1.\" $NetBSD: lseek.2,v 1.6 1995/02/27 12:34:09 cgd Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)lseek.2 8.3 (Berkeley) 4/19/94 35.\" 36.Dd April 19, 1994 37.Dt LSEEK 2 38.Os BSD 4 39.Sh NAME 40.Nm lseek 41.Nd reposition read/write file offset 42.Sh SYNOPSIS 43.Fd #include <unistd.h> 44.Ft off_t 45.Fo lseek 46.Fa "int fildes" 47.Fa "off_t offset" 48.Fa "int whence" 49.Fc 50.Sh DESCRIPTION 51The 52.Fn lseek 53function repositions the offset of the file descriptor 54.Fa fildes 55to the 56argument 57.Fa offset , 58according to the directive 59.Fa whence . 60The argument 61.Fa fildes 62must be an open 63file descriptor. 64.Fn lseek 65repositions the file pointer 66.Fa fildes 67as follows: 68.Bl -item -offset indent 69.It 70If 71.Fa whence 72is 73.Dv SEEK_SET , 74the offset is set to 75.Fa offset 76bytes. 77.It 78If 79.Fa whence 80is 81.Dv SEEK_CUR , 82the offset is set to its current location plus 83.Fa offset 84bytes. 85.It 86If 87.Fa whence 88is 89.Dv SEEK_END , 90the offset is set to the size of the 91file plus 92.Fa offset 93bytes. 94.It 95If 96.Fa whence 97is 98.Dv SEEK_HOLE , 99the offset is set to the start of the next hole greater than or equal 100to the supplied 101.Fa offset . 102The definition of a hole is provided below. 103.It 104If 105.Fa whence 106is 107.Dv SEEK_DATA , 108the offset is set to the start of the next non-hole file region greater 109than or equal to the supplied 110.Fa offset . 111.El 112.Pp 113The 114.Fn lseek 115function allows the file offset to be set beyond the end 116of the existing end-of-file of the file. If data is later written 117at this point, subsequent reads of the data in the gap return 118bytes of zeros (until data is actually written into the gap). 119.Pp 120Some devices are incapable of seeking. The value of the pointer 121associated with such a device is undefined. 122.Pp 123A 124.Qq hole 125is defined as a contiguous range of bytes in a file, all having the value of 126zero, but not all zeros in a file are guaranteed to be represented as holes 127returned with 128.Dv SEEK_HOLE . 129File systems are allowed to expose ranges of zeros with 130.Dv SEEK_HOLE , 131but not required to. 132Applications can use 133.Dv SEEK_HOLE 134to optimise their behavior for ranges of zeros, but must not depend on it to 135find all such ranges in a file. 136Each file is presented as having a zero-size virtual hole at the very 137end of the file. 138The existence of a hole at the end of every data region allows for easy 139programming and also provides compatibility to the original implementation 140in Solaris. 141It also causes the current file size (i.e., end-of-file offset) to be returned 142to indicate that there are no more holes past the supplied 143.Fa offset . 144Applications should use 145.Fn fpathconf _PC_MIN_HOLE_SIZE 146or 147.Fn pathconf _PC_MIN_HOLE_SIZE 148to determine if a file system supports 149.Dv SEEK_HOLE . 150See 151.Xr pathconf 2 . 152.Pp 153For file systems that do not supply information about holes, the file will be 154represented as one entire data region. 155.Sh RETURN VALUES 156Upon successful completion, 157.Fn lseek 158returns the resulting offset location as measured in bytes from the 159beginning of the file. 160Otherwise, 161a value of -1 is returned and 162.Va errno 163is set to indicate 164the error. 165.Sh ERRORS 166.Fn lseek 167will fail and the file pointer will remain unchanged if: 168.Bl -tag -width Er 169.\" ========== 170.It Bq Er EBADF 171.Em Fildes 172is not an open file descriptor. 173.\" ========== 174.It Bq Er EINVAL 175.Fa Whence 176is not a proper value. 177.\" ========== 178.It Bq Er EINVAL 179The seek location (calculated from 180.Fa offset 181and 182.Fa whence ) 183is negative. 184.\" ========== 185.It Bq Er ENXIO 186For 187.Dv SEEK_DATA , 188there are no more data regions past the supplied offset. 189Due to existence of the hole at the end of the file, for 190.Dv SEEK_HOLE 191this error is only returned when the 192.Fa offset 193already points to the end-of-file position. 194.It Bq Er EOVERFLOW 195The seek location is too large to be stored 196in an object of type off_t. 197.\" ========== 198.It Bq Er ESPIPE 199.Em Fildes 200is associated with a pipe, socket, or FIFO. 201.El 202.Sh SEE ALSO 203.Xr dup 2 , 204.Xr open 2 205.Sh BUGS 206This document's use of 207.Fa whence 208is incorrect English, but is maintained for historical reasons. 209.Sh STANDARDS 210The 211.Fn lseek 212function conforms to 213.St -p1003.1-88 . 214