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SENDFILE(2)		       System Calls Manual		     SENDFILE(2)

NAME
     sendfile -- send a file to a socket

LIBRARY
     Standard C Library (libc, -lc)

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

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

DESCRIPTION
     The  sendfile() system call sends a regular file 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
     structure:

	   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  flags  argument has one possible value: SF_NODISKIO.	This flag causes
     any sendfile() call which would block on disk I/O to instead return  EBUSY.
     Busy  servers  may  benefit  by transferring requests that would block to a
     separate I/O worker thread.

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

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

TUNING
     Internally,  this	system	call  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, send-
     file() 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 necessary buffers to become available be-
     fore 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 accordingly
     to  avoid	blocking!   Busy  installations that make extensive use of send-
     file()  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  respec-
     tively.  These values may also be viewed through netstat -m.

RETURN VALUES
     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
     error.

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

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

     [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 SF_NODISKIO is
			specified.)

     [ENOTSOCK] 	The s argument is not a socket.

     [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.

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

     [EPIPE]		The socket peer has closed the connection.

     [EIO]		An error occurred while reading from fd.

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

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

     [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.

SEE ALSO
     netstat(1), open(2), send(2), socket(2), writev(2), tuning(7)

HISTORY
     The sendfile() system call first appeared in FreeBSD 3.0.	This manual page
     first appeared in FreeBSD 3.1.

AUTHORS
     The  sendfile()  system  call and this manual page were written by David G.
     Lawrence <dg@dglawrence.com>.

FreeBSD 5.5			October 16, 2004		     SENDFILE(2)
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