b75a1171d8
referencing the files VM pages are returned from the network stack, making changes to the file safe. This flag does not guarantee that the data has been transmitted to the other end.
309 lines
7.3 KiB
Groff
309 lines
7.3 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 24, 2006
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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 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
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.Fa flags
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argument is a bitmap of these values:
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.Bl -item -offset indent
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.It
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.Dv SF_NODISKIO .
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This flag causes any
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.Fn sendfile
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call which would block on disk I/O to instead
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return
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.Er EBUSY .
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Busy servers may benefit by transferring requests that would
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block to a separate I/O worker thread.
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.It
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.Dv SF_MNOWAIT .
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(description missing)
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.It
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.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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.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 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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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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Completing the entire transfer would have required disk I/O, so
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it was aborted.
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Partial data may have been sent.
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(This error can only occur when
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.Dv SF_NODISKIO
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is specified.)
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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 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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.Sh AUTHORS
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The
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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 dg@dglawrence.com .
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