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SENDFILE(2)		  FreeBSD System Calls Manual		   SENDFILE(2)

     sendfile -- send a	file to	a socket

     Standard C	Library	(libc, -lc)

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <sys/uio.h>

     sendfile(int fd, int s, off_t offset, size_t nbytes,
	 struct	sf_hdtr	*hdtr, off_t *sbytes, int flags);

     The sendfile() system call	sends a	regular	file or	shared memory object
     specified by descriptor fd	out a stream socket specified by descriptor s.

     The offset	argument specifies where to begin in the file.	Should offset
     fall beyond the end of file, the system will return success and report 0
     bytes sent	as described below.  The 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.

     An	optional header	and/or trailer can be sent before and after the	file
     data by specifying	a pointer to a struct sf_hdtr, which has the following

	   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 */

     The headers and trailers pointers,	if non-NULL, point to arrays of	struct
     iovec structures.	See the	writev() system	call for information on	the
     iovec structure.  The number of iovecs in these arrays is specified by
     hdr_cnt and trl_cnt.

     If	non-NULL, the system will write	the total number of bytes sent on the
     socket to the variable pointed to by sbytes.

     The least significant 16 bits of flags argument is	a bitmap of these val-

	   SF_NODISKIO	      This flag	causes sendfile	to return EBUSY	in-
			      stead of blocking	when a busy page is encoun-
			      tered.  This rare	situation can happen if	some
			      other process is now working with	the same re-
			      gion of the file.	 It is advised to retry	the
			      operation	after a	short period.

			      Note that	in older FreeBSD versions the
			      SF_NODISKIO had slightly different notion.  The
			      flag prevented sendfile to run I/O operations in
			      case if an invalid (not cached) page is encoun-
			      tered, thus avoiding blocking on I/O.  Starting
			      with FreeBSD 11 sendfile sending files off the
			      ffs(7) filesystem	does not block on I/O (see
			      IMPLEMENTATION NOTES ), so the condition no
			      longer applies.  However,	it is safe if an ap-
			      plication	utilizes SF_NODISKIO and on EBUSY per-
			      forms the	same action as it did in older FreeBSD
			      versions,	e.g., aio_read(2), read(2) or sendfile
			      in a different context.

	   SF_NOCACHE	      The data sent to socket will not be cached by
			      the virtual memory system, and will be freed di-
			      rectly to	the pool of free pages.

	   SF_SYNC	      sendfile sleeps until the	network	stack no
			      longer references	the VM pages of	the file, mak-
			      ing subsequent modifications to it safe.	Please
			      note that	this is	not a guarantee	that the data
			      has actually been	sent.

	   SF_USER_READAHEAD  sendfile has some	internal heuristics to do
			      readahead	when sending data.  This flag forces
			      sendfile to override any heuristically calcu-
			      lated readahead and use exactly the application
			      specified	readahead.  See	SETTING	READAHEAD for
			      more details on readahead.

     When using	a socket marked	for non-blocking I/O, sendfile() may send
     fewer bytes than requested.  In this case,	the number of bytes success-
     fully written is returned in *sbytes (if specified), and the error	EAGAIN
     is	returned.

     sendfile uses internal heuristics based on	request	size and file system
     layout to do readahead.  Additionally application may request extra
     readahead.	 The most significant 16 bits of flags specify amount of pages
     that sendfile may read ahead when reading the file.  A macro SF_FLAGS()
     is	provided to combine readahead amount and flags.	 An example showing
     specifying	readahead of 16	pages and SF_NOCACHE flag:


     sendfile will use either application specified readahead or internally
     calculated, whichever is bigger.  Setting flag SF_USER_READAHEAD would
     turn off any heuristics and set maximum possible readahead	length to the
     number of pages specified via flags.

     The FreeBSD implementation	of sendfile() does not block on	disk I/O when
     it	sends a	file off the ffs(7) filesystem.	 The syscall returns success
     before the	actual I/O completes, and data is put into the socket later
     unattended.  However, the order of	data in	the socket is preserved, so it
     is	safe to	do further writes to the socket.

     The FreeBSD implementation	of sendfile() is "zero-copy", meaning that it
     has been optimized	so that	copying	of the file data is avoided.

   physical paging buffers
     sendfile()	uses vnode pager to read file pages into memory.  The pager
     uses a pool of physical buffers to	run its	I/O operations.	 When system
     runs out of pbufs,	sendfile will block and	report state "zonelimit".
     Size of the pool can be tuned with	vm.vnode_pbufs loader.conf(5) tunable
     and can be	checked	with sysctl(8) OID of the same name at runtime.

   sendfile(2) buffers
     On	some architectures, this system	call internally	uses a special
     sendfile()	buffer (struct sf_buf) to handle sending file data to the
     client.  If the sending socket is blocking, and there are not enough
     sendfile()	buffers	available, sendfile() will block and report a state of
     "sfbufa".	If the sending socket is non-blocking and there	are not	enough
     sendfile()	buffers	available, the call will block and wait	for the	neces-
     sary buffers to become available before finishing the call.

     The number	of sf_buf's allocated should be	proportional to	the number of
     nmbclusters used to send data to a	client via sendfile().	Tune accord-
     ingly to avoid blocking!  Busy installations that make extensive use of
     sendfile()	may want to increase these values to be	inline with their
     kern.ipc.nmbclusters (see tuning(7) for details).

     The number	of sendfile() buffers available	is determined at boot time by
     either the	kern.ipc.nsfbufs loader.conf(5)	variable or the	NSFBUFS	kernel
     configuration tunable.  The number	of sendfile() buffers scales with
     kern.maxusers.  The kern.ipc.nsfbufsused and kern.ipc.nsfbufspeak read-
     only sysctl(8) variables show current and peak sendfile() buffers usage
     respectively.  These values may also be viewed through netstat -m.

     If	sysctl(8) OID kern.ipc.nsfbufs doesn't exist, your architecture	does
     not need to use sendfile()	buffers	because	their task can be efficiently
     performed by the generic virtual memory structures.

     The sendfile() function returns the value 0 if successful;	otherwise the
     value -1 is returned and the global variable errno	is set to indicate the

     [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 *sbytes.

     [EBADF]		The fd argument	is not a valid file descriptor.

     [EBADF]		The s argument is not a	valid socket descriptor.

     [EBUSY]		A busy page was	encountered and	SF_NODISKIO had	been
			specified.  Partial data may have been sent.

     [EFAULT]		An invalid address was specified for an	argument.

     [EINTR]		A signal interrupted sendfile()	before it could	be
			completed.  If specified, the number of	bytes success-
			fully sent will	be returned in *sbytes.

     [EINVAL]		The fd argument	is not a regular file.

     [EINVAL]		The s argument is not a	SOCK_STREAM type socket.

     [EINVAL]		The offset argument is negative.

     [EIO]		An error occurred while	reading	from fd.

     [EINTEGRITY]	Corrupted data was detected while reading from fd.

     [ENOTCAPABLE]	The fd or the s	argument has insufficient rights.

     [ENOBUFS]		The system was unable to allocate an internal buffer.

     [ENOTCONN]		The s argument points to an unconnected	socket.

     [ENOTSOCK]		The s argument is not a	socket.

     [EOPNOTSUPP]	The file system	for descriptor fd does not support

     [EPIPE]		The socket peer	has closed the connection.

     netstat(1), open(2), send(2), socket(2), writev(2), loader.conf(5),
     tuning(7),	sysctl(8)

     K.	Elmeleegy, A. Chanda, A. L. Cox, and W.	Zwaenepoel, "A Portable	Kernel
     Abstraction for Low-Overhead Ephemeral Mapping Management", The
     Proceedings of the	2005 USENIX Annual Technical Conference, pp 223-236,

     The sendfile() system call	first appeared in FreeBSD 3.0.	This manual
     page first	appeared in FreeBSD 3.1.  In FreeBSD 10	support	for sending
     shared memory descriptors had been	introduced.  In	FreeBSD	11 a non-
     blocking implementation had been introduced.

     The initial implementation	of sendfile() system call and this manual page
     were written by David G. Lawrence <>.  The FreeBSD 11
     implementation was	written	by
     Gleb Smirnoff <>.

     The sendfile() system call	will not fail, i.e., return -1 and set errno
     to	EFAULT,	if provided an invalid address for sbytes.  The	sendfile()
     system call does not support SCTP sockets,	it will	return -1 and set
     errno to EINVAL.

FreeBSD	13.0			March 30, 2020			  FreeBSD 13.0


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