Section: POSIX Programmer's Manual (3P)
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.
--- move the read/write file offset
off_t lseek(int fildes, off_t offset, int whence);
function shall set the file offset for the open file description
associated with the file descriptor
is SEEK_SET, the file offset shall be set to
is SEEK_CUR, the file offset shall be set to its current location plus
is SEEK_END, the file offset shall be set to the size of the file plus
The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END
are defined in
The behavior of
on devices which are incapable of seeking is implementation-defined.
The value of the file offset associated with such a device is
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.
function shall not, by itself, extend the size of a file.
refers to a shared memory object, the result of the
function is unspecified.
refers to a typed memory object, the result of the
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,
shall be set to indicate the error, and the file offset shall remain
function shall fail if:
argument is not an open file descriptor.
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
argument is associated with a pipe, FIFO, or socket.
The following sections are informative.
The ISO C standard includes the functions
which work on very large files by use of a special positioning type.
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
several functions originally derived from the ISO C standard, such as
and so on, may do so (by causing calls on
An invalid file offset that would cause
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
from returning a negative offset. Therefore, an application was
required to clear
prior to the call and check
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
be set to
when the resulting file offset would be negative for a regular file,
block special file, or directory.
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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
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