fseeko(3p) — Linux manual page


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

PROLOG         top

       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         top

       fseek, fseeko — reposition a file-position indicator in a stream

SYNOPSIS         top

       #include <stdio.h>

       int fseek(FILE *stream, long offset, int whence);
       int fseeko(FILE *stream, off_t offset, int whence);

DESCRIPTION         top

       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

       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         top

       The fseek() and fseeko() functions shall return 0 if they succeed.

       Otherwise, they shall return −1 and set errno to indicate the error.

ERRORS         top

       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

       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

              For fseek(), the resulting file offset would be a value which
              cannot be represented correctly in an object of type long.

              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         top




RATIONALE         top




SEE ALSO         top

       Section 2.5, Standard I/O Streams, fopen(3p), fsetpos(3p), ftell(3p),
       getrlimit(3p), lseek(3p), rewind(3p), ulimit(3p), ungetc(3p),

       The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                           FSEEK(3P)