1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
|
'\" t
.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\" @(#)fseek.3 6.11 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
.\"
.TH fseek 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
fgetpos, fseek, fsetpos, ftell, rewind \- reposition a stream
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "int fseek(FILE *" stream ", long " offset ", int " whence );
.BI "long ftell(FILE *" stream );
.PP
.BI "void rewind(FILE *" stream );
.PP
.BI "int fgetpos(FILE *restrict " stream ", fpos_t *restrict " pos );
.BI "int fsetpos(FILE *" stream ", const fpos_t *" pos );
.fi
.SH DESCRIPTION
The
.BR fseek ()
function sets the file position indicator for the stream pointed to by
.IR stream .
The new position, measured in bytes, is obtained by adding
.I offset
bytes to the position specified by
.IR whence .
If
.I whence
is set to
.BR SEEK_SET ,
.BR SEEK_CUR ,
or
.BR SEEK_END ,
the offset is relative to the start of the file, the current position
indicator, or end-of-file, respectively.
A successful call to the
.BR fseek ()
function clears the end-of-file indicator for the stream and undoes
any effects of the
.BR ungetc (3)
function on the same stream.
.PP
The
.BR ftell ()
function obtains the current value of the file position indicator for the
stream pointed to by
.IR stream .
.PP
The
.BR rewind ()
function sets the file position indicator for the stream pointed to by
.I stream
to the beginning of the file.
It is equivalent to:
.PP
.RS
(void) fseek(stream, 0L, SEEK_SET)
.RE
.PP
except that the error indicator for the stream is also cleared (see
.BR clearerr (3)).
.PP
The
.BR fgetpos ()
and
.BR fsetpos ()
functions are alternate interfaces equivalent to
.BR ftell ()
and
.BR fseek ()
(with
.I whence
set to
.BR SEEK_SET ),
setting and storing the current value of the file offset into or from the
object referenced by
.IR pos .
On some non-UNIX systems, an
.I fpos_t
object may be a complex object and these routines may be the only way to
portably reposition a text stream.
.PP
If the stream refers to a regular file
and the resulting stream offset is beyond the size of the file,
subsequent writes will extend the file with a hole, up to the offset,
before committing any data.
See
.BR lseek (2)
for details on file seeking semantics.
.SH RETURN VALUE
The
.BR rewind ()
function returns no value.
Upon successful completion,
.BR fgetpos (),
.BR fseek (),
.BR fsetpos ()
return 0,
and
.BR ftell ()
returns the current offset.
Otherwise, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
The
.I whence
argument to
.BR fseek ()
was not
.BR SEEK_SET ,
.BR SEEK_END ,
or
.BR SEEK_CUR .
Or: the resulting file offset would be negative.
.TP
.B ESPIPE
The file descriptor underlying
.I stream
is not seekable (e.g., it refers to a pipe, FIFO, or socket).
.PP
The functions
.BR fgetpos (),
.BR fseek (),
.BR fsetpos (),
and
.BR ftell ()
may also fail and set
.I errno
for any of the errors specified for the routines
.BR fflush (3),
.BR fstat (2),
.BR lseek (2),
and
.BR malloc (3).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR fseek (),
.BR ftell (),
.BR rewind (),
.BR fgetpos (),
.BR fsetpos ()
T} Thread safety MT-Safe
.TE
.sp 1
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89.
.SH SEE ALSO
.BR lseek (2),
.BR fseeko (3)
|
