man2/lseek.2

   1 .\" Copyright (c) 1980, 1991 Regents of the University of California.
   2 .\" and Copyright (c) 2011, Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\" All rights reserved.
   4 .\"
   5 .\" SPDX-License-Identifier: BSD-4-Clause-UC
   6 .\"
   7 .\"     @(#)lseek.2     6.5 (Berkeley) 3/10/91
   8 .\"
   9 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
  10 .\" Modified 1995-06-10 by Andries Brouwer <aeb@cwi.nl>
  11 .\" Modified 1996-10-31 by Eric S. Raymond <esr@thyrsus.com>
  12 .\" Modified 1998-01-17 by Michael Haardt
  13 .\"   <michael@cantor.informatik.rwth-aachen.de>
  14 .\" Modified 2001-09-24 by Michael Haardt <michael@moria.de>
  15 .\" Modified 2003-08-21 by Andries Brouwer <aeb@cwi.nl>
  16 .\" 2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE
  17 .\"
  18 .TH LSEEK 2 2021-03-22 "Linux" "Linux Programmer's Manual"
  19 .SH NAME
  20 lseek \- reposition read/write file offset
  21 .SH LIBRARY
  22 Standard C library
  23 .RI ( libc ", " \-lc )
  24 .SH SYNOPSIS
  25 .nf
  26 .B #include <unistd.h>
  27 .PP
  28 .BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
  29 .fi
  30 .SH DESCRIPTION
  31 .BR lseek ()
  32 repositions the file offset of the open file description
  33 associated with the file descriptor
  34 .I fd
  35 to the argument
  36 .I offset
  37 according to the directive
  38 .I whence
  39 as follows:
  40 .TP
  41 .B SEEK_SET
  42 The file offset is set to
  43 .I offset
  44 bytes.
  45 .TP
  46 .B SEEK_CUR
  47 The file offset is set to its current location plus
  48 .I offset
  49 bytes.
  50 .TP
  51 .B SEEK_END
  52 The file offset is set to the size of the file plus
  53 .I offset
  54 bytes.
  55 .PP
  56 .BR lseek ()
  57 allows the file offset to be set beyond the end
  58 of the file (but this does not change the size of the file).
  59 If data is later written at this point, subsequent reads of the data
  60 in the gap (a "hole") return null bytes (\(aq\e0\(aq) until
  61 data is actually written into the gap.
  62 .SS Seeking file data and holes
  63 Since version 3.1, Linux supports the following additional values for
  64 .IR whence :
  65 .TP
  66 .B SEEK_DATA
  67 Adjust the file offset to the next location
  68 in the file greater than or equal to
  69 .I offset
  70 containing data.
  71 If
  72 .I offset
  73 points to data,
  74 then the file offset is set to
  75 .IR offset .
  76 .TP
  77 .B SEEK_HOLE
  78 Adjust the file offset to the next hole in the file
  79 greater than or equal to
  80 .IR offset .
  81 If
  82 .I offset
  83 points into the middle of a hole,
  84 then the file offset is set to
  85 .IR offset .
  86 If there is no hole past
  87 .IR offset ,
  88 then the file offset is adjusted to the end of the file
  89 (i.e., there is an implicit hole at the end of any file).
  90 .PP
  91 In both of the above cases,
  92 .BR lseek ()
  93 fails if
  94 .I offset
  95 points past the end of the file.
  96 .PP
  97 These operations allow applications to map holes in a sparsely
  98 allocated file.
  99 This can be useful for applications such as file backup tools,
 100 which can save space when creating backups and preserve holes,
 101 if they have a mechanism for discovering holes.
 102 .PP
 103 For the purposes of these operations, a hole is a sequence of zeros that
 104 (normally) has not been allocated in the underlying file storage.
 105 However, a filesystem is not obliged to report holes,
 106 so these operations are not a guaranteed mechanism for
 107 mapping the storage space actually allocated to a file.
 108 (Furthermore, a sequence of zeros that actually has been written
 109 to the underlying storage may not be reported as a hole.)
 110 In the simplest implementation,
 111 a filesystem can support the operations by making
 112 .B SEEK_HOLE
 113 always return the offset of the end of the file,
 114 and making
 115 .B SEEK_DATA
 116 always return
 117 .I offset
 118 (i.e., even if the location referred to by
 119 .I offset
 120 is a hole,
 121 it can be considered to consist of data that is a sequence of zeros).
 122 .\" https://lkml.org/lkml/2011/4/22/79
 123 .\" http://lwn.net/Articles/440255/
 124 .\" http://blogs.oracle.com/bonwick/entry/seek_hole_and_seek_data
 125 .PP
 126 The
 127 .B _GNU_SOURCE
 128 feature test macro must be defined in order to obtain the definitions of
 129 .B SEEK_DATA
 130 and
 131 .B SEEK_HOLE
 132 from
 133 .IR <unistd.h> .
 134 .PP
 135 The
 136 .B SEEK_HOLE
 137 and
 138 .B SEEK_DATA
 139 operations are supported for the following filesystems:
 140 .IP * 3
 141 Btrfs (since Linux 3.1)
 142 .IP * 3
 143 OCFS (since Linux 3.2)
 144 .\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1
 145 .IP *
 146 XFS (since Linux 3.5)
 147 .IP *
 148 ext4 (since Linux 3.8)
 149 .IP *
 150 .BR tmpfs (5)
 151 (since Linux 3.8)
 152 .IP *
 153 NFS (since Linux 3.18)
 154 .\" commit 1c6dcbe5ceff81c2cf8d929646af675cd59fe7c0
 155 .\" commit 24bab491220faa446d945624086d838af41d616c
 156 .IP *
 157 FUSE (since Linux 4.5)
 158 .\" commit 0b5da8db145bfd44266ac964a2636a0cf8d7c286
 159 .IP *
 160 GFS2 (since Linux 4.15)
 161 .\" commit 3a27411cb4bc3ce31db228e3569ad01b462a4310
 162 .SH RETURN VALUE
 163 Upon successful completion,
 164 .BR lseek ()
 165 returns the resulting offset location as measured in bytes from the
 166 beginning of the file.
 167 On error, the value \fI(off_t)\ \-1\fP is returned and
 168 .I errno
 169 is set to indicate the error.
 170 .SH ERRORS
 171 .TP
 172 .B EBADF
 173 .I fd
 174 is not an open file descriptor.
 175 .TP
 176 .B EINVAL
 177 .I whence
 178 is not valid.
 179 Or: the resulting file offset would be negative,
 180 or beyond the end of a seekable device.
 181 .\" Some systems may allow negative offsets for character devices
 182 .\" and/or for remote filesystems.
 183 .TP
 184 .B ENXIO
 185 .I whence
 186 is
 187 .B SEEK_DATA
 188 or
 189 .BR SEEK_HOLE ,
 190 and
 191 .I offset
 192 is beyond the end of the file, or
 193 .I whence
 194 is
 195 .B SEEK_DATA
 196 and
 197 .I offset
 198 is within a hole at the end of the file.
 199 .TP
 200 .B EOVERFLOW
 201 .\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
 202 The resulting file offset cannot be represented in an
 203 .IR off_t .
 204 .TP
 205 .B ESPIPE
 206 .I fd
 207 is associated with a pipe, socket, or FIFO.
 208 .SH CONFORMING TO
 209 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 210 .PP
 211 .B SEEK_DATA
 212 and
 213 .B SEEK_HOLE
 214 are nonstandard extensions also present in Solaris,
 215 FreeBSD, and DragonFly BSD;
 216 they are proposed for inclusion in the next POSIX revision (Issue 8).
 217 .\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
 218 .SH NOTES
 219 See
 220 .BR open (2)
 221 for a discussion of the relationship between file descriptors,
 222 open file descriptions, and files.
 223 .PP
 224 If the
 225 .B O_APPEND
 226 file status flag is set on the open file description,
 227 then a
 228 .BR write (2)
 229 .I always
 230 moves the file offset to the end of the file, regardless of the use of
 231 .BR lseek ().
 232 .PP
 233 The
 234 .I off_t
 235 data type is a signed integer data type specified by POSIX.1.
 236 .PP
 237 Some devices are incapable of seeking and POSIX does not specify which
 238 devices must support
 239 .BR lseek ().
 240 .PP
 241 On Linux, using
 242 .BR lseek ()
 243 on a terminal device fails with the error
 244 \fBESPIPE\fP.
 245 .\" Other systems return the number of written characters,
 246 .\" using SEEK_SET to set the counter. (Of written characters.)
 247 .SH SEE ALSO
 248 .BR dup (2),
 249 .BR fallocate (2),
 250 .BR fork (2),
 251 .BR open (2),
 252 .BR fseek (3),
 253 .BR lseek64 (3),
 254 .BR posix_fallocate (3)