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       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.

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 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 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.

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 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.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .