mirror of https://github.com/mkerrisk/man-pages
161 lines
5.0 KiB
Groff
161 lines
5.0 KiB
Groff
'\" t
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.\" Copyright (c) 1980, 1991 Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)lseek.2 6.5 (Berkeley) 3/10/91
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.\"
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.\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
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.\" Modified 1995-06-10 by Andries Brouwer <aeb@cwi.nl>
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.\" Modified 1996-10-31 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 1998-01-17 by Michael Haardt
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.\" <michael@cantor.informatik.rwth-aachen.de>
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.\" Modified 2001-09-24 by Michael Haardt <michael@moria.de>
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.\" Modified 2003-08-21 by Andries Brouwer <aeb@cwi.nl>
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.\"
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.TH LSEEK 2 2001-09-24 "Linux" "Linux Programmer's Manual"
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.SH NAME
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lseek \- reposition read/write file offset
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.SH SYNOPSIS
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.B #include <sys/types.h>
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.br
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.B #include <unistd.h>
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.sp
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.BI "off_t lseek(int " fildes ", off_t " offset ", int " whence );
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.SH DESCRIPTION
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The
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.BR lseek ()
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function repositions the offset of the open file associated with the
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file descriptor
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.I fildes
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to the argument
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.I offset
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according to the directive
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.I whence
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as follows:
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.TP
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.B SEEK_SET
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The offset is set to
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.I offset
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bytes.
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.TP
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.B SEEK_CUR
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The offset is set to its current location plus
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.I offset
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bytes.
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.TP
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.B SEEK_END
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The offset is set to the size of the file plus
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.I offset
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bytes.
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.PP
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The
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.BR lseek ()
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function allows the file offset to be set beyond the end
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of the file (but this does not change the size of the file).
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If data is later written at this point, subsequent reads of the data
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in the gap (a "hole") return null bytes ('\\0') until
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data is actually written into the gap.
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.SH "RETURN VALUE"
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Upon successful completion,
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.BR lseek ()
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returns the resulting offset location as measured in bytes from the
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beginning of the file.
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Otherwise, a value of \fI(off_t)\-1\fP is returned and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EBADF
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.I fildes
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is not an open file descriptor.
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.TP
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.B EINVAL
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.I whence
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is not one of SEEK_SET, SEEK_CUR, SEEK_END;
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or the resulting file offset would be negative,
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or beyond the end of a seekable device.
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.\" Some systems may allow negative offsets for character devices
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.\" and/or for remote filesystems.
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.TP
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.B EOVERFLOW
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.\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
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The resulting file offset cannot be represented in an
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.IR off_t .
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.TP
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.B ESPIPE
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.I fildes
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is associated with a pipe, socket, or FIFO.
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.SH "CONFORMING TO"
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SVr4, 4.3BSD, POSIX.1-2001.
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.SH RESTRICTIONS
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Some devices are incapable of seeking and POSIX does not specify which
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devices must support
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.BR lseek ().
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Linux specific restrictions: using \fBlseek\fP() on a tty device returns
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\fBESPIPE\fP.
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.\" Other systems return the number of written characters,
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.\" using SEEK_SET to set the counter. (Of written characters.)
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.SH NOTES
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This document's use of
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.I whence
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is incorrect English, but maintained for historical reasons.
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When converting old code, substitute values for \fIwhence\fP with the
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following macros:
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.TS
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c c
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l l.
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old new
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0 SEEK_SET
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1 SEEK_CUR
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2 SEEK_END
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L_SET SEEK_SET
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L_INCR SEEK_CUR
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L_XTND SEEK_END
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.TE
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.PP
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SVr1-3 returns \fIlong\fP instead of \fIoff_t\fP, BSD returns \fIint\fP.
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.PP
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Note that file descriptors created by
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.BR dup (2)
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or
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.BR fork (2)
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share the current file position pointer, so seeking on such files may be
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subject to race conditions.
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.SH "SEE ALSO"
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.BR dup (2),
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.BR fork (2),
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.BR open (2),
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.BR fseek (3),
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.BR lseek64 (3),
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.BR posix_fallocate (3)
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