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1.\" $NetBSD: lseek.2,v 1.6 1995/02/27 12:34:09 cgd Exp $
2.\"
3.\" Copyright (c) 1980, 1991, 1993
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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 .