PROLOG

       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.1‐2017 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.

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.

SEE ALSO

       Section 2.5, Standard I/O Streams, fopen(), fsetpos(), ftell(), getrlimit(), lseek(), rewind(), ulimit(),
       ungetc(), write()

       The Base Definitions volume of POSIX.1‐2017, <stdio.h>

COPYRIGHT

       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(3POSIX)