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36 .\" @(#)lseek.2 6.5 (Berkeley) 3/10/91
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
47 .TH LSEEK 2 2014-01-22 "Linux" "Linux Programmer's Manual"
49 lseek \- reposition read/write file offset
51 .B #include <sys/types.h>
53 .B #include <unistd.h>
55 .BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
59 function repositions the offset of the open file associated with the
64 according to the directive
74 The offset is set to its current location plus
79 The offset is set to the size of the file plus
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
95 Adjust the file offset to the next location
96 in the file greater than or equal to
102 then the file offset is set to
106 Adjust the file offset to the next hole in the file
107 greater than or equal to
111 points into the middle of a hole,
112 then the file offset is set to
114 If there is no hole past
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).
119 In both of the above cases,
123 points past the end of the file.
125 These operations allow applications to map holes in a sparsely
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.
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
141 always return the offset of the end of the file,
146 (i.e., even if the location referred to by
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
156 feature test macro must be defined in order to obtain the definitions of
167 operations are supported for the following filesystems:
169 Btrfs (since Linux 3.1)
171 OCFS (since Linux 3.2)
172 .\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1
174 XFS (since Linux 3.5)
176 ext4 (since Linux 3.8)
178 tmpfs (since Linux 3.8)
180 Upon successful completion,
182 returns the resulting offset location as measured in bytes from the
183 beginning of the file.
184 On error, the value \fI(off_t)\ \-1\fP is returned and
186 is set to indicate the error.
191 is not an open file descriptor.
196 Or: the resulting file offset would be negative,
197 or beyond the end of a seekable device.
198 .\" Some systems may allow negative offsets for character devices
199 .\" and/or for remote filesystems.
202 .\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
203 The resulting file offset cannot be represented in an
208 is associated with a pipe, socket, or FIFO.
216 and the current file offset is beyond the end of the file.
218 SVr4, 4.3BSD, POSIX.1-2001.
223 are nonstandard extensions also present in Solaris,
224 FreeBSD, and DragonFly BSD;
225 they are proposed for inclusion in the next POSIX revision (Issue 8).
226 .\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
228 Some devices are incapable of seeking and POSIX does not specify which
234 on a terminal device returns
236 .\" Other systems return the number of written characters,
237 .\" using SEEK_SET to set the counter. (Of written characters.)
239 When converting old code, substitute values for \fIwhence\fP with the
253 .\" SVr1-3 returns \fIlong\fP instead of \fIoff_t\fP,
254 .\" (ancient) BSD returns \fIint\fP.
256 Note that file descriptors created by
260 share the current file position pointer, so seeking on such files may be
261 subject to race conditions.
268 .BR posix_fallocate (3)
270 This page is part of release 3.68 of the Linux
273 A description of the project,
274 information about reporting bugs,
275 and the latest version of this page,
277 \%http://www.kernel.org/doc/man\-pages/.