FSEEK(P) POSIX Programmer's Manual FSEEK(P)
fseek, fseeko - reposition a file-position indicator in a stream
int fseek(FILE *stream, long offset, int whence);
int fseeko(FILE *stream, off_t offset, int whence);
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 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 fseeko() function shall be equivalent to the fseek() function except that the offset
argument is of type off_t.
The fseek() and fseeko() functions shall return 0 if they succeed.
Otherwise, they shall return -1 and set errno to indicate the error.
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 process 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 process' file size limit.
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
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 process is neither ignoring nor blocking SIGTTOU, and the process group
of the process is orphaned. This error may also be returned under implementation-
ENOSPC There was no free space remaining on the device containing the file.
ENXIO A request was made of a nonexistent device, or the request was outside the capabil-
ities of the device.
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 repre-
sented 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 or FIFO.
The following sections are informative.
fopen() , fsetpos() , ftell() , getrlimit() , lseek() , rewind() , ulimit() , ungetc() ,
write() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 FSEEK(P)