4.2BSD/usr/man/man2/lseek.2

.TH LSEEK 2 "7 July 1983"
.UC 4
.SH NAME
lseek \- move read/write pointer
.SH SYNOPSIS
.nf
.ft B
.ta 1.25i 1.6i
#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)
int pos;
int d, offset, 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, a non-negative integer,
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.
.TP 15
[EINVAL]
The resulting file pointer would be negative.
.SH "SEE ALSO"
dup(2), open(2)
.SH BUGS
This document's use of
.I whence
is incorrect English, but maintained for historical reasons.