FSEEK(3)                   POSIX Programmer's Manual                  FSEEK(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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(3)
