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.
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
* 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
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,
−1 shall be returned, errno shall be set to indicate the error, and
the file offset shall remain unchanged.
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.
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
The following sections are informative.
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 POSIX.1‐2008 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 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,
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
IEEE/The Open Group 2013 LSEEK(3P)