SENDFILE(2) FreeBSD System Calls Manual SENDFILE(2)
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 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
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 least significant 16 bits of flags argument is a bitmap of these
values:
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 does not 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.
SF_USER_READAHEAD sendfile has some internal heuristics to do
readahead when sending data. This flag forces
sendfile to override any heuristically
calculated readahead and use exactly the
application specified readahead. See SETTING
READAHEAD for more details on readahead.
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.
SETTING READAHEAD
sendfile uses internal heuristics based on request size and file system
layout to do readahead. Additionally application may request extra
readahead. 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. An example showing
specifying readahead of 16 pages and SF_NOCACHE flag:
SF_FLAGS(16, SF_NOCACHE)
sendfile will use either application specified readahead or internally
calculated, whichever is bigger. Setting flag SF_USER_READAHEAD would
turn off any heuristics and set maximum possible readahead length to the
number of pages specified via flags.
IMPLEMENTATION NOTES
The FreeBSD implementation of sendfile() does not 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.
TUNING
physical paging buffers
sendfile() uses vnode pager to read file pages into memory. The pager
uses a pool of physical buffers to run its I/O operations. When system
runs out of pbufs, sendfile will block and report state "zonelimit".
Size of the pool can be tuned with vm.vnode_pbufs loader.conf(5) tunable
and can be checked with sysctl(8) OID of the same name at runtime.
sendfile(2) buffers
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 sysctl(8) OID kern.ipc.nsfbufs doesn't exist, 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] 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.
[EINTEGRITY] Corrupted data was detected while reading from fd.
[ENOTCAPABLE] The fd or the s argument has insufficient rights.
[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.
SEE ALSO
netstat(1), open(2), send(2), socket(2), writev(2), loader.conf(5),
tuning(7), sysctl(8)
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. In FreeBSD 10 support for sending
shared memory descriptors had been introduced. In FreeBSD 11 a non-
blocking implementation had been introduced.
AUTHORS
The initial implementation of sendfile() system call and this manual page
were written by David G. Lawrence <dg@dglawrence.com>. The FreeBSD 11
implementation was written by
Gleb Smirnoff <glebius@FreeBSD.org>.
BUGS
The sendfile() system call will not fail, i.e., return -1 and set errno
to EFAULT, if provided an invalid address for sbytes. The sendfile()
system call does not support SCTP sockets, it will return -1 and set
errno to EINVAL.
FreeBSD 13.1-RELEASE-p6 March 30, 2020 FreeBSD 13.1-RELEASE-p6
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