.\" $NetBSD: lseek.2,v 1.6 1995/02/27 12:34:09 cgd Exp $ .\" .\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)lseek.2 8.3 (Berkeley) 4/19/94 .\" .Dd April 19, 1994 .Dt LSEEK 2 .Os BSD 4 .Sh NAME .Nm lseek .Nd reposition read/write file offset .Sh SYNOPSIS .Fd #include <unistd.h> .Ft off_t .Fo lseek .Fa "int fildes" .Fa "off_t offset" .Fa "int whence" .Fc .Sh DESCRIPTION The .Fn lseek function repositions the offset of the file descriptor .Fa fildes to the argument .Fa offset , according to the directive .Fa whence . The argument .Fa fildes must be an open file descriptor. .Fn lseek repositions the file pointer .Fa fildes as follows: .Bl -item -offset indent .It If .Fa whence is .Dv SEEK_SET , the offset is set to .Fa offset bytes. .It If .Fa whence is .Dv SEEK_CUR , the offset is set to its current location plus .Fa offset bytes. .It If .Fa whence is .Dv SEEK_END , the offset is set to the size of the file plus .Fa offset bytes. .It If .Fa whence is .Dv SEEK_HOLE , the offset is set to the start of the next hole greater than or equal to the supplied .Fa offset . The definition of a hole is provided below. .It If .Fa whence is .Dv SEEK_DATA , the offset is set to the start of the next non-hole file region greater than or equal to the supplied .Fa offset . .El .Pp The .Fn lseek function allows the file offset to be set beyond the end of the existing end-of-file of the file. If data is later written at this point, subsequent reads of the data in the gap return bytes of zeros (until data is actually written into the gap). .Pp Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined. .Pp A .Qq hole is defined as a contiguous range of bytes in a file, all having the value of zero, but not all zeros in a file are guaranteed to be represented as holes returned with .Dv SEEK_HOLE . File systems are allowed to expose ranges of zeros with .Dv SEEK_HOLE , but not required to. Applications can use .Dv SEEK_HOLE to optimise their behavior for ranges of zeros, but must not depend on it to find all such ranges in a file. Each file is presented as having a zero-size virtual hole at the very end of the file. The existence of a hole at the end of every data region allows for easy programming and also provides compatibility to the original implementation in Solaris. It also causes the current file size (i.e., end-of-file offset) to be returned to indicate that there are no more holes past the supplied .Fa offset . Applications should use .Fn fpathconf _PC_MIN_HOLE_SIZE or .Fn pathconf _PC_MIN_HOLE_SIZE to determine if a file system supports .Dv SEEK_HOLE . See .Xr pathconf 2 . .Pp For file systems that do not supply information about holes, the file will be represented as one entire data region. .Sh RETURN VALUES Upon successful completion, .Fn lseek returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a value of -1 is returned and .Va errno is set to indicate the error. .Sh ERRORS .Fn lseek will fail and the file pointer will remain unchanged if: .Bl -tag -width Er .\" ========== .It Bq Er EBADF .Em Fildes is not an open file descriptor. .\" ========== .It Bq Er EINVAL .Fa Whence is not a proper value. .\" ========== .It Bq Er EINVAL The seek location (calculated from .Fa offset and .Fa whence ) is negative. .\" ========== .It Bq Er ENXIO For .Dv SEEK_DATA , there are no more data regions past the supplied offset. Due to existence of the hole at the end of the file, for .Dv SEEK_HOLE this error is only returned when the .Fa offset already points to the end-of-file position. .It Bq Er EOVERFLOW The seek location is too large to be stored in an object of type off_t. .\" ========== .It Bq Er ESPIPE .Em Fildes is associated with a pipe, socket, or FIFO. .El .Sh SEE ALSO .Xr dup 2 , .Xr open 2 .Sh BUGS This document's use of .Fa whence is incorrect English, but is maintained for historical reasons. .Sh STANDARDS The .Fn lseek function conforms to .St -p1003.1-88 .