FSEEK(3P) POSIX Programmer's Manual FSEEK(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
fseek, fseeko — reposition a file-position indicator in a stream
int fseek(FILE *stream, long offset, int whence);
int fseeko(FILE *stream, off_t offset, int whence);
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of POSIX.1‐2008
defers to the ISO C standard.
The fseek() function shall set the file-position indicator for the
stream pointed to by stream. If a read or write error occurs, the
error indicator for the stream shall be set and fseek() fails.
The new position, measured in bytes from the beginning of the file,
shall be obtained by adding offset to the position specified by whence.
The specified point is the beginning of the file for SEEK_SET, the cur‐
rent value of the file-position indicator for SEEK_CUR, or end-of-file
If the stream is to be used with wide-character input/output functions,
the application shall ensure that offset is either 0 or a value
returned by an earlier call to ftell() on the same stream and whence is
A successful call to fseek() shall clear the end-of-file indicator for
the stream and undo any effects of ungetc() and ungetwc() on the same
stream. After an fseek() call, the next operation on an update stream
may be either input or output.
If the most recent operation, other than ftell(), on a given stream is
fflush(), the file offset in the underlying open file description shall
be adjusted to reflect the location specified by fseek().
The fseek() function shall allow the file-position indicator to be set
beyond the end of existing data in the file. If data is later written
at this point, subsequent reads of data in the gap shall return bytes
with the value 0 until data is actually written into the gap.
The behavior of fseek() on devices which are incapable of seeking is
implementation-defined. The value of the file offset associated with
such a device is undefined.
If the stream is writable and buffered data had not been written to the
underlying file, fseek() shall cause the unwritten data to be written
to the file and shall mark the last data modification and last file
status change timestamps of the file for update.
In a locale with state-dependent encoding, whether fseek() restores the
stream's shift state is implementation-defined.
The fseeko() function shall be equivalent to the fseek() function
except that the offset argument is of type off_t.
The fseek() and fseeko() functions shall return 0 if they succeed.
Otherwise, they shall return −1 and set errno to indicate the error.
The fseek() and fseeko() functions shall fail if, either the stream is
unbuffered or the stream's buffer needed to be flushed, and the call to
fseek() or fseeko() causes an underlying lseek() or write() to be
EAGAIN The O_NONBLOCK flag is set for the file descriptor and the
thread would be delayed in the write operation.
EBADF The file descriptor underlying the stream file is not open for
writing or the stream's buffer needed to be flushed and the file
is not open.
EFBIG An attempt was made to write a file that exceeds the maximum
EFBIG An attempt was made to write a file that exceeds the file size
limit of the process.
EFBIG The file is a regular file and an attempt was made to write at
or beyond the offset maximum associated with the corresponding
EINTR The write operation was terminated due to the receipt of a sig‐
nal, and no data was transferred.
EINVAL The whence argument is invalid. The resulting file-position
indicator would be set to a negative value.
EIO A physical I/O error has occurred, or the process is a member of
a background process group attempting to perform a write() to
its controlling terminal, TOSTOP is set, the calling thread is
not blocking SIGTTOU, the process is not ignoring SIGTTOU, and
the process group of the process is orphaned. This error may
also be returned under implementation-defined conditions.
ENOSPC There was no free space remaining on the device containing the
For fseek(), the resulting file offset would be a value which
cannot be represented correctly in an object of type long.
For fseeko(), the resulting file offset would be a value which
cannot be represented correctly in an object of type off_t.
EPIPE An attempt was made to write to a pipe or FIFO that is not open
for reading by any process; a SIGPIPE signal shall also be sent
to the thread.
ESPIPE The file descriptor underlying stream is associated with a pipe,
FIFO, or socket.
The fseek() and fseeko() functions may fail if:
ENXIO A request was made of a nonexistent device, or the request was
outside the capabilities of the device.
The following sections are informative.
Section 2.5, Standard I/O Streams, fopen(), fsetpos(), ftell(), getr‐
limit(), lseek(), rewind(), ulimit(), ungetc(), write()
The Base Definitions volume of POSIX.1‐2008, <stdio.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
IEEE/The Open Group 2013 FSEEK(3P)