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2 .\" and Copyright (c) 2011, Michael Kerrisk <mtk.manpages@gmail.com>
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35 .\" @(#)lseek.2 6.5 (Berkeley) 3/10/91
37 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
38 .\" Modified 1995-06-10 by Andries Brouwer <aeb@cwi.nl>
39 .\" Modified 1996-10-31 by Eric S. Raymond <esr@thyrsus.com>
40 .\" Modified 1998-01-17 by Michael Haardt
41 .\" <michael@cantor.informatik.rwth-aachen.de>
42 .\" Modified 2001-09-24 by Michael Haardt <michael@moria.de>
43 .\" Modified 2003-08-21 by Andries Brouwer <aeb@cwi.nl>
44 .\" 2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE
46 .TH LSEEK 2 2021-03-22 "Linux" "Linux Programmer's Manual"
48 lseek \- reposition read/write file offset
51 .B #include <unistd.h>
53 .BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
57 repositions the file offset of the open file description
58 associated with the file descriptor
62 according to the directive
67 The file offset is set to
72 The file offset is set to its current location plus
77 The file offset is set to the size of the file plus
82 allows the file offset to be set beyond the end
83 of the file (but this does not change the size of the file).
84 If data is later written at this point, subsequent reads of the data
85 in the gap (a "hole") return null bytes (\(aq\e0\(aq) until
86 data is actually written into the gap.
87 .SS Seeking file data and holes
88 Since version 3.1, Linux supports the following additional values for
92 Adjust the file offset to the next location
93 in the file greater than or equal to
99 then the file offset is set to
103 Adjust the file offset to the next hole in the file
104 greater than or equal to
108 points into the middle of a hole,
109 then the file offset is set to
111 If there is no hole past
113 then the file offset is adjusted to the end of the file
114 (i.e., there is an implicit hole at the end of any file).
116 In both of the above cases,
120 points past the end of the file.
122 These operations allow applications to map holes in a sparsely
124 This can be useful for applications such as file backup tools,
125 which can save space when creating backups and preserve holes,
126 if they have a mechanism for discovering holes.
128 For the purposes of these operations, a hole is a sequence of zeros that
129 (normally) has not been allocated in the underlying file storage.
130 However, a filesystem is not obliged to report holes,
131 so these operations are not a guaranteed mechanism for
132 mapping the storage space actually allocated to a file.
133 (Furthermore, a sequence of zeros that actually has been written
134 to the underlying storage may not be reported as a hole.)
135 In the simplest implementation,
136 a filesystem can support the operations by making
138 always return the offset of the end of the file,
143 (i.e., even if the location referred to by
146 it can be considered to consist of data that is a sequence of zeros).
147 .\" https://lkml.org/lkml/2011/4/22/79
148 .\" http://lwn.net/Articles/440255/
149 .\" http://blogs.oracle.com/bonwick/entry/seek_hole_and_seek_data
153 feature test macro must be defined in order to obtain the definitions of
164 operations are supported for the following filesystems:
166 Btrfs (since Linux 3.1)
168 OCFS (since Linux 3.2)
169 .\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1
171 XFS (since Linux 3.5)
173 ext4 (since Linux 3.8)
178 NFS (since Linux 3.18)
179 .\" commit 1c6dcbe5ceff81c2cf8d929646af675cd59fe7c0
180 .\" commit 24bab491220faa446d945624086d838af41d616c
182 FUSE (since Linux 4.5)
183 .\" commit 0b5da8db145bfd44266ac964a2636a0cf8d7c286
185 GFS2 (since Linux 4.15)
186 .\" commit 3a27411cb4bc3ce31db228e3569ad01b462a4310
188 Upon successful completion,
190 returns the resulting offset location as measured in bytes from the
191 beginning of the file.
192 On error, the value \fI(off_t)\ \-1\fP is returned and
194 is set to indicate the error.
199 is not an open file descriptor.
204 Or: the resulting file offset would be negative,
205 or beyond the end of a seekable device.
206 .\" Some systems may allow negative offsets for character devices
207 .\" and/or for remote filesystems.
217 is beyond the end of the file, or
223 is within a hole at the end of the file.
226 .\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
227 The resulting file offset cannot be represented in an
232 is associated with a pipe, socket, or FIFO.
234 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
239 are nonstandard extensions also present in Solaris,
240 FreeBSD, and DragonFly BSD;
241 they are proposed for inclusion in the next POSIX revision (Issue 8).
242 .\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
246 for a discussion of the relationship between file descriptors,
247 open file descriptions, and files.
251 file status flag is set on the open file description,
255 moves the file offset to the end of the file, regardless of the use of
260 data type is a signed integer data type specified by POSIX.1.
262 Some devices are incapable of seeking and POSIX does not specify which
268 on a terminal device fails with the error
270 .\" Other systems return the number of written characters,
271 .\" using SEEK_SET to set the counter. (Of written characters.)
279 .BR posix_fallocate (3)