#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h> int
Fo sendfile
Fa int fd int s off_t offset size_t nbytes
Fa struct sf_hdtr *hdtr off_t *sbytes int flags
Fc
DESCRIPTION
The
sendfile ();
system call
sends a regular file specified by descriptor
Fa fd
out a stream socket specified by descriptor
Fa s .
The
Fa offset
argument specifies where to begin in the file.
Should
Fa offset
fall beyond the end of file, the system will return
success and report 0 bytes sent as described below.
The
Fa 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
Fa headers
and
Fa 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
Fa hdr_cnt
and
Fa trl_cnt .
If
non- NULL
the system will write the total number of bytes sent on the socket to the
variable pointed to by
Fa sbytes .
The
Fa flags
argument has one possible value:
SF_NODISKIO
This flag causes any
sendfile ();
call which would block on disk I/O to instead
return
Er 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
Fa *sbytes
(if specified),
and the error
Er EAGAIN
is returned.
IMPLEMENTATION NOTES
The
Fx 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
(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.conf5
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
Rv -std sendfile
ERRORS
Bq Er 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
Fa *sbytes .
Bq Er EBADF
The
Fa fd
argument
is not a valid file descriptor.
Bq Er EBADF
The
Fa s
argument
is not a valid socket descriptor.
Bq Er 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.)
Bq Er EFAULT
An invalid address was specified for an argument.
Bq Er EINTR
A signal interrupted
sendfile ();
before it could be completed.
If specified, the number
of bytes successfully sent will be returned in
Fa *sbytes .
Bq Er EINVAL
The
Fa fd
argument
is not a regular file.
Bq Er EINVAL
The
Fa s
argument
is not a SOCK_STREAM type socket.
Bq Er EINVAL
The
Fa offset
argument
is negative.
Bq Er EIO
An error occurred while reading from
Fa fd .
Bq Er ENOTCONN
The
Fa s
argument
points to an unconnected socket.
Bq Er ENOTSOCK
The
Fa s
argument
is not a socket.
Bq Er EOPNOTSUPP
The file system for descriptor
Fa fd
does not support
sendfile (.);
K. Elmeleegy
A. Chanda
A. L. Cox
W. Zwaenepoel
A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping ManagementThe Proceedings of the 2005 USENIX Annual Technical Conference
pp 223-236
2005
HISTORY
The
sendfile ();
system call
first appeared in
Fx 3.0 .
This manual page first appeared in
Fx 3.1 .
AUTHORS
The
sendfile ();
system call
and this manual page were written by
An David G. Lawrence Aq dg@dglawrence.com .
