1.\" $NetBSD: lseek.2,v 1.16 2002/02/08 01:28:18 ross 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 39.Sh NAME 40.Nm lseek , 41.Nm seek 42.Nd reposition read/write file offset 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.Fd #include \*[Lt]unistd.h\*[Gt] 47.Ft off_t 48.Fn lseek "int fildes" "off_t offset" "int whence" 49.Sh DESCRIPTION 50The 51.Fn lseek 52function repositions the offset of the file descriptor 53.Fa fildes 54to the 55argument 56.Fa offset 57according to the directive 58.Fa whence . 59The argument 60.Fa fildes 61must be an open 62file descriptor. 63.Fn lseek 64repositions the file pointer 65.Fa fildes 66as follows: 67.Bl -item -offset indent 68.It 69If 70.Fa whence 71is 72.Dv SEEK_SET , 73the offset is set to 74.Fa offset 75bytes. 76.It 77If 78.Fa whence 79is 80.Dv SEEK_CUR , 81the offset is set to its current location plus 82.Fa offset 83bytes. 84.It 85If 86.Fa whence 87is 88.Dv SEEK_END , 89the offset is set to the size of the 90file plus 91.Fa offset 92bytes. 93.El 94.Pp 95The 96.Fn lseek 97function allows the file offset to be set beyond the end 98of the existing end-of-file of the file. If data is later written 99at this point, subsequent reads of the data in the gap return 100bytes of zeros (until data is actually written into the gap). 101.Pp 102Some devices are incapable of seeking. The value of the pointer 103associated with such a device is undefined. 104.Sh RETURN VALUES 105Upon successful completion, 106.Fn lseek 107returns the resulting offset location as measured in bytes from the 108beginning of the file. 109Otherwise, 110a value of -1 is returned and 111.Va errno 112is set to indicate 113the error. 114.Sh ERRORS 115.Fn lseek 116will fail and the file pointer will remain unchanged if: 117.Bl -tag -width Er 118.It Bq Er EBADF 119.Em Fildes 120is not an open file descriptor. 121.It Bq Er ESPIPE 122.Em Fildes 123is associated with a pipe, socket, or FIFO. 124.It Bq Er EINVAL 125.Fa whence 126is not a proper value, or the resulting file offset would be invalid. 127.El 128.Sh SEE ALSO 129.Xr dup 2 , 130.Xr open 2 131.Sh STANDARDS 132The 133.Fn lseek 134function conforms to 135.St -p1003.1-90 . 136.Sh BUGS 137This document's use of 138.Fa whence 139is incorrect English, but is maintained for historical reasons. 140