man2/lseek.2

   1 '\" t
   2 .\" Copyright (c) 1980, 1991 Regents of the University of California.
   3 .\" and Copyright (c) 2011, Michael Kerrisk <mtk.manpages@gmail.com>
   4 .\" All rights reserved.
   5 .\"
   6 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
   7 .\" Redistribution and use in source and binary forms, with or without
   8 .\" modification, are permitted provided that the following conditions
   9 .\" are met:
  10 .\" 1. Redistributions of source code must retain the above copyright
  11 .\"    notice, this list of conditions and the following disclaimer.
  12 .\" 2. Redistributions in binary form must reproduce the above copyright
  13 .\"    notice, this list of conditions and the following disclaimer in the
  14 .\"    documentation and/or other materials provided with the distribution.
  15 .\" 3. All advertising materials mentioning features or use of this software
  16 .\"    must display the following acknowledgement:
  17 .\"     This product includes software developed by the University of
  18 .\"     California, Berkeley and its contributors.
  19 .\" 4. Neither the name of the University nor the names of its contributors
  20 .\"    may be used to endorse or promote products derived from this software
  21 .\"    without specific prior written permission.
  22 .\"
  23 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  24 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  25 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  26 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  27 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  28 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  29 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  30 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  31 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  32 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  33 .\" SUCH DAMAGE.
  34 .\" %%%LICENSE_END
  35 .\"
  36 .\"     @(#)lseek.2     6.5 (Berkeley) 3/10/91
  37 .\"
  38 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
  39 .\" Modified 1995-06-10 by Andries Brouwer <aeb@cwi.nl>
  40 .\" Modified 1996-10-31 by Eric S. Raymond <esr@thyrsus.com>
  41 .\" Modified 1998-01-17 by Michael Haardt
  42 .\"   <michael@cantor.informatik.rwth-aachen.de>
  43 .\" Modified 2001-09-24 by Michael Haardt <michael@moria.de>
  44 .\" Modified 2003-08-21 by Andries Brouwer <aeb@cwi.nl>
  45 .\" 2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE
  46 .\"
  47 .TH LSEEK 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  48 .SH NAME
  49 lseek \- reposition read/write file offset
  50 .SH SYNOPSIS
  51 .B #include <sys/types.h>
  52 .br
  53 .B #include <unistd.h>
  54 .PP
  55 .BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
  56 .SH DESCRIPTION
  57 .BR lseek ()
  58 repositions the file offset of the open file description
  59 associated with the file descriptor
  60 .I fd
  61 to the argument
  62 .I offset
  63 according to the directive
  64 .I whence
  65 as follows:
  66 .TP
  67 .B SEEK_SET
  68 The file offset is set to
  69 .I offset
  70 bytes.
  71 .TP
  72 .B SEEK_CUR
  73 The file offset is set to its current location plus
  74 .I offset
  75 bytes.
  76 .TP
  77 .B SEEK_END
  78 The file offset is set to the size of the file plus
  79 .I offset
  80 bytes.
  81 .PP
  82 .BR lseek ()
  83 allows the file offset to be set beyond the end
  84 of the file (but this does not change the size of the file).
  85 If data is later written at this point, subsequent reads of the data
  86 in the gap (a "hole") return null bytes (\(aq\\0\(aq) until
  87 data is actually written into the gap.
  88 .SS Seeking file data and holes
  89 Since version 3.1, Linux supports the following additional values for
  90 .IR whence :
  91 .TP
  92 .B SEEK_DATA
  93 Adjust the file offset to the next location
  94 in the file greater than or equal to
  95 .I offset
  96 containing data.
  97 If
  98 .I offset
  99 points to data,
 100 then the file offset is set to
 101 .IR offset .
 102 .TP
 103 .B SEEK_HOLE
 104 Adjust the file offset to the next hole in the file
 105 greater than or equal to
 106 .IR offset .
 107 If
 108 .I offset
 109 points into the middle of a hole,
 110 then the file offset is set to
 111 .IR offset .
 112 If there is no hole past
 113 .IR offset ,
 114 then the file offset is adjusted to the end of the file
 115 (i.e., there is an implicit hole at the end of any file).
 116 .PP
 117 In both of the above cases,
 118 .BR lseek ()
 119 fails if
 120 .I offset
 121 points past the end of the file.
 122 .PP
 123 These operations allow applications to map holes in a sparsely
 124 allocated file.
 125 This can be useful for applications such as file backup tools,
 126 which can save space when creating backups and preserve holes,
 127 if they have a mechanism for discovering holes.
 128 .PP
 129 For the purposes of these operations, a hole is a sequence of zeros that
 130 (normally) has not been allocated in the underlying file storage.
 131 However, a filesystem is not obliged to report holes,
 132 so these operations are not a guaranteed mechanism for
 133 mapping the storage space actually allocated to a file.
 134 (Furthermore, a sequence of zeros that actually has been written
 135 to the underlying storage may not be reported as a hole.)
 136 In the simplest implementation,
 137 a filesystem can support the operations by making
 138 .BR SEEK_HOLE
 139 always return the offset of the end of the file,
 140 and making
 141 .BR SEEK_DATA
 142 always return
 143 .IR offset
 144 (i.e., even if the location referred to by
 145 .I offset
 146 is a hole,
 147 it can be considered to consist of data that is a sequence of zeros).
 148 .\" https://lkml.org/lkml/2011/4/22/79
 149 .\" http://lwn.net/Articles/440255/
 150 .\" http://blogs.oracle.com/bonwick/entry/seek_hole_and_seek_data
 151 .PP
 152 The
 153 .BR _GNU_SOURCE
 154 feature test macro must be defined in order to obtain the definitions of
 155 .BR SEEK_DATA
 156 and
 157 .BR SEEK_HOLE
 158 from
 159 .IR <unistd.h> .
 160 .PP
 161 The
 162 .BR SEEK_HOLE
 163 and
 164 .BR SEEK_DATA
 165 operations are supported for the following filesystems:
 166 .IP * 3
 167 Btrfs (since Linux 3.1)
 168 .IP * 3
 169 OCFS (since Linux 3.2)
 170 .\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1
 171 .IP *
 172 XFS (since Linux 3.5)
 173 .IP *
 174 ext4 (since Linux 3.8)
 175 .IP *
 176 .BR tmpfs (5)
 177 (since Linux 3.8)
 178 .IP *
 179 NFS (since Linux 3.18)
 180 .\" commit 1c6dcbe5ceff81c2cf8d929646af675cd59fe7c0
 181 .\" commit 24bab491220faa446d945624086d838af41d616c
 182 .IP *
 183 FUSE (since Linux 4.5)
 184 .\" commit 0b5da8db145bfd44266ac964a2636a0cf8d7c286
 185 .SH RETURN VALUE
 186 Upon successful completion,
 187 .BR lseek ()
 188 returns the resulting offset location as measured in bytes from the
 189 beginning of the file.
 190 On error, the value \fI(off_t)\ \-1\fP is returned and
 191 .I errno
 192 is set to indicate the error.
 193 .SH ERRORS
 194 .TP
 195 .B EBADF
 196 .I fd
 197 is not an open file descriptor.
 198 .TP
 199 .B EINVAL
 200 .I whence
 201 is not valid.
 202 Or: the resulting file offset would be negative,
 203 or beyond the end of a seekable device.
 204 .\" Some systems may allow negative offsets for character devices
 205 .\" and/or for remote filesystems.
 206 .TP
 207 .B ENXIO
 208 .I whence
 209 is
 210 .B SEEK_DATA
 211 or
 212 .BR SEEK_HOLE ,
 213 and the file offset is beyond the end of the file.
 214 .TP
 215 .B EOVERFLOW
 216 .\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
 217 The resulting file offset cannot be represented in an
 218 .IR off_t .
 219 .TP
 220 .B ESPIPE
 221 .I fd
 222 is associated with a pipe, socket, or FIFO.
 223 .SH CONFORMING TO
 224 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 225 .PP
 226 .BR SEEK_DATA
 227 and
 228 .BR SEEK_HOLE
 229 are nonstandard extensions also present in Solaris,
 230 FreeBSD, and DragonFly BSD;
 231 they are proposed for inclusion in the next POSIX revision (Issue 8).
 232 .\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
 233 .SH NOTES
 234 See
 235 .BR open (2)
 236 for a discussion of the relationship between file descriptors,
 237 open file descriptions, and files.
 238 .PP
 239 If the
 240 .B O_APPEND
 241 file status flag is set on the open file description,
 242 then a
 243 .BR write (2)
 244 .I always
 245 moves the file offset to the end of the file, regardless of the use of
 246 .BR lseek ().
 247 .PP
 248 The
 249 .I off_t
 250 data type is a signed integer data type specified by POSIX.1.
 251 .PP
 252 Some devices are incapable of seeking and POSIX does not specify which
 253 devices must support
 254 .BR lseek ().
 255 .PP
 256 On Linux, using
 257 .BR lseek ()
 258 on a terminal device fails with the error
 259 \fBESPIPE\fP.
 260 .\" Other systems return the number of written characters,
 261 .\" using SEEK_SET to set the counter. (Of written characters.)
 262 .SH SEE ALSO
 263 .BR dup (2),
 264 .BR fallocate (2),
 265 .BR fork (2),
 266 .BR open (2),
 267 .BR fseek (3),
 268 .BR lseek64 (3),
 269 .BR posix_fallocate (3)