.\" Copyright (c) 2003, David G. Lawrence .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice unmodified, this list of conditions, and the following .\" disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD: head/lib/libc/sys/sendfile.2 293439 2016-01-08 20:34:57Z glebius $ .\" .Dd January 7, 2016 .Dt SENDFILE 2 .Os .Sh NAME .Nm sendfile .Nd send a file to a socket .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/types.h .In sys/socket.h .In sys/uio.h .Ft 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 .Sh DESCRIPTION The .Fn sendfile system call sends a regular file or shared memory object specified by descriptor .Fa fd out a stream socket specified by descriptor .Fa s . .Pp 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. .Pp 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: .Pp .Bd -literal -offset indent -compact 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 */ }; .Ed .Pp The .Fa headers and .Fa trailers pointers, if .Pf non- Dv NULL , point to arrays of .Vt "struct iovec" structures. See the .Fn 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 . .Pp If .Pf non- Dv NULL , the system will write the total number of bytes sent on the socket to the variable pointed to by .Fa sbytes . .Pp The least significant 16 bits of .Fa flags argument is a bitmap of these values: .Bl -tag -offset indent .It Dv SF_NODISKIO This flag causes .Nm to return .Er 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. .Pp Note that in older .Fx versions the .Dv SF_NODISKIO had slightly different notion. The flag prevented .Nm to run I/O operations in case if an invalid (not cached) page is encountered, thus avoiding blocking on I/O. Starting with .Fx 11 .Nm sending files off the .Xr ffs 7 filesystem doesn't block on I/O (see .Sx IMPLEMENTATION NOTES ), so the condition no longer applies. However, it is safe if an application utilizes .Dv SF_NODISKIO and on .Er EBUSY performs the same action as it did in older .Fx versions, e.g. .Xr aio_read 2, .Xr read 2 or .Nm in a different context. .It Dv 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. .It Dv SF_SYNC .Nm 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. .El .Pp The most significant 16 bits of .Fa flags specify amount of pages that .Nm may read ahead when reading the file. A macro .Fn SF_FLAGS is provided to combine readahead amount and flags. Example shows specifing readahead of 16 pages and .Dv SF_NOCACHE flag: .Pp .Bd -literal -offset indent -compact SF_FLAGS(16, SF_NOCACHE) .Ed .Pp When using a socket marked for non-blocking I/O, .Fn 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. .Sh IMPLEMENTATION NOTES The .Fx implementation of .Fn sendfile doesn't block on disk I/O when it sends a file off the .Xr 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. .Pp The .Fx implementation of .Fn sendfile is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided. .Sh TUNING On some architectures, this system call internally uses a special .Fn sendfile buffer .Pq Vt "struct sf_buf" to handle sending file data to the client. If the sending socket is blocking, and there are not enough .Fn sendfile buffers available, .Fn sendfile will block and report a state of .Dq Li sfbufa . If the sending socket is non-blocking and there are not enough .Fn sendfile buffers available, the call will block and wait for the necessary buffers to become available before finishing the call. .Pp 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 .Fn sendfile . Tune accordingly to avoid blocking! Busy installations that make extensive use of .Fn sendfile may want to increase these values to be inline with their .Va kern.ipc.nmbclusters (see .Xr tuning 7 for details). .Pp The number of .Fn sendfile buffers available is determined at boot time by either the .Va kern.ipc.nsfbufs .Xr loader.conf 5 variable or the .Dv NSFBUFS kernel configuration tunable. The number of .Fn sendfile buffers scales with .Va kern.maxusers . The .Va kern.ipc.nsfbufsused and .Va kern.ipc.nsfbufspeak read-only .Xr sysctl 8 variables show current and peak .Fn sendfile buffers usage respectively. These values may also be viewed through .Nm netstat Fl m . .Pp If a value of zero is reported for .Va kern.ipc.nsfbufs , your architecture does not need to use .Fn sendfile buffers because their task can be efficiently performed by the generic virtual memory structures. .Sh RETURN VALUES .Rv -std sendfile .Sh ERRORS .Bl -tag -width Er .It 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 . .It Bq Er EBADF The .Fa fd argument is not a valid file descriptor. .It Bq Er EBADF The .Fa s argument is not a valid socket descriptor. .It Bq Er EBUSY A busy page was encountered and .Dv SF_NODISKIO had been specified. Partial data may have been sent. .It Bq Er EFAULT An invalid address was specified for an argument. .It Bq Er EINTR A signal interrupted .Fn sendfile before it could be completed. If specified, the number of bytes successfully sent will be returned in .Fa *sbytes . .It Bq Er EINVAL The .Fa fd argument is not a regular file. .It Bq Er EINVAL The .Fa s argument is not a SOCK_STREAM type socket. .It Bq Er EINVAL The .Fa offset argument is negative. .It Bq Er EIO An error occurred while reading from .Fa fd . .It Bq Er ENOBUFS The system was unable to allocate an internal buffer. .It Bq Er ENOTCONN The .Fa s argument points to an unconnected socket. .It Bq Er ENOTSOCK The .Fa s argument is not a socket. .It Bq Er EOPNOTSUPP The file system for descriptor .Fa fd does not support .Fn sendfile . .It Bq Er EPIPE The socket peer has closed the connection. .El .Sh SEE ALSO .Xr netstat 1 , .Xr open 2 , .Xr send 2 , .Xr socket 2 , .Xr writev 2 , .Xr tuning 7 .Rs .%A K. Elmeleegy .%A A. Chanda .%A A. L. Cox .%A W. Zwaenepoel .%T A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management .%J The Proceedings of the 2005 USENIX Annual Technical Conference .%P pp 223-236 .%D 2005 .Re .Sh HISTORY The .Fn sendfile system call first appeared in .Fx 3.0 . This manual page first appeared in .Fx 3.1 . In .Fx 10 support for sending shared memory descriptors had been introduced. In .Fx 11 a non-blocking implementation had been introduced. .Sh AUTHORS The initial implementation of .Fn sendfile system call and this manual page were written by .An David G. Lawrence Aq Mt dg@dglawrence.com . The .Fx 11 implementation was written by .An Gleb Smirnoff Aq Mt glebius@FreeBSD.org .