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SENDFILE(2) System Calls Manual SENDFILE(2)


sendfilesend 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 () 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 () 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:

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.

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.
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.
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, () 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.


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 () is provided to combine readahead amount and flags. An example showing specifying readahead of 16 pages and SF_NOCACHE flag:


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.


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.


On some architectures, this system call internally uses a special () 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 (). 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 () 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 () 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.


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.
A busy page was encountered and SF_NODISKIO had been specified. Partial data may have been sent.
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.
An error occurred while reading from fd.
Corrupted data was detected while reading from fd.
The fd or the s argument has insufficient rights.
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)

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. 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 <>.


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.

March 30, 2020 Debian