400 lines
9.7 KiB
Groff
400 lines
9.7 KiB
Groff
.\" Copyright (c) 2003, David G. Lawrence
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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 unmodified, this list of conditions, and the following
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.\" 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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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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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.\" $FreeBSD$
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.\"
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.Dd November 17, 2016
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.Dt SENDFILE 2
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.Os
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.Sh NAME
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.Nm sendfile
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.Nd send a file to a socket
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/socket.h
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.In sys/uio.h
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.Ft int
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.Fo sendfile
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.Fa "int fd" "int s" "off_t offset" "size_t nbytes"
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.Fa "struct sf_hdtr *hdtr" "off_t *sbytes" "int flags"
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.Fc
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.Sh DESCRIPTION
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The
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.Fn sendfile
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system call
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sends a regular file or shared memory object specified by descriptor
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.Fa fd
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out a stream socket specified by descriptor
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.Fa s .
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.Pp
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The
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.Fa offset
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argument specifies where to begin in the file.
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Should
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.Fa offset
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fall beyond the end of file, the system will return
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success and report 0 bytes sent as described below.
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The
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.Fa nbytes
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argument specifies how many bytes of the file should be sent, with 0 having the special
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meaning of send until the end of file has been reached.
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.Pp
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An optional header and/or trailer can be sent before and after the file data by specifying
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a pointer to a
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.Vt "struct sf_hdtr" ,
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which has the following structure:
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.Pp
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.Bd -literal -offset indent -compact
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struct sf_hdtr {
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struct iovec *headers; /* pointer to header iovecs */
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int hdr_cnt; /* number of header iovecs */
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struct iovec *trailers; /* pointer to trailer iovecs */
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int trl_cnt; /* number of trailer iovecs */
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};
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.Ed
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.Pp
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The
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.Fa headers
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and
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.Fa trailers
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pointers, if
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.Pf non- Dv NULL ,
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point to arrays of
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.Vt "struct iovec"
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structures.
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See the
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.Fn writev
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system call for information on the iovec structure.
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The number of iovecs in these
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arrays is specified by
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.Fa hdr_cnt
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and
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.Fa trl_cnt .
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.Pp
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If
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.Pf non- Dv NULL ,
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the system will write the total number of bytes sent on the socket to the
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variable pointed to by
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.Fa sbytes .
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.Pp
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The least significant 16 bits of
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.Fa flags
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argument is a bitmap of these values:
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.Bl -tag -offset indent
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.It Dv SF_NODISKIO
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This flag causes
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.Nm
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to return
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.Er EBUSY
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instead of blocking when a busy page is encountered.
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This rare situation can happen if some other process is now working
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with the same region of the file.
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It is advised to retry the operation after a short period.
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.Pp
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Note that in older
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.Fx
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versions the
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.Dv SF_NODISKIO
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had slightly different notion.
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The flag prevented
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.Nm
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to run I/O operations in case if an invalid (not cached) page is encountered,
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thus avoiding blocking on I/O.
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Starting with
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.Fx 11
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.Nm
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sending files off the
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.Xr ffs 7
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filesystem does not block on I/O
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(see
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.Sx IMPLEMENTATION NOTES
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), so the condition no longer applies.
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However, it is safe if an application utilizes
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.Dv SF_NODISKIO
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and on
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.Er EBUSY
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performs the same action as it did in
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older
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.Fx
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versions, e.g.,
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.Xr aio_read 2 ,
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.Xr read 2
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or
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.Nm
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in a different context.
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.It Dv SF_NOCACHE
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The data sent to socket will not be cached by the virtual memory system,
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and will be freed directly to the pool of free pages.
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.It Dv SF_SYNC
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.Nm
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sleeps until the network stack no longer references the VM pages
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of the file, making subsequent modifications to it safe.
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Please note that this is not a guarantee that the data has actually
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been sent.
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.It Dv SF_USER_READAHEAD
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.Nm
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has some internal heuristics to do readahead when sending data.
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This flag forces
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.Nm
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to override any heuristically calculated readahead and use exactly the
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application specified readahead.
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See
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.Sx SETTING READAHEAD
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for more details on readahead.
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.El
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.Pp
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When using a socket marked for non-blocking I/O,
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.Fn sendfile
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may send fewer bytes than requested.
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In this case, the number of bytes successfully
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written is returned in
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.Fa *sbytes
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(if specified),
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and the error
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.Er EAGAIN
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is returned.
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.Sh SETTING READAHEAD
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.Nm
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uses internal heuristics based on request size and file system layout
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to do readahead.
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Additionally application may request extra readahead.
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The most significant 16 bits of
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.Fa flags
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specify amount of pages that
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.Nm
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may read ahead when reading the file.
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A macro
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.Fn SF_FLAGS
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is provided to combine readahead amount and flags.
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An example showing specifying readahead of 16 pages and
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.Dv SF_NOCACHE
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flag:
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.Pp
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.Bd -literal -offset indent -compact
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SF_FLAGS(16, SF_NOCACHE)
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.Ed
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.Pp
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.Nm
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will use either application specified readahead or internally calculated,
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whichever is bigger.
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Setting flag
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.Dv SF_USER_READAHEAD
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would turn off any heuristics and set maximum possible readahead length to
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the number of pages specified via flags.
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.Sh IMPLEMENTATION NOTES
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The
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.Fx
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implementation of
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.Fn sendfile
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does not block on disk I/O when it sends a file off the
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.Xr ffs 7
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filesystem.
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The syscall returns success before the actual I/O completes, and data
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is put into the socket later unattended.
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However, the order of data in the socket is preserved, so it is safe
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to do further writes to the socket.
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.Pp
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The
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.Fx
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implementation of
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.Fn sendfile
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is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided.
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.Sh TUNING
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On some architectures, this system call internally uses a special
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.Fn sendfile
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buffer
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.Pq Vt "struct sf_buf"
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to handle sending file data to the client.
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If the sending socket is
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blocking, and there are not enough
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.Fn sendfile
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buffers available,
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.Fn sendfile
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will block and report a state of
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.Dq Li sfbufa .
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If the sending socket is non-blocking and there are not enough
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.Fn sendfile
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buffers available, the call will block and wait for the
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necessary buffers to become available before finishing the call.
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.Pp
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The number of
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.Vt sf_buf Ns 's
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allocated should be proportional to the number of nmbclusters used to
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send data to a client via
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.Fn sendfile .
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Tune accordingly to avoid blocking!
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Busy installations that make extensive use of
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.Fn sendfile
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may want to increase these values to be inline with their
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.Va kern.ipc.nmbclusters
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(see
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.Xr tuning 7
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for details).
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.Pp
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The number of
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.Fn sendfile
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buffers available is determined at boot time by either the
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.Va kern.ipc.nsfbufs
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.Xr loader.conf 5
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variable or the
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.Dv NSFBUFS
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kernel configuration tunable.
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The number of
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.Fn sendfile
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buffers scales with
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.Va kern.maxusers .
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The
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.Va kern.ipc.nsfbufsused
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and
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.Va kern.ipc.nsfbufspeak
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read-only
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.Xr sysctl 8
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variables show current and peak
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.Fn sendfile
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buffers usage respectively.
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These values may also be viewed through
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.Nm netstat Fl m .
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.Pp
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If a value of zero is reported for
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.Va kern.ipc.nsfbufs ,
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your architecture does not need to use
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.Fn sendfile
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buffers because their task can be efficiently performed
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by the generic virtual memory structures.
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.Sh RETURN VALUES
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.Rv -std sendfile
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.Sh ERRORS
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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The socket is marked for non-blocking I/O and not all data was sent due to
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the socket buffer being filled.
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If specified, the number of bytes successfully sent will be returned in
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.Fa *sbytes .
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.It Bq Er EBADF
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The
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.Fa fd
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argument
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is not a valid file descriptor.
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.It Bq Er EBADF
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The
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.Fa s
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argument
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is not a valid socket descriptor.
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.It Bq Er EBUSY
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A busy page was encountered and
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.Dv SF_NODISKIO
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had been specified.
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Partial data may have been sent.
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.It Bq Er EFAULT
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An invalid address was specified for an argument.
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.It Bq Er EINTR
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A signal interrupted
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.Fn sendfile
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before it could be completed.
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If specified, the number
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of bytes successfully sent will be returned in
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.Fa *sbytes .
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.It Bq Er EINVAL
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The
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.Fa fd
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argument
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is not a regular file.
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.It Bq Er EINVAL
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The
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.Fa s
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argument
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is not a SOCK_STREAM type socket.
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.It Bq Er EINVAL
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The
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.Fa offset
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argument
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is negative.
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.It Bq Er EIO
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An error occurred while reading from
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.Fa fd .
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.It Bq Er ENOBUFS
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The system was unable to allocate an internal buffer.
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.It Bq Er ENOTCONN
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The
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.Fa s
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argument
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points to an unconnected socket.
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.It Bq Er ENOTSOCK
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The
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.Fa s
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argument
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is not a socket.
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.It Bq Er EOPNOTSUPP
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The file system for descriptor
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.Fa fd
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does not support
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.Fn sendfile .
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.It Bq Er EPIPE
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The socket peer has closed the connection.
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.El
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.Sh SEE ALSO
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.Xr netstat 1 ,
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.Xr open 2 ,
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.Xr send 2 ,
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.Xr socket 2 ,
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.Xr writev 2 ,
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.Xr tuning 7
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.Rs
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.%A K. Elmeleegy
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.%A A. Chanda
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.%A A. L. Cox
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.%A W. Zwaenepoel
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.%T A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management
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.%J The Proceedings of the 2005 USENIX Annual Technical Conference
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.%P pp 223-236
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.%D 2005
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.Re
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.Sh HISTORY
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The
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.Fn sendfile
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system call
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first appeared in
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.Fx 3.0 .
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This manual page first appeared in
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.Fx 3.1 .
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In
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.Fx 10
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support for sending shared memory descriptors had been introduced.
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In
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.Fx 11
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a non-blocking implementation had been introduced.
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.Sh AUTHORS
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The initial implementation of
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.Fn sendfile
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system call
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and this manual page were written by
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.An David G. Lawrence Aq Mt dg@dglawrence.com .
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The
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.Fx 11
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implementation was written by
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.An Gleb Smirnoff Aq Mt glebius@FreeBSD.org .
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