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.\" @(#)lseek.2 6.5 (Berkeley) 3/10/91
.\" Modified 1993-07-23 by Rik Faith
.\" Modified 1995-06-10 by Andries Brouwer
.\" Modified 1996-10-31 by Eric S. Raymond
.\" Modified 1998-01-17 by Michael Haardt
.\" Modified 2001-09-24 by Michael Haardt
.\" Modified 2003-08-21 by Andries Brouwer
.TH LSEEK 2 2001-09-24 "Linux" "Linux Programmer's Manual"
lseek \- reposition read/write file offset
.BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
.BR lseek ()
function repositions the offset of the open file associated with the
to the argument
according to the directive
The offset is set to
The offset is set to its current location plus
The offset is set to the size of the file plus
.BR lseek ()
function allows the file offset to be set beyond the end
of the file (but this does not change the size of the file).
If data is later written at this point, subsequent reads of the data
in the gap (a "hole") return null bytes ('\\0') until
data is actually written into the gap.
.SH "RETURN VALUE"
Upon successful completion,
.BR lseek ()
returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise, a value of \fI(off_t)\ \-1\fP is returned and
is set to indicate the error.
is not an open file descriptor.
is not one of
.BR SEEK_SET ,
.BR SEEK_CUR ,
.BR SEEK_END ;
or the resulting file offset would be negative,
or beyond the end of a seekable device.
.\" Some systems may allow negative offsets for character devices
.\" and/or for remote filesystems.
.\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
The resulting file offset cannot be represented in an
.IR off_t .
is associated with a pipe, socket, or FIFO.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
This document's use of
is incorrect English, but maintained for historical reasons.
Some devices are incapable of seeking and POSIX does not specify which
devices must support
.BR lseek ().
On Linux, using
.BR lseek ()
on a tty device returns
.\" Other systems return the number of written characters,
.\" using SEEK_SET to set the counter. (Of written characters.)
When converting old code, substitute values for \fIwhence\fP with the
SVr1-3 returns \fIlong\fP instead of \fIoff_t\fP, BSD returns \fIint\fP.
Note that file descriptors created by
.BR dup (2)
.BR fork (2)
share the current file position pointer, so seeking on such files may be
subject to race conditions.
.SH "SEE ALSO"
.BR dup (2),
.BR fork (2),
.BR open (2),
.BR fseek (3),
.BR lseek64 (3),
.BR posix_fallocate (3)
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