fseek (3p) - Linux Man Pages
fseek: reposition a file-position indicator in a stream
PROLOGThis 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.
fseek, fseeko - reposition a file-position indicator in a stream
int fseek(FILE *stream, long offset, int
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 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 fseek() and fseeko() functions shall return 0 if they succeed.
The fseek() and fseeko()
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