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
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.
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.
The ISO C standard includes the functions fgetpos() and
fsetpos(), which work on very large files by use of a special
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
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
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.
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