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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 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
     necessary 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
     accordingly 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
     efficiently 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
                        successfully 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〉.