sendfile man page on FreeBSD

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SENDFILE(2)		    BSD 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 is a bitmap of these values:

	   SF_NODISKIO.	 This flag causes any sendfile() call which would
	   block on disk I/O to instead return EBUSY.  Busy servers may bene‐
	   fit by transferring requests that would block to a separate I/O
	   worker thread.

	   SF_MNOWAIT.	Do not wait for some kernel resource to become avail‐
	   able, in particular, mbuf and sf_buf.  The flag does not make the
	   sendfile() syscall truly non-blocking, since other resources are
	   still allocated in a blocking fashion.

	   SF_SYNC.  sendfile sleeps until the network stack no longer refer‐
	   ences 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.

     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.

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
     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 a value of zero is reported for kern.ipc.nsfbufs, your architecture
     does not need to use sendfile() buffers because their task can be effi‐
     ciently performed by the generic virtual memory structures.

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
     [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]		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.)

     [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.

     [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
			sendfile().

     [EPIPE]		The socket peer has closed the connection.

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

     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,
     2005.

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⟩.

BSD				January 7, 2010				   BSD
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