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Man Pages

Manual Reference Pages  -  SENDFILE (2)


sendfile - send a file to a socket


Implementation Notes
Return Values
See Also


.Lb libc


.In sys/types.h
.In sys/socket.h
.In sys/uio.h int
.Fo 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
.Vt 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
.Vt 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,
.Vt mbuf and
.Vt 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.


.Fx 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 (Vt 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
.Vt sf_buf Ns ’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.


.Rv -std sendfile


  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.
  The fd argument is not a valid file descriptor.
  The s argument is not a valid socket descriptor.
  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.)
  An invalid address was specified for an argument.
  A signal interrupted sendfile before it could be completed. If specified, the number of bytes successfully sent will be returned in *sbytes.
  The fd argument is not a regular file.
  The s argument is not a SOCK_STREAM type socket.
  The offset argument is negative.
[EIO] An error occurred while reading from fd.
  The system was unable to allocate an internal buffer.
  The s argument points to an unconnected socket.
  The s argument is not a socket.
  The file system for descriptor fd does not support sendfile.
  The socket peer has closed the connection.


netstat(1), open(2), send(2), socket(2), writev(2), tuning(7)
.Rs A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management


The sendfile system call first appeared in
.Fx 3.0 . This manual page first appeared in
.Fx 3.1 .


The sendfile system call and this manual page were written by
.An David G. Lawrence Aq .
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