4.3BSD/usr/man/man2/lseek.2

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)lseek.2	6.3 (Berkeley) 2/24/86
.\"
.TH LSEEK 2 "February 24, 1986"
.UC 4
.SH NAME
lseek \- move read/write pointer
.SH SYNOPSIS
.nf
.ft B
#include <sys/file.h>
.PP
.nf
.ft B
.ta 1.25i 1.6i 1.8i
#define L_SET	0	/* set the seek pointer */
#define L_INCR	1	/* increment the seek pointer */
#define L_XTND	2	/* extend the file size */
.PP
.ft B
pos = lseek(d, offset, whence)
off_t pos;
int d;
off_t offset;
int whence;
.fi
.ft R
.SH DESCRIPTION
The descriptor 
.I d
refers to a file or device open for reading and/or writing.
.I Lseek
sets the file pointer of
.I d
as follows:
.IP
If
.I whence
is L_SET, the pointer is set to
.I offset
bytes.
.IP
If
.I whence
is L_INCR, the pointer is set to its current location plus
.IR offset .
.IP
If
.I whence
is L_XTND, the pointer is set to the size of the
file plus
.IR offset .
.PP
Upon successful completion, the resulting pointer location
as measured in bytes from beginning of the file is returned.
Some devices are incapable of seeking.  The value of the pointer
associated with such a device is undefined.
.SH NOTES
Seeking far beyond the end of a file, then writing,
creates a gap or \*(lqhole\*(rq, which occupies no
physical space and reads as zeros.
.SH "RETURN VALUE
Upon successful completion,
the current file pointer value is returned.
Otherwise,
a value of \-1 is returned and \fIerrno\fP is set to indicate
the error.
.SH "ERRORS
.I Lseek
will fail and the file pointer will remain unchanged if:
.TP 15
[EBADF]
.I Fildes
is not an open file descriptor.
.TP 15
[ESPIPE]
.I Fildes
is associated with a pipe or a socket.
.TP 15
[EINVAL]
.I Whence
is not a proper value.
.SH "SEE ALSO"
dup(2), open(2)
.SH BUGS
This document's use of
.I whence
is incorrect English, but maintained for historical reasons.