man2/read.2

   1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   2 .\"             and Copyright (C) 1993 Michael Haardt, Ian Jackson.
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  26 .\" Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faith@cs.unc.edu>
  27 .\" Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
  28 .\"   <michael@cantor.informatik.rwth-aachen.de>
  29 .\" Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aeb@cwi.nl>
  30 .\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
  31 .\" Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esr@thyrsus.com>
  32 .\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
  33 .\"   <michael@cantor.informatik.rwth-aachen.de>
  34 .\"
  35 .TH READ 2 2014-05-04 "Linux" "Linux Programmer's Manual"
  36 .SH NAME
  37 read \- read from a file descriptor
  38 .SH SYNOPSIS
  39 .nf
  40 .B #include <unistd.h>
  41 .sp
  42 .BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
  43 .fi
  44 .SH DESCRIPTION
  45 .BR read ()
  46 attempts to read up to
  47 .I count
  48 bytes from file descriptor
  49 .I fd
  50 into the buffer starting at
  51 .IR buf .
  52
  53 On files that support seeking,
  54 the read operation commences at the current file offset,
  55 and the file offset is incremented by the number of bytes read.
  56 If the current file offset is at or past the end of file,
  57 no bytes are read, and
  58 .BR read ()
  59 returns zero.
  60
  61 If
  62 .I count
  63 is zero,
  64 .BR read ()
  65 .I may
  66 detect the errors described below.
  67 In the absence of any errors,
  68 or if
  69 .BR read ()
  70 does not check for errors, a
  71 .BR read ()
  72 with a
  73 .I count
  74 of 0 returns zero and has no other effects.
  75
  76 If
  77 .I count
  78 is greater than
  79 .BR SSIZE_MAX ,
  80 the result is unspecified.
  81 .SH RETURN VALUE
  82 On success, the number of bytes read is returned (zero indicates end of
  83 file), and the file position is advanced by this number.
  84 It is not an error if this number is smaller than the number of bytes
  85 requested; this may happen for example because fewer bytes are actually
  86 available right now (maybe because we were close to end-of-file, or
  87 because we are reading from a pipe, or from a terminal), or because
  88 .BR read ()
  89 was interrupted by a signal.
  90 On error, \-1 is returned, and
  91 .I errno
  92 is set appropriately.
  93 In this case, it is left unspecified whether
  94 the file position (if any) changes.
  95 .SH ERRORS
  96 .TP
  97 .B EAGAIN
  98 The file descriptor
  99 .I fd
 100 refers to a file other than a socket and has been marked nonblocking
 101 .RB ( O_NONBLOCK ),
 102 and the read would block.
 103 See
 104 .BR open (2)
 105 for further details on the
 106 .BR O_NONBLOCK
 107 flag.
 108 .TP
 109 .BR EAGAIN " or " EWOULDBLOCK
 110 .\" Actually EAGAIN on Linux
 111 The file descriptor
 112 .I fd
 113 refers to a socket and has been marked nonblocking
 114 .RB ( O_NONBLOCK ),
 115 and the read would block.
 116 POSIX.1-2001 allows either error to be returned for this case,
 117 and does not require these constants to have the same value,
 118 so a portable application should check for both possibilities.
 119 .TP
 120 .B EBADF
 121 .I fd
 122 is not a valid file descriptor or is not open for reading.
 123 .TP
 124 .B EFAULT
 125 .I buf
 126 is outside your accessible address space.
 127 .TP
 128 .B EINTR
 129 The call was interrupted by a signal before any data was read; see
 130 .BR signal (7).
 131 .TP
 132 .B EINVAL
 133 .I fd
 134 is attached to an object which is unsuitable for reading;
 135 or the file was opened with the
 136 .B O_DIRECT
 137 flag, and either the address specified in
 138 .IR buf ,
 139 the value specified in
 140 .IR count ,
 141 or the current file offset is not suitably aligned.
 142 .TP
 143 .B EINVAL
 144 .I fd
 145 was created via a call to
 146 .BR timerfd_create (2)
 147 and the wrong size buffer was given to
 148 .BR read ();
 149 see
 150 .BR timerfd_create (2)
 151 for further information.
 152 .TP
 153 .B EIO
 154 I/O error.
 155 This will happen for example when the process is in a
 156 background process group, tries to read from its controlling terminal,
 157 and either it is ignoring or blocking
 158 .B SIGTTIN
 159 or its process group
 160 is orphaned.
 161 It may also occur when there is a low-level I/O error
 162 while reading from a disk or tape.
 163 .TP
 164 .B EISDIR
 165 .I fd
 166 refers to a directory.
 167 .PP
 168 Other errors may occur, depending on the object connected to
 169 .IR fd .
 170 POSIX allows a
 171 .BR read ()
 172 that is interrupted after reading some data
 173 to return \-1 (with
 174 .I errno
 175 set to
 176 .BR EINTR )
 177 or to return the number of bytes already read.
 178 .SH CONFORMING TO
 179 SVr4, 4.3BSD, POSIX.1-2001.
 180 .SH NOTES
 181 On NFS filesystems, reading small amounts of data will update the
 182 timestamp only the first time, subsequent calls may not do so.
 183 This is caused
 184 by client side attribute caching, because most if not all NFS clients
 185 leave
 186 .I st_atime
 187 (last file access time)
 188 updates to the server, and client side reads satisfied from the
 189 client's cache will not cause
 190 .I st_atime
 191 updates on the server as there are no
 192 server-side reads.
 193 UNIX semantics can be obtained by disabling client-side attribute caching,
 194 but in most situations this will substantially
 195 increase server load and decrease performance.
 196 .SH BUGS
 197 According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
 198 ("Thread Interactions with Regular File Operations"):
 199
 200 .RS 4
 201 All of the following functions shall be atomic with respect to
 202 each other in the effects specified in POSIX.1-2008 when they
 203 operate on regular files or symbolic links: ...
 204 .RE
 205
 206 Among the APIs subsequently listed are
 207 .BR read ()
 208 and
 209 .BR readv (2).
 210 And among the effects that should be atomic across threads (and processes)
 211 are updates of the file offset.
 212 However, on Linux before version 3.14,
 213 this was not the case: if two processes that share
 214 an open file description (see
 215 .BR open (2))
 216 perform a
 217 .BR read ()
 218 (or
 219 .BR readv (2))
 220 at the same time, then the I/O operations were not atomic
 221 with respect updating the file offset,
 222 with the result that the reads in the two processes
 223 might (incorrectly) overlap in the blocks of data that they obtained.
 224 This problem was fixed in Linux 3.14.
 225 .\" http://thread.gmane.org/gmane.linux.kernel/1649458
 226 .\"    From: Michael Kerrisk (man-pages <mtk.manpages <at> gmail.com>
 227 .\"    Subject: Update of file offset on write() etc. is non-atomic with I/O
 228 .\"    Date: 2014-02-17 15:41:37 GMT
 229 .\"    Newsgroups: gmane.linux.kernel, gmane.linux.file-systems
 230 .\" commit 9c225f2655e36a470c4f58dbbc99244c5fc7f2d4
 231 .\"    Author: Linus Torvalds <torvalds@linux-foundation.org>
 232 .\"    Date:   Mon Mar 3 09:36:58 2014 -0800
 233 .\"
 234 .\"        vfs: atomic f_pos accesses as per POSIX
 235 .SH SEE ALSO
 236 .BR close (2),
 237 .BR fcntl (2),
 238 .BR ioctl (2),
 239 .BR lseek (2),
 240 .BR open (2),
 241 .BR pread (2),
 242 .BR readdir (2),
 243 .BR readlink (2),
 244 .BR readv (2),
 245 .BR select (2),
 246 .BR write (2),
 247 .BR fread (3)