lseek.2

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 2 代码 · 共 106 行

.\" SCCSID: @(#)lseek.2	2.1	3/10/87
.TH lseek 2
.SH Name
lseek, tell \- move read or write pointer
.SH Syntax
.nf
#include <sys/types.h>
#include <unistd.h>
.PP
pos = lseek(\fId, offset, whence\fP)
.br
off_t pos;
.br
int \fId, whence\fP;
.br
off_t \fIoffset\fP;
.PP
pos = tell(\fId\fP)
.br
off_t pos;
.br
int \fId\fP;
.fi
.SH Description
.NXR "lseek system call"
.NXR "tell system call"
.NXR "file pointer" "moving"
The system call
.PN lseek
moves the file pointer associated with a
file or device open for reading or writing.  
.PP
The descriptor 
.I d
refers to a file or device open for reading or writing.
The
.PN lseek
system call sets the file pointer of
.I d
as follows:
.IP \(bu 5
If
.I whence
is SEEK_SET, the pointer is set to
.I offset
bytes.
.IP \(bu 5
If
.I whence
is SEEK_CUR the pointer is set to its current location plus
.IR offset .
.IP \(bu 5
If
.I whence
is SEEK_END, the pointer is set to the size of the
file plus
.IR offset .
.PP
Seeking beyond the end of a file and then writing to the file 
creates a gap or hole that does not occupy physical space
and reads as zeros.
.PP
The 
.PN tell
system call returns the offset of the current byte relative to the
beginning of the file associated with the file descriptor.
.SH Environment
.SS System Five
If you compile a program in the System Five environment,
an invalid 
.I whence 
argument causes SIGSYS to be sent.  This complies with
the behavior described in the System V Interface 
Definition (SVID), Issue 1.
.SH Return Values
Upon successful completion, a long integer
(the current file pointer value) is returned.  This pointer
is measured in bytes from the beginning of the file, where
the first byte is byte 0.  (Note that some devices are incapable
of seeking.  The value of the pointer associated with such
a device is undefined.)
If a value of \-1 is returned, \fIerrno\fP is set to indicate
the error.
.SH Diagnostics
The
.PN lseek
system call fails and the file pointer remains unchanged under the
following conditions:
.TP 15
[EBADF] 
The
.I fildes 
is not an open file descriptor.  
.TP 15
[EINVAL]
The
.I whence
is not a proper value.
.TP 15
[ESPIPE]
The
.I fildes
is associated with a pipe or a socket.
.SH See Also
dup(2), open(2)