NAME
lseek - move the read/write file offset
SYNOPSIS
#include <unistd.h>
off_t lseek(int fildes, off_t offset, int whence);
DESCRIPTION
The lseek() function shall set the file offset for the open file
description associated with the file descriptor fildes, as follows:
* If whence is SEEK_SET, the file offset shall be set to offset bytes.
* If whence is SEEK_CUR, the file offset shall be set to its current
location plus offset.
* If whence is SEEK_END, the file offset shall be set to the size of
the file plus offset.
The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in
<unistd.h>.
The behavior of lseek() on devices which are incapable of seeking is
implementation-defined. The value of the file offset associated with
such a device is undefined.
The lseek() function shall allow the file offset to be set beyond the
end of the 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 lseek() function shall not, by itself, extend the size of a file.
If fildes refers to a shared memory object, the result of the lseek()
function is unspecified.
If fildes refers to a typed memory object, the result of the lseek()
function is unspecified.
RETURN VALUE
Upon successful completion, the resulting offset, as measured in bytes
from the beginning of the file, shall be returned. Otherwise,
(off_t)-1 shall be returned, errno shall be set to indicate the error,
and the file offset shall remain unchanged.
ERRORS
The lseek() function shall fail if:
EBADF The fildes argument is not an open file descriptor.
EINVAL The whence argument is not a proper value, or the resulting file
offset would be negative for a regular file, block special file,
or directory.
EOVERFLOW
The resulting file offset would be a value which cannot be
represented correctly in an object of type off_t.
ESPIPE The fildes argument is associated with a pipe, FIFO, or socket.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
The ISO C standard includes the functions fgetpos() and fsetpos(),
which work on very large files by use of a special positioning type.
Although lseek() may position the file offset beyond the end of the
file, this function does not itself extend the size of the file. While
the only function in IEEE Std 1003.1-2001 that may directly extend the
size of the file is write(), truncate(), and ftruncate(), several
functions originally derived from the ISO C standard, such as fwrite(),
fprintf(), and so on, may do so (by causing calls on write()).
An invalid file offset that would cause [EINVAL] to be returned may be
both implementation-defined and device-dependent (for example, memory
may have few invalid values). A negative file offset may be valid for
some devices in some implementations.
The POSIX.1-1990 standard did not specifically prohibit lseek() from
returning a negative offset. Therefore, an application was required to
clear errno prior to the call and check errno upon return to determine
whether a return value of ( off_t)-1 is a negative offset or an
indication of an error condition. The standard developers did not wish
to require this action on the part of a conforming application, and
chose to require that errno be set to [EINVAL] when the resulting file
offset would be negative for a regular file, block special file, or
directory.
FUTURE DIRECTIONS
None.
SEE ALSO
open() , the Base Definitions volume of IEEE Std 1003.1-2001,
<sys/types.h>, <unistd.h>
COPYRIGHT
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 http://www.opengroup.org/unix/online.html .