lseek.2

操作系统设计与实现源码

.\" 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/types.h>
#include <unistd.h>

.ta +1.8i +0.6i
#define SEEK_SET	0	/* offset is absolute */
#define SEEK_CUR	1	/* relative to current position */
#define SEEK_END	2	/* relative to end of file */

off_t lseek(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.
.B Lseek
sets the file pointer of
.I d
as follows:
.IP
If
.I whence
is SEEK_SET, the pointer is set to
.I offset
bytes.
.IP
If
.I whence
is SEEK_CUR, the pointer is set to its current location plus
.IR offset .
.IP
If
.I whence
is SEEK_END, 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 \fBerrno\fP is set to indicate
the error.
.SH "ERRORS
.B 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"
.BR fcntl (2),
.BR open (2).
.SH BUGS
This document's use of
.I whence
is incorrect English, but maintained for historical reasons.