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     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 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 benefit by transferring requests that would
           block to a separate I/O worker thread.

           SF_MNOWAIT.  Do not wait for some kernel resource to become available, 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 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.

     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() 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 error.


     [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

     [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 sendfile().

     [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, 2005.


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


     The sendfile() system call and this manual page were written by David G. Lawrence