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+.\" Copyright (c) 2003, David G. Lawrence
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice unmodified, this list of conditions, and the following
+.\"    disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd January 7, 2010
+.Dt SENDFILE 2
+.Os
+.Sh NAME
+.Nm sendfile
+.Nd send a file to a socket
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In sys/types.h
+.In sys/socket.h
+.In sys/uio.h
+.Ft int
+.Fo sendfile
+.Fa "int fd" "int s" "off_t offset" "size_t nbytes"
+.Fa "struct sf_hdtr *hdtr" "off_t *sbytes" "int flags"
+.Fc
+.Sh DESCRIPTION
+The
+.Fn sendfile
+system call
+sends a regular file specified by descriptor
+.Fa fd
+out a stream socket specified by descriptor
+.Fa s .
+.Pp
+The
+.Fa offset
+argument specifies where to begin in the file.
+Should
+.Fa offset
+fall beyond the end of file, the system will return
+success and report 0 bytes sent as described below.
+The
+.Fa nbytes
+argument specifies how many bytes of the file should be sent, with 0 having the special
+meaning of send until the end of file has been reached.
+.Pp
+An optional header and/or trailer can be sent before and after the file data by specifying
+a pointer to a
+.Vt "struct sf_hdtr" ,
+which has the following structure:
+.Pp
+.Bd -literal -offset indent -compact
+struct sf_hdtr {
+	struct iovec *headers;	/* pointer to header iovecs */
+	int hdr_cnt;		/* number of header iovecs */
+	struct iovec *trailers;	/* pointer to trailer iovecs */
+	int trl_cnt;		/* number of trailer iovecs */
+};
+.Ed
+.Pp
+The
+.Fa headers
+and
+.Fa trailers
+pointers, if
+.Pf non- Dv NULL ,
+point to arrays of
+.Vt "struct iovec"
+structures.
+See the
+.Fn writev
+system call for information on the iovec structure.
+The number of iovecs in these
+arrays is specified by
+.Fa hdr_cnt
+and
+.Fa trl_cnt .
+.Pp
+If
+.Pf non- Dv NULL ,
+the system will write the total number of bytes sent on the socket to the
+variable pointed to by
+.Fa sbytes .
+.Pp
+The
+.Fa flags
+argument is a bitmap of these values:
+.Bl -item -offset indent
+.It
+.Dv SF_NODISKIO .
+This flag causes any
+.Fn sendfile
+call which would block on disk I/O to instead
+return
+.Er EBUSY .
+Busy servers may benefit by transferring requests that would
+block to a separate I/O worker thread.
+.It
+.Dv SF_MNOWAIT .
+Do not wait for some kernel resource to become available,
+in particular,
+.Vt mbuf
+and
+.Vt sf_buf .
+The flag does not make the
+.Fn sendfile
+syscall truly non-blocking, since other resources are still allocated
+in a blocking fashion.
+.It
+.Dv SF_SYNC .
+.Nm
+sleeps until the network stack no longer references the VM pages
+of the file, making subsequent modifications to it safe.
+Please note that this is not a guarantee that the data has actually
+been sent.
+.El
+.Pp
+When using a socket marked for non-blocking I/O,
+.Fn sendfile
+may send fewer bytes than requested.
+In this case, the number of bytes successfully
+written is returned in
+.Fa *sbytes
+(if specified),
+and the error
+.Er EAGAIN
+is returned.
+.Sh IMPLEMENTATION NOTES
+The
+.Fx
+implementation of
+.Fn sendfile
+is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided.
+.Sh TUNING
+On some architectures, this system call internally uses a special
+.Fn sendfile
+buffer
+.Pq Vt "struct sf_buf"
+to handle sending file data to the client.
+If the sending socket is
+blocking, and there are not enough
+.Fn sendfile
+buffers available,
+.Fn sendfile
+will block and report a state of
+.Dq Li sfbufa .
+If the sending socket is non-blocking and there are not enough
+.Fn sendfile
+buffers available, the call will block and wait for the
+necessary buffers to become available before finishing the call.
+.Pp
+The number of
+.Vt sf_buf Ns 's
+allocated should be proportional to the number of nmbclusters used to
+send data to a client via
+.Fn sendfile .
+Tune accordingly to avoid blocking!
+Busy installations that make extensive use of
+.Fn sendfile
+may want to increase these values to be inline with their
+.Va kern.ipc.nmbclusters
+(see
+.Xr tuning 7
+for details).
+.Pp
+The number of
+.Fn sendfile
+buffers available is determined at boot time by either the
+.Va kern.ipc.nsfbufs
+.Xr loader.conf 5
+variable or the
+.Dv NSFBUFS
+kernel configuration tunable.
+The number of
+.Fn sendfile
+buffers scales with
+.Va kern.maxusers .
+The
+.Va kern.ipc.nsfbufsused
+and
+.Va kern.ipc.nsfbufspeak
+read-only
+.Xr sysctl 8
+variables show current and peak
+.Fn sendfile
+buffers usage respectively.
+These values may also be viewed through
+.Nm netstat Fl m .
+.Pp
+If a value of zero is reported for
+.Va kern.ipc.nsfbufs ,
+your architecture does not need to use
+.Fn sendfile
+buffers because their task can be efficiently performed
+by the generic virtual memory structures.
+.Sh RETURN VALUES
+.Rv -std sendfile
+.Sh ERRORS
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+The socket is marked for non-blocking I/O and not all data was sent due to
+the socket buffer being filled.
+If specified, the number of bytes successfully sent will be returned in
+.Fa *sbytes .
+.It Bq Er EBADF
+The
+.Fa fd
+argument
+is not a valid file descriptor.
+.It Bq Er EBADF
+The
+.Fa s
+argument
+is not a valid socket descriptor.
+.It Bq Er EBUSY
+Completing the entire transfer would have required disk I/O, so
+it was aborted.
+Partial data may have been sent.
+(This error can only occur when
+.Dv SF_NODISKIO
+is specified.)
+.It Bq Er EFAULT
+An invalid address was specified for an argument.
+.It Bq Er EINTR
+A signal interrupted
+.Fn sendfile
+before it could be completed.
+If specified, the number
+of bytes successfully sent will be returned in
+.Fa *sbytes .
+.It Bq Er EINVAL
+The
+.Fa fd
+argument
+is not a regular file.
+.It Bq Er EINVAL
+The
+.Fa s
+argument
+is not a SOCK_STREAM type socket.
+.It Bq Er EINVAL
+The
+.Fa offset
+argument
+is negative.
+.It Bq Er EIO
+An error occurred while reading from
+.Fa fd .
+.It Bq Er ENOBUFS
+The system was unable to allocate an internal buffer.
+.It Bq Er ENOTCONN
+The
+.Fa s
+argument
+points to an unconnected socket.
+.It Bq Er ENOTSOCK
+The
+.Fa s
+argument
+is not a socket.
+.It Bq Er EOPNOTSUPP
+The file system for descriptor
+.Fa fd
+does not support
+.Fn sendfile .
+.It Bq Er EPIPE
+The socket peer has closed the connection.
+.El
+.Sh SEE ALSO
+.Xr netstat 1 ,
+.Xr open 2 ,
+.Xr send 2 ,
+.Xr socket 2 ,
+.Xr writev 2 ,
+.Xr tuning 7
+.Rs
+.%A K. Elmeleegy
+.%A A. Chanda
+.%A A. L. Cox
+.%A W. Zwaenepoel
+.%T A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management
+.%J The Proceedings of the 2005 USENIX Annual Technical Conference
+.%P pp 223-236
+.%D 2005
+.Re
+.Sh HISTORY
+The
+.Fn sendfile
+system call
+first appeared in
+.Fx 3.0 .
+This manual page first appeared in
+.Fx 3.1 .
+.Sh AUTHORS
+The
+.Fn sendfile
+system call
+and this manual page were written by
+.An David G. Lawrence Aq dg@dglawrence.com .