man2/lseek.2

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  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 2016-03-15 "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 .sp
  55 .BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
  56 .SH DESCRIPTION
  57 The
  58 .BR lseek ()
  59 function repositions the file offset of the open file description
  60 associated with the file descriptor
  61 .I fd
  62 to the argument
  63 .I offset
  64 according to the directive
  65 .I whence
  66 as follows:
  67 .TP
  68 .B SEEK_SET
  69 The file offset is set to
  70 .I offset
  71 bytes.
  72 .TP
  73 .B SEEK_CUR
  74 The file offset is set to its current location plus
  75 .I offset
  76 bytes.
  77 .TP
  78 .B SEEK_END
  79 The file offset is set to the size of the file plus
  80 .I offset
  81 bytes.
  82 .PP
  83 The
  84 .BR lseek ()
  85 function allows the file offset to be set beyond the end
  86 of the file (but this does not change the size of the file).
  87 If data is later written at this point, subsequent reads of the data
  88 in the gap (a "hole") return null bytes (\(aq\\0\(aq) until
  89 data is actually written into the gap.
  90 .SS Seeking file data and holes
  91 Since version 3.1, Linux supports the following additional values for
  92 .IR whence :
  93 .TP
  94 .B SEEK_DATA
  95 Adjust the file offset to the next location
  96 in the file greater than or equal to
  97 .I offset
  98 containing data.
  99 If
 100 .I offset
 101 points to data,
 102 then the file offset is set to
 103 .IR offset .
 104 .TP
 105 .B SEEK_HOLE
 106 Adjust the file offset to the next hole in the file
 107 greater than or equal to
 108 .IR offset .
 109 If
 110 .I offset
 111 points into the middle of a hole,
 112 then the file offset is set to
 113 .IR offset .
 114 If there is no hole past
 115 .IR offset ,
 116 then the file offset is adjusted to the end of the file
 117 (i.e., there is an implicit hole at the end of any file).
 118 .PP
 119 In both of the above cases,
 120 .BR lseek ()
 121 fails if
 122 .I offset
 123 points past the end of the file.
 124
 125 These operations allow applications to map holes in a sparsely
 126 allocated file.
 127 This can be useful for applications such as file backup tools,
 128 which can save space when creating backups and preserve holes,
 129 if they have a mechanism for discovering holes.
 130
 131 For the purposes of these operations, a hole is a sequence of zeros that
 132 (normally) has not been allocated in the underlying file storage.
 133 However, a filesystem is not obliged to report holes,
 134 so these operations are not a guaranteed mechanism for
 135 mapping the storage space actually allocated to a file.
 136 (Furthermore, a sequence of zeros that actually has been written
 137 to the underlying storage may not be reported as a hole.)
 138 In the simplest implementation,
 139 a filesystem can support the operations by making
 140 .BR SEEK_HOLE
 141 always return the offset of the end of the file,
 142 and making
 143 .BR SEEK_DATA
 144 always return
 145 .IR offset
 146 (i.e., even if the location referred to by
 147 .I offset
 148 is a hole,
 149 it can be considered to consist of data that is a sequence of zeros).
 150 .\" https://lkml.org/lkml/2011/4/22/79
 151 .\" http://lwn.net/Articles/440255/
 152 .\" http://blogs.oracle.com/bonwick/entry/seek_hole_and_seek_data
 153
 154 The
 155 .BR _GNU_SOURCE
 156 feature test macro must be defined in order to obtain the definitions of
 157 .BR SEEK_DATA
 158 and
 159 .BR SEEK_HOLE
 160 from
 161 .IR <unistd.h> .
 162
 163 The
 164 .BR SEEK_HOLE
 165 and
 166 .BR SEEK_DATA
 167 operations are supported for the following filesystems:
 168 .IP * 3
 169 Btrfs (since Linux 3.1)
 170 .IP * 3
 171 OCFS (since Linux 3.2)
 172 .\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1
 173 .IP *
 174 XFS (since Linux 3.5)
 175 .IP *
 176 ext4 (since Linux 3.8)
 177 .IP *
 178 tmpfs (since Linux 3.8)
 179 .IP *
 180 NFS (since Linux 3.18)
 181 .\" commit 1c6dcbe5ceff81c2cf8d929646af675cd59fe7c0
 182 .\" commit 24bab491220faa446d945624086d838af41d616c
 183 .IP *
 184 FUSE (since Linux 4.5)
 185 .\" commit 0b5da8db145bfd44266ac964a2636a0cf8d7c286
 186 .SH RETURN VALUE
 187 Upon successful completion,
 188 .BR lseek ()
 189 returns the resulting offset location as measured in bytes from the
 190 beginning of the file.
 191 On error, the value \fI(off_t)\ \-1\fP is returned and
 192 .I errno
 193 is set to indicate the error.
 194 .SH ERRORS
 195 .TP
 196 .B EBADF
 197 .I fd
 198 is not an open file descriptor.
 199 .TP
 200 .B EINVAL
 201 .I whence
 202 is not valid.
 203 Or: the resulting file offset would be negative,
 204 or beyond the end of a seekable device.
 205 .\" Some systems may allow negative offsets for character devices
 206 .\" and/or for remote filesystems.
 207 .TP
 208 .B EOVERFLOW
 209 .\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
 210 The resulting file offset cannot be represented in an
 211 .IR off_t .
 212 .TP
 213 .B ESPIPE
 214 .I fd
 215 is associated with a pipe, socket, or FIFO.
 216 .TP
 217 .B ENXIO
 218 .I whence
 219 is
 220 .B SEEK_DATA
 221 or
 222 .BR SEEK_HOLE ,
 223 and the file offset is beyond the end of the file.
 224 .SH CONFORMING TO
 225 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 226
 227 .BR SEEK_DATA
 228 and
 229 .BR SEEK_HOLE
 230 are nonstandard extensions also present in Solaris,
 231 FreeBSD, and DragonFly BSD;
 232 they are proposed for inclusion in the next POSIX revision (Issue 8).
 233 .\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
 234 .SH NOTES
 235 See
 236 .BR open (2)
 237 for a discussion of the relationship between file descriptors,
 238 open file descriptions, and files.
 239
 240 Some devices are incapable of seeking and POSIX does not specify which
 241 devices must support
 242 .BR lseek ().
 243
 244 On Linux, using
 245 .BR lseek ()
 246 on a terminal device returns
 247 \fBESPIPE\fP.
 248 .\" Other systems return the number of written characters,
 249 .\" using SEEK_SET to set the counter. (Of written characters.)
 250
 251 When converting old code, substitute values for \fIwhence\fP with the
 252 following macros:
 253 .TS
 254 c c
 255 l l.
 256 old     new
 257 0       SEEK_SET
 258 1       SEEK_CUR
 259 2       SEEK_END
 260 L_SET   SEEK_SET
 261 L_INCR  SEEK_CUR
 262 L_XTND  SEEK_END
 263 .TE
 264 .\" .PP
 265 .\" SVr1-3 returns \fIlong\fP instead of \fIoff_t\fP,
 266 .\" (ancient) BSD returns \fIint\fP.
 267 .PP
 268 Note that file descriptors created by
 269 .BR dup (2)
 270 or
 271 .BR fork (2)
 272 refer to the same open file descriptions (and thus file offsets),
 273 so seeking on such files may be subject to race conditions.
 274 .SH SEE ALSO
 275 .BR dup (2),
 276 .BR fork (2),
 277 .BR open (2),
 278 .BR fseek (3),
 279 .BR lseek64 (3),
 280 .BR posix_fallocate (3)