤â¤¦¤È¤¹¤ë¡£
.PP
.\"O If
.\"O .I count
.\"O is zero,
.\"O .BR read ()
.\"O returns zero and has no other results.
.\"O If
.\"O .I count
.\"O is greater than
.\"O .BR SSIZE_MAX ,
.\"O the result is unspecified.
.I count
¤¬ 0 ¤Ê¤é¤Ð¡¢
.BR read ()
¤Ï 0 ¤òÊÖ¤·¡¢Â¾¤Ë²¿¤âµ¯¤­¤Ê¤¤¡£
.I count
¤¬
.B SSIZE_MAX
¤è¤êÂ礭¤±¤ì¤Ð¡¢·ë²Ì¤ÏÆÃÄê¤Ç¤­¤Ê¤¤¡£
.\"O .SH "RETURN VALUE"
.SH ÊÖ¤êÃÍ
.\"O On success, the number of bytes read is returned (zero indicates end of
.\"O file), and the file position is advanced by this number.
.\"O It is not an error if this number is smaller than the number of bytes
.\"O requested; this may happen for example because fewer bytes are actually
.\"O available right now (maybe because we were close to end-of-file, or
.\"O because we are reading from a pipe, or from a terminal), or because
.\"O .BR read ()
.\"O was interrupted by a signal.
À®¸ù¤·¤¿¾ì¹ç¡¢Æɤ߹þ¤ó¤À¥Ð¥¤¥È¿ô¤òÊÖ¤¹ (0 ¤Ï¥Õ¥¡¥¤¥ë¤Î½ª¤ê¤ò°ÕÌ£¤¹¤ë)¡£
¥Õ¥¡¥¤¥ë°ÌÃ֤Ϥ³¤Î¿ô¤À¤±¿Ê¤á¤é¤ì¤ë¡£
¤³¤Î¿ô¤¬Í׵ᤷ¤¿¿ô¤è¤ê¾®¤µ¤«¤Ã¤¿¤È¤·¤Æ¤â¥¨¥é¡¼¤Ç¤Ï¤Ê¤¤;
Î㤨¤Ðº£¤¹¤°¤Ë¤Ï¼ÂºÝ¤Ë¤½¤ì¤À¤±¤Î¿ô¤·¤«¤Ê¤¤¾ì¹ç (¥Õ¥¡¥¤¥ë¤ÎºÇ¸å¤Ë¶á¤¤¤Î¤«¤â
¤·¤ì¤Ê¤¤¤·¡¢¥Ñ¥¤¥× (pipe) ¤äüËö (terminal) ¤«¤éÆɤ߹þ¤ó¤Ç¤¤¤ë¤«¤â¤·¤ì¤Ê¤¤) ¤ä
.BR read ()
¤¬¥·¥°¥Ê¥ë (signal) ¤Ë¤è¤Ã¤Æ³ä¤ê¹þ¤Þ¤ì¤¿¾ì¹ç¤Ë¤³¤ì¤Ïµ¯¤³¤ê¤¨¤ë¡£
.\"O On error, \-1 is returned, and
.\"O .I errno
.\"O is set appropriately.
.\"O In this case it is left unspecified whether
.\"O the file position (if any) changes.
¥¨¥é¡¼¤Î¾ì¹ç¤Ï¡¢\-1 ¤¬ÊÖ¤µ¤ì¡¢
.I errno
¤¬Å¬ÀÚ¤ËÀßÄꤵ¤ì¤ë¡£¤³¤Î¾ì¹ç¤Ï¥Õ¥¡¥¤¥ë°ÌÃÖ¤¬Êѹ¹¤µ¤ì¤ë¤«¤É¤¦¤«¤Ï
ÉÔÄê¤Ç¤¢¤ë¡£
.SH ¥¨¥é¡¼
.TP
.B EAGAIN
.\"O The file descriptor
.\"O .I fd
.\"O refers to a file other than a socket and has been marked nonblocking
.\"O .RB ( O_NONBLOCK ),
.\"O and the read would block.
¥Õ¥¡¥¤¥ë¡¦¥Ç¥£¥¹¥¯¥ê¥×¥¿¡¼
.I fd
¤¬¥½¥±¥Ã¥È°Ê³°¤Î¥Õ¥¡¥¤¥ë¤ò»²¾È¤·¤Æ¤¤¤Æ¡¢
ÈóÄä»ß (nonblocking) ¥â¡¼¥É
.RB ( O_NONBLOCK )
¤ËÀßÄꤵ¤ì¤Æ¤ª¤ê¡¢Æɤ߹þ¤ß¤ò¹Ô¤¦¤ÈÄä»ß¤¹¤ë¾õ¶·¤Ë¤¢¤ë¡£
.TP
.\"O .BR EAGAIN " or " EWOULDBLOCK
.BR EAGAIN " ¤Þ¤¿¤Ï " EWOULDBLOCK
.\" Actually EAGAIN on Linux
.\"O The file descriptor
.\"O .I fd
.\"O refers to a socket and has been marked nonblocking
.\"O .RB ( O_NONBLOCK ),
.\"O and the read would block.
.\"O POSIX.1-2001 allows either error to be returned for this case,
.\"O and does not require these constants to have the same value,
.\"O so a portable application should check for both possibilities.
¥Õ¥¡¥¤¥ë¡¦¥Ç¥£¥¹¥¯¥ê¥×¥¿¡¼
.I fd
¤¬¥½¥±¥Ã¥È¤ò»²¾È¤·¤Æ¤¤¤Æ¡¢ÈóÄä»ß (nonblocking) ¥â¡¼¥É
.RB ( O_NONBLOCK )
¤ËÀßÄꤵ¤ì¤Æ¤ª¤ê¡¢Æɤ߹þ¤ß¤ò¹Ô¤¦¤ÈÄä»ß¤¹¤ë¾õ¶·¤Ë¤¢¤ë¡£
POSIX.1-2001 ¤Ï¡¢¤³¤Î¾ì¹ç¤Ë¤É¤Á¤é¤Î¥¨¥é¡¼¤òÊÖ¤¹¤³¤È¤âǧ¤á¤Æ¤ª¤ê¡¢
¤³¤ì¤é 2 ¤Ä¤ÎÄê¿ô¤¬Æ±¤¸Ãͤò»ý¤Ä¤³¤È¤âµá¤á¤Æ¤¤¤Ê¤¤¡£
¤·¤¿¤¬¤Ã¤Æ¡¢°Ü¿¢À­¤¬É¬Íפʥ¢¥×¥ê¥±¡¼¥·¥ç¥ó¤Ç¤Ï¡¢Î¾Êý¤Î²ÄǽÀ­¤ò
³Îǧ¤¹¤Ù¤­¤Ç¤¢¤ë¡£
.TP
.B EBADF
.\"O .I fd
.\"O is not a valid file descriptor or is not open for reading.
.I fd
¤¬Í­¸ú¤Ê¥Õ¥¡¥¤¥ë¡¦¥Ç¥£¥¹¥¯¥ê¥×¥¿¡¼¤Ç¤Ê¤¤¤«¡¢Æɤ߹þ¤ß¤Î¤¿¤á¤Ë
¥ª¡¼¥×¥ó (open) ¤µ¤ì¤Æ¤¤¤Ê¤¤¡£
.TP
.B EFAULT
.\"O .I buf
.\"O is outside your accessible address space.
.I buf
¤¬¥¢¥¯¥»¥¹²Äǽ¤Ê¥¢¥É¥ì¥¹¶õ´Ö¤Î³°¤Ë¤¢¤ë¡£
.TP
.B EINTR
.\"O The call was interrupted by a signal before any data was read; see
.\"O .BR signal (7).
²¿¤Î¥Ç¡¼¥¿¤âÆɤ߹þ¤Þ¤Ê¤¤¤¦¤Á¤Ë¥·¥°¥Ê¥ë¤Ë³ä¤ê¹þ¤Þ¤ì¤¿¡£
.BR signal (7)
»²¾È¡£
.TP
.B EINVAL
.\"O .I fd
.\"O is attached to an object which is unsuitable for reading;
.\"O or the file was opened with the
.\"O .B O_DIRECT
.\"O flag, and either the address specified in
.\"O .IR buf ,
.\"O the value specified in
.\"O .IR count ,
.\"O or the current file offset is not suitably aligned.
.I fd
¤ÏÆɤ߹þ¤ß¤ËŬ¤·¤Æ¤¤¤Ê¤¤¥ª¥Ö¥¸¥§¥¯¥È¤ò»²¾È¤·¤Æ¤¤¤ë¡£
¤â¤·¤¯¤Ï¡¢¥Õ¥¡¥¤¥ë¤¬
.B O_DIRECT
¥Õ¥é¥°¤ò»ØÄꤷ¤Æ¥ª¡¼¥×¥ó¤µ¤ì¤Æ¤¤¤ë¤¬¡¢
.I buf
¤Ë»ØÄꤵ¤ì¤¿¥¢¥É¥ì¥¹¡¢
.I count
¤Ë»ØÄꤵ¤ì¤¿ÃÍ¡¢
¸½ºß¤Î¥Õ¥¡¥¤¥ë¥ª¥Õ¥»¥Ã¥È¤Î¤¤¤º¤ì¤«¤Î
¥¢¥é¥¤¥ó¥á¥ó¥È¤¬ÉÔŬÀڤǤ¢¤ë¡£
.TP
.B EINVAL
.\"O .I fd
.\"O was created via a call to
.\"O .BR timerfd_create (2)
.\"O and the wrong size buffer was given to
.\"O .BR read ();
.\"O see
.\"O .BR timerfd_create (2)
.\"O for further information.
.I fd
¤¬
.BR timerfd_create (2)
¤Î¸Æ¤Ó½Ð¤·¤ÇºîÀ®¤µ¤ì¤¿¤¬¡¢
.BR read ()
¤Ë´Ö°ã¤Ã¤¿¥µ¥¤¥º¤Î¥Ð¥Ã¥Õ¥¡¤¬ÅϤµ¤ì¤¿¡£
¤µ¤é¤Ê¤ë¾ðÊó¤Ï
.BR timerfd_create (2)
¤ò»²¾È¤Î¤³¤È¡£
.TP
.B EIO
.\"O I/O error.
.\"O This will happen for example when the process is in a
.\"O background process group, tries to read from its controlling tty,
.\"O and either it is ignoring or blocking
.\"O .B SIGTTIN
.\"O or its process group
.\"O is orphaned.
.\"O It may also occur when there is a low-level I/O error
.\"O while reading from a disk or tape.
I/O ¥¨¥é¡¼¡£¤³¤ì¤ÏÎ㤨¤Ð¥×¥í¥»¥¹¤¬¥Ð¥Ã¥¯¥°¥é¥ó¥É¡¦¥×¥í¥»¥¹¡¦¥°¥ë¡¼¥×¤Ç¡¢
¤½¤ì¤òÀ©¸æ¤·¤Æ¤¤¤ë tty ¤«¤éÆɤ߹þ¤â¤¦¤È¤·¡¢
.B SIGTTIN
¤¬Ìµ»ë (ignore) ¤Þ¤¿¤Ï¶Ø»ß (blocking) ¤µ¤ì¤Æ¤¤¤ë¾ì¹ç¤ä¡¢
¤½¤Î¥×¥í¥»¥¹¡¦¥°¥ë¡¼¥×¤¬¸ÉΩ (orphan) ¤·¤Æ¤¤¤ë¾ì¹ç¤Ëµ¯¤³¤ë¡£
¤Þ¤¿¥Ç¥£¥¹¥¯¤ä¥Æ¡¼¥×¤òÆɤó¤Ç¤¤¤ë»þ¤ËÄã¥ì¥Ù¥ë I/O ¥¨¥é¡¼
¤¬È¯À¸¤·¤¿¾ì¹ç¤Ë¤âµ¯¤³¤ë¡£
.TP
.B EISDIR
.\"O .I fd
.\"O refers to a directory.
.I fd
¤¬¥Ç¥£¥ì¥¯¥È¥ê¤ò»²¾È¤·¤Æ¤¤¤ë¡£
.PP
.\"O Other errors may occur, depending on the object connected to
.\"O .IR fd .
.I fd
¤¬Àܳ¤·¤Æ¤¤¤ë¥ª¥Ö¥¸¥§¥¯¥È¤Ë¤è¤Ã¤Æ¤Ï¾¤Î¥¨¥é¡¼¤âµ¯¤³¤ê¤¨¤ë¡£
.\"O POSIX allows a
.\"O .BR read ()
.\"O that is interrupted after reading some data
.\"O to return \-1 (with
.\"O .I errno
.\"O set to
.\"O .BR EINTR )
.\"O or to return the number of bytes already read.
POSIX ¤Ç¤Ï¡¢
¤¤¤¯¤é¤«¤Î¥Ç¡¼¥¿¤òÆɤó¤À¸å¤Ë³ä¤ê¹þ¤ß¤¬µ¯¤³¤Ã¤¿¾ì¹ç¡¢
.BR read ()
¤Ï
.RI ( errno
¤Ë
.B EINTR
¤òÀßÄꤷ¤Æ) \-1 ¤òÊÖ¤·¤Æ¤â¤è¤¤¤·¡¢
´û¤ËÆɤ߹þ¤ó¤À¥Ð¥¤¥È¿ô¤òÊÖ¤·¤Æ¤â¤è¤¤¡£
.\"O .SH "CONFORMING TO"
.SH ½àµò
SVr4, 4.3BSD, POSIX.1-2001.
.\"O .SH NOTES
.SH Ãí°Õ
.\"O On NFS file systems, reading small amounts of data will only update the
.\"O timestamp the first time, subsequent calls may not do so.
.\"O This is caused
.\"O by client side attribute caching, because most if not all NFS clients
.\"O leave st_atime (last file access time)
.\"O updates to the server and client side reads satisfied from the
.\"O client's cache will not cause st_atime updates on the server as there are no
.\"O server side reads.
.\"O Unix semantics can be obtained by disabling client
.\"O side attribute caching, but in most situations this will substantially
.\"O increase server load and decrease performance.
NFS ¤Ë¤ª¤¤¤Æ¡£¾¯Î̤Υǡ¼¥¿¤òÆɤ߹þ¤à¾ì¹ç¡¢ºÇ½é¤Î»þ¤Î¤ß¤Ë¥¿¥¤¥à
¥¹¥¿¥ó¥×¤¬¹¹¿·¤µ¤ì¡¢Â³¤¯¥³¡¼¥ë¤Ç¤Ï¹¹¿·¤µ¤ì¤Ê¤¤¤À¤í¤¦¡£
¤³¤ì¤Ï¥¯¥é¥¤¥¢¥ó¥È¦¤Ç°À­¤Î¥­¥ã¥Ã¥·¥ó¥°¤ò¹Ô¤Ê¤¦¤¿¤á¤Ç¤¢¤ë¡£
¤Ê¤¼¤Ê¤é¤Ð¡¢¤â¤·Á´¤Æ¤Î NFS ¥¯¥é¥¤¥¢¥ó¥È¤¬ st_atime (ºÇ½ª¥Õ¥¡¥¤¥ë¥¢¥¯¥»¥¹»þ¹ï)
¤Î¹¹¿·¤ò¥µ¡¼¥Ð¡¼¤ËÁ÷¤é¤º¡¢¥¯¥é¥¤¥¢¥ó¥È¦¤Ç¥­¥ã¥Ã¥·¥å¤òÆɤळ¤È¤ËËþ­¤·¤Æ
¤¤¤ì¤Ð¡¢¥µ¡¼¥Ð¡¼Â¦¤Ç¤Î read ¤ÏȯÀ¸¤·¤Ê¤¤¤Î¤Ç st_atime ¤Î¹¹¿·¤Ï¹Ô¤Ê¤ï¤ì¤«¤é¤À¡£
Unix ¤ÎÊý¼°¤Ç¤Ï¡¢¥¯¥é¥¤¥¢¥ó¥È¦¤Î°À­¤Î¥­¥ã¥Ã¥·¥ó¥°¤ò̵¸ú¤Ë¤¹¤ë¤³¤È¤Ç¡¢
¤³¤ì¤òÆÀ¤ë¤³¤È¤¬¤Ç¤­¤ë¡£¤·¤«¤·¤Û¤È¤ó¤É¤Î¾õ¶·¤Ç¤Ï¤³¤ì¤Ï³¤¯¥µ¡¼¥Ð¡¼¤Î
Éé²Ù¤òÁý²Ã¤µ¤»¡¢¥Ñ¥Õ¥©¡¼¥Þ¥ó¥¹¤ÎÄã²¼¤ò¤â¤¿¤é¤¹¡£
.PP
.\"O Many file systems and disks were considered to be fast enough that the
.\"O implementation of
.\"O .B O_NONBLOCK
.\"O was deemed unnecessary.
.\"O So,
.\"O .B O_NONBLOCK
.\"O may not be available on files
.\"O and/or disks.
¿¤¯¤Î¥Õ¥¡¥¤¥ë¥·¥¹¥Æ¥à¤ä¥Ç¥£¥¹¥¯¤Ï
.B O_NONBLOCK
¤Î¼ÂÁõ¤Ï¤·¤Ê¤¯¤Æ¤âºÑ¤à¤¯¤é¤¤¤Ë½½Ê¬¤Ë¹â®¤Ç¤¢¤ë¤È¹Í¤¨¤é¤ì¤Æ¤¤¤ë¡£
¤½¤ì¤Ç¤½¤Î¤è¤¦¤Ê¥Õ¥¡¥¤¥ë¤ä¥Ç¥£¥¹¥¯¤Ë¤Ï
.B O_NONBLOCK
¤ÏÍøÍѤǤ­¤Ê¤¤¤«¤â¤·¤ì¤Ê¤¤¡£
.\"O .SH "SEE ALSO"
.SH ´ØÏ¢¹àÌÜ
.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)