man2/lseek.2

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  34 .\"     @(#)lseek.2     6.5 (Berkeley) 3/10/91
  35 .\"
  36 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
  37 .\" Modified 1995-06-10 by Andries Brouwer <aeb@cwi.nl>
  38 .\" Modified 1996-10-31 by Eric S. Raymond <esr@thyrsus.com>
  39 .\" Modified 1998-01-17 by Michael Haardt
  40 .\"   <michael@cantor.informatik.rwth-aachen.de>
  41 .\" Modified 2001-09-24 by Michael Haardt <michael@moria.de>
  42 .\" Modified 2003-08-21 by Andries Brouwer <aeb@cwi.nl>
  43 .\" 2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE
  44 .\"
  45 .TH LSEEK 2 2011-09-20 "Linux" "Linux Programmer's Manual"
  46 .SH NAME
  47 lseek \- reposition read/write file offset
  48 .SH SYNOPSIS
  49 .B #include <sys/types.h>
  50 .br
  51 .B #include <unistd.h>
  52 .sp
  53 .BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
  54 .SH DESCRIPTION
  55 The
  56 .BR lseek ()
  57 function repositions the offset of the open file associated with the
  58 file descriptor
  59 .I fd
  60 to the argument
  61 .I offset
  62 according to the directive
  63 .I whence
  64 as follows:
  65 .TP
  66 .B SEEK_SET
  67 The offset is set to
  68 .I offset
  69 bytes.
  70 .TP
  71 .B SEEK_CUR
  72 The offset is set to its current location plus
  73 .I offset
  74 bytes.
  75 .TP
  76 .B SEEK_END
  77 The offset is set to the size of the file plus
  78 .I offset
  79 bytes.
  80 .PP
  81 The
  82 .BR lseek ()
  83 function 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
 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
 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 file system 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 file system 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 .SH "RETURN VALUE"
 152 Upon successful completion,
 153 .BR lseek ()
 154 returns the resulting offset location as measured in bytes from the
 155 beginning of the file.
 156 On error, the value \fI(off_t)\ \-1\fP is returned and
 157 .I errno
 158 is set to indicate the error.
 159 .SH ERRORS
 160 .TP
 161 .B EBADF
 162 .I fd
 163 is not an open file descriptor.
 164 .TP
 165 .B EINVAL
 166 .I whence
 167 is not valid.
 168 Or: the resulting file offset would be negative,
 169 or beyond the end of a seekable device.
 170 .\" Some systems may allow negative offsets for character devices
 171 .\" and/or for remote file systems.
 172 .TP
 173 .B EOVERFLOW
 174 .\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
 175 The resulting file offset cannot be represented in an
 176 .IR off_t .
 177 .TP
 178 .B ESPIPE
 179 .I fd
 180 is associated with a pipe, socket, or FIFO.
 181 .TP
 182 .B ENXIO
 183 .I whence
 184 is
 185 .B SEEK_DATA
 186 or
 187 .BR SEEK_HOLE ,
 188 and the current file offset is beyond the end of the file.
 189 .SH "CONFORMING TO"
 190 SVr4, 4.3BSD, POSIX.1-2001.
 191
 192 .BR SEEK_DATA
 193 and
 194 .BR SEEK_HOLE
 195 are nonstandard extensions also present in Solaris;
 196 they are proposed for inclusion in the next POSIX revision (Issue 8).
 197 .\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
 198 .SH NOTES
 199 Some devices are incapable of seeking and POSIX does not specify which
 200 devices must support
 201 .BR lseek ().
 202
 203 On Linux, using
 204 .BR lseek ()
 205 on a tty device returns
 206 \fBESPIPE\fP.
 207 .\" Other systems return the number of written characters,
 208 .\" using SEEK_SET to set the counter. (Of written characters.)
 209
 210 When converting old code, substitute values for \fIwhence\fP with the
 211 following macros:
 212 .TS
 213 c c
 214 l l.
 215 old     new
 216 0       SEEK_SET
 217 1       SEEK_CUR
 218 2       SEEK_END
 219 L_SET   SEEK_SET
 220 L_INCR  SEEK_CUR
 221 L_XTND  SEEK_END
 222 .TE
 223 .\" .PP
 224 .\" SVr1-3 returns \fIlong\fP instead of \fIoff_t\fP,
 225 .\" (ancient) BSD returns \fIint\fP.
 226 .PP
 227 Note that file descriptors created by
 228 .BR dup (2)
 229 or
 230 .BR fork (2)
 231 share the current file position pointer, so seeking on such files may be
 232 subject to race conditions.
 233 .SH "SEE ALSO"
 234 .BR dup (2),
 235 .BR fork (2),
 236 .BR open (2),
 237 .BR fseek (3),
 238 .BR lseek64 (3),
 239 .BR posix_fallocate (3)