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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

           SF_NODISKIO   This flag causes sendfile to return EBUSY instead of
                         blocking when a busy page is encountered.  This rare
                         situation can happen if some other process is now
                         working with the same region 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 encountered, thus avoiding
                         blocking on I/O.  Starting with FreeBSD 11 sendfile
                         sending files off the ffs(7) filesystem doesn't block
                         on I/O (see IMPLEMENTATION NOTES ), so the condition
                         no longer applies.  However, it is safe if an
                         application utilizes SF_NODISKIO and on EBUSY
                         performs 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 directly to
                         the pool of free pages.

           SF_SYNC       sendfile 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.

     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.  Example shows specifing
     readahead of 16 pages and SF_NOCACHE flag:

                   SF_FLAGS(16, SF_NOCACHE)

     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.

     The FreeBSD implementation of sendfile() doesn't 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.

     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.

     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
                        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.

     [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), 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,

     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 <>.

FreeBSD 11.0-PRERELEASE         January 7, 2016        FreeBSD 11.0-PRERELEASE


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