1FSEEK(3P) POSIX Programmer's Manual FSEEK(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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13 fseek, fseeko — reposition a file-position indicator in a stream
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16 #include <stdio.h>
17 int fseek(FILE *stream, long offset, int whence);
19 int fseeko(FILE *stream, off_t offset, int whence);
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22 The functionality described on this reference page is aligned with the
23 ISO C standard. Any conflict between the requirements described here
24 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
25 defers to the ISO C standard.
26 The fseek() function shall set the file-position indicator for the
28 stream pointed to by stream. If a read or write error occurs, the
29 error indicator for the stream shall be set and fseek() fails.
30 The new position, measured in bytes from the beginning of the file,
32 shall be obtained by adding offset to the position specified by whence.
33 The specified point is the beginning of the file for SEEK_SET, the cur‐
34 rent value of the file-position indicator for SEEK_CUR, or end-of-file
35 for SEEK_END.
36 If the stream is to be used with wide-character input/output functions,
38 the application shall ensure that offset is either 0 or a value
39 returned by an earlier call to ftell() on the same stream and whence is
40 SEEK_SET.
41 A successful call to fseek() shall clear the end-of-file indicator for
43 the stream and undo any effects of ungetc() and ungetwc() on the same
44 stream. After an fseek() call, the next operation on an update stream
45 may be either input or output.
46 If the most recent operation, other than ftell(), on a given stream is
48 fflush(), the file offset in the underlying open file description shall
49 be adjusted to reflect the location specified by fseek().
50 The fseek() function shall allow the file-position indicator to be set
52 beyond the end of existing data in the file. If data is later written
53 at this point, subsequent reads of data in the gap shall return bytes
54 with the value 0 until data is actually written into the gap.
55 The behavior of fseek() on devices which are incapable of seeking is
57 implementation-defined. The value of the file offset associated with
58 such a device is undefined.
59 If the stream is writable and buffered data had not been written to the
61 underlying file, fseek() shall cause the unwritten data to be written
62 to the file and shall mark the last data modification and last file
63 status change timestamps of the file for update.
64 In a locale with state-dependent encoding, whether fseek() restores the
66 stream's shift state is implementation-defined.
67 The fseeko() function shall be equivalent to the fseek() function
69 except that the offset argument is of type off_t.
70
72 The fseek() and fseeko() functions shall return 0 if they succeed.
73 Otherwise, they shall return −1 and set errno to indicate the error.
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77 The fseek() and fseeko() functions shall fail if, either the stream is
78 unbuffered or the stream's buffer needed to be flushed, and the call to
79 fseek() or fseeko() causes an underlying lseek() or write() to be
80 invoked, and:
81 EAGAIN The O_NONBLOCK flag is set for the file descriptor and the
83 thread would be delayed in the write operation.
84 EBADF The file descriptor underlying the stream file is not open for
86 writing or the stream's buffer needed to be flushed and the file
87 is not open.
88 EFBIG An attempt was made to write a file that exceeds the maximum
90 file size.
91 EFBIG An attempt was made to write a file that exceeds the file size
93 limit of the process.
94 EFBIG The file is a regular file and an attempt was made to write at
96 or beyond the offset maximum associated with the corresponding
97 stream.
98 EINTR The write operation was terminated due to the receipt of a sig‐
100 nal, and no data was transferred.
101 EINVAL The whence argument is invalid. The resulting file-position
103 indicator would be set to a negative value.
104 EIO A physical I/O error has occurred, or the process is a member of
106 a background process group attempting to perform a write() to
107 its controlling terminal, TOSTOP is set, the calling thread is
108 not blocking SIGTTOU, the process is not ignoring SIGTTOU, and
109 the process group of the process is orphaned. This error may
110 also be returned under implementation-defined conditions.
111 ENOSPC There was no free space remaining on the device containing the
113 file.
114 EOVERFLOW
116 For fseek(), the resulting file offset would be a value which
117 cannot be represented correctly in an object of type long.
118 EOVERFLOW
120 For fseeko(), the resulting file offset would be a value which
121 cannot be represented correctly in an object of type off_t.
122 EPIPE An attempt was made to write to a pipe or FIFO that is not open
124 for reading by any process; a SIGPIPE signal shall also be sent
125 to the thread.
126 ESPIPE The file descriptor underlying stream is associated with a pipe,
128 FIFO, or socket.
129 The fseek() and fseeko() functions may fail if:
131 ENXIO A request was made of a nonexistent device, or the request was
133 outside the capabilities of the device.
134 The following sections are informative.
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138 None.
139
141 None.
142
144 None.
145
147 None.
148
150 Section 2.5, Standard I/O Streams, fopen(), fsetpos(), ftell(), getr‐
151 limit(), lseek(), rewind(), ulimit(), ungetc(), write()
152 The Base Definitions volume of POSIX.1‐2008, <stdio.h>
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156 Portions of this text are reprinted and reproduced in electronic form
157 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
158 -- Portable Operating System Interface (POSIX), The Open Group Base
159 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
160 cal and Electronics Engineers, Inc and The Open Group. (This is
161 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
162 event of any discrepancy between this version and the original IEEE and
163 The Open Group Standard, the original IEEE and The Open Group Standard
164 is the referee document. The original Standard can be obtained online
165 at http://www.unix.org/online.html .
166 Any typographical or formatting errors that appear in this page are
168 most likely to have been introduced during the conversion of the source
169 files to man page format. To report such errors, see https://www.ker‐
170 nel.org/doc/man-pages/reporting_bugs.html .
171IEEE/The Open Group 2013 FSEEK(3P)