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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FSEEK(3P) POSIX Programmer's Manual FSEEK(3P)
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
#include <stdio.h>
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.1‐2017 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 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 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 file size.
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 stream.
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 file.
EOVERFLOW
For fseek(), the resulting file offset would be a value
which cannot be represented correctly in an object of type
long.
EOVERFLOW
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.
None.
None.
None.
None.
Section 2.5, Standard I/O Streams, fopen(3p), fsetpos(3p),
ftell(3p), getrlimit(3p), lseek(3p), rewind(3p), ulimit(3p),
ungetc(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 FSEEK(3P)
Pages that refer to this page: stdio.h(0p), fmemopen(3p), ftell(3p), rewind(3p), stdin(3p), ungetc(3p), ungetwc(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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