fseek(3)fseek(3)NAME
fseek, fseeko, fseek_unlocked, rewind, ftell, ftello, fgetpos, fsetpos
- Reposition the file pointer of a stream
SYNOPSIS
#include <stdio.h>
int fseek(
FILE *stream,
long int offset,
int whence ); int fseeko(
FILE *stream,
off_t offset,
int whence ); int fseek_unlocked(
FILE *stream,
long int offset,
int whence ); void rewind(
FILE *stream ); long int ftell(
FILE *stream ); off_t ftello(
FILE *stream ); int fsetpos(
FILE *stream,
const fpos_t *position ); int fgetpos(
FILE *stream,
fpos_t *position );
LIBRARY
Standard C Library (libc)
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
fseek(), fgetpos(), fsetpos(), ftell(), rewind(): XSH5.0, XNS5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Specifies the I/O stream. Determines the position of the next opera‐
tion. Determines the value for the file pointer associated with the
stream parameter. Specifies the value of the file position indicator.
DESCRIPTION
The fseek() function sets the position of the next input or output
operation on the I/O stream specified by the stream parameter. The
position of the next operation is determined by the offset parameter,
which can be either positive or negative.
The fseek() function sets the file pointer associated with the speci‐
fied stream as follows: If the whence parameter is SEEK_SET(0), the
pointer is set to the value of the offset parameter. If the whence
parameter is SEEK_CUR(1), the pointer is set to its current location
plus the value of the offset parameter. If the whence parameter is
SEEK_END(2), the pointer is set to the size of the file plus the value
of the offset parameter.
The fseek() function fails if attempted on a file that was not opened
with the fopen() function. In particular, the fseek() function cannot
be used on a terminal or on a file opened with the popen() function.
A successful call to the fseek() function clears the End-of-File indi‐
cator for the stream and undoes any effects of the ungetc() function on
the same stream. After a call to the fseek() function, the next opera‐
tion on an update stream may be either input or output.
If the stream is writable, and buffered data was not written to the
underlying file, the fseek() function causes the unwritten data to be
written to the file and marks the st_ctime and st_mtime fields of the
file for update.
If the most recent operation (ignoring any ftell() operations) on a
given stream was fflush(), then the fseek() function causes the file
offset in the underlying open file descriptor to be adjusted to reflect
the location specified by the fseek() function.
The fseek() function allows 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 will return bytes
with the value 0 (zero) until data is actually written into the gap.
The rewind() function is equivalent to: (void) fseek (stream, 0L,
SEEK_SET) Except that rewind() also clears the error indicator.
The ftell() function obtains the current value of the file position
indicator for the specified stream.
The fgetpos() and fsetpos() functions are similar to the ftell() and
fseek() functions, respectively. The fgetpos() function stores the cur‐
rent value of the file position indicator for the stream pointed to by
the stream parameter in the object pointed to by the position parame‐
ter. The fsetpos function sets the file position indicator according to
the value of the position parameter that was returned by a prior call
to the fgetpos() function.
A successful call to the fsetpos() function clears the EOF indicator
and undoes any effects of the ungetc() function.
[Tru64 UNIX] The fseek_unlocked() function is functionally identical
to the fseek() function, except that fseek_unlocked() may be safely
used only within a scope that is protected by the flockfile() and fun‐
lockfile() functions used as a pair. The caller must ensure that the
stream is locked before using these functions.
The fseeko() and ftello() functions behave identically to fseek() and
ftell().
RETURN VALUES
Upon successful completion, the fseek() and fseek_unlocked() functions
return a value of 0 (zero). If the fseek() or fseek_unlocked() function
fails, a value of -1 is returned and errno is set to indicate the
error.
The rewind() function does not return a value. You can detect errors by
first clearing errno and then calling the rewindfunction. If errno is
then nonzero, assume an error has occurred.
Upon successful completion, the ftell() function returns the offset of
the current byte relative to the beginning of the file associated with
the named stream. Otherwise, a value of -1 is returned and errno is set
to indicate the error.
Upon successful completion, the fgetpos() and fsetpos() functions
return a value of 0 (zero). If the fgetpos() or fsetpos() function
fails, a value of -1 is returned and errno is set to indicate the
error.
ERRORS
The fseek() or fseek_unlocked() function fails if either the stream is
unbuffered or the stream's buffer needed to be flushed and the call to
fseek() or fseek_unlocked() caused an underlying lseek() or write()
function to be invoked. In addition, if any of the following conditions
occurs, the fseek() or fseek_unlocked() function sets errno to the
value that corresponds to the condition. The O_NONBLOCK option is set
for the file descriptor underlying the stream parameter and the process
would be delayed in the write operation. The file descriptor underly‐
ing the stream parameter is not a valid file descriptor open for writ‐
ing. An attempt was made to write to a file that exceeds the process's
file size limit or the maximum file size. (See the ulimit(3) reference
page.)
The file is a regular file and an attempt was made to write at
or beyond the offset maximum associated with the corresponding
stream. The write operation was terminated by a signal, and
either none, some, or all the data was transferred. If buffered
I/O is being used, it is recommended that you call the fflush()
function before the fseek() function to guarantee that the buf‐
fer characters were written. The whence parameter is an invalid
value or the resulting file offset would be invalid. A physical
I/O error has occurred or the following set of conditions has
occurred: the process is a member of a background process group
attempting to write to its controlling terminal, the TOSTOP sig‐
nal is set, the process is neither ignoring nor blocking SIGT‐
TOU, and the process group of the process is orphaned. No free
space remained on the device containing the file. The file
descriptor underlying stream is associated with a pipe or FIFO.
An attempt was made to write to a pipe or FIFO that is not open
for reading by any process. A SIGPIPE signal will also be sent
to the process. A request was made of a nonexistent device or
the request was outside the capabilities of the device.
The rewind() function fails under the same conditions as the fseek()
function, with the exception of [EINVAL], which does not apply.
If the following conditions occur, the fgetpos(), fsetpos() or ftell()
function sets errno to the value that corresponds to the condition.
The file descriptor underlying the stream parameter is not a valid file
descriptor. [Tru64 UNIX] The stream parameter does not point to a
valid FILE structure or the position parameter is negative. An illegal
attempt was made to get or set the file position of a pipe or FIFO.
[XNS5.0] The file descriptor parameter underlying stream is
associated with a socket.
SEE ALSO
Functions: lseek(2), fopen(3)
Standards: standards(5)fseek(3)
