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.
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 current value of the file-position indicator for
SEEK_CUR, or end-of-file for SEEK_END.
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 SEEK_SET.
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
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 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 invoked, and:
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
signal, 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.
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 Electrical 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
IEEE/The Open Group 2013 FSEEK(3P)