FSEEK(P) POSIX Programmer's Manual FSEEK(P)NAME
fseek, fseeko - reposition a file-position indicator in a stream
SYNOPSIS
#include <stdio.h>
int fseek(FILE *stream, long offset, int whence);
int fseeko(FILE *stream, off_t offset, int whence);
DESCRIPTION
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
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 st_ctime and st_mtime fields 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.
RETURN VALUE
The fseek() and fseeko() functions shall return 0 if they succeed.
Otherwise, they shall return -1 and set errno to indicate the error.
ERRORS
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
process 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 process'
file size limit.
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 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 process is neither
ignoring nor blocking 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.
ENXIO A request was made of a nonexistent device, or the request was
outside the capabilities of the device.
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
or FIFO.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSOfopen() , fsetpos() , ftell() , getrlimit() , lseek() , rewind() ,
ulimit() , ungetc() , write() , the Base Definitions volume of
IEEE Std 1003.1-2001, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 FSEEK(P)
