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33 .Nd send a file to a socket
42 .Fa "int fd" "int s" "off_t offset" "size_t nbytes"
43 .Fa "struct sf_hdtr *hdtr" "off_t *sbytes" "int flags"
49 sends a regular file specified by descriptor
51 out a stream socket specified by descriptor
56 argument specifies where to begin in the file.
59 fall beyond the end of file, the system will return
60 success and report 0 bytes sent as described below.
63 argument specifies how many bytes of the file should be sent, with 0 having the special
64 meaning of send until the end of file has been reached.
66 An optional header and/or trailer can be sent before and after the file data by specifying
68 .Vt "struct sf_hdtr" ,
69 which has the following structure:
71 .Bd -literal -offset indent -compact
73 struct iovec *headers; /* pointer to header iovecs */
74 int hdr_cnt; /* number of header iovecs */
75 struct iovec *trailers; /* pointer to trailer iovecs */
76 int trl_cnt; /* number of trailer iovecs */
91 system call for information on the iovec structure.
92 The number of iovecs in these
93 arrays is specified by
100 the system will write the total number of bytes sent on the socket to the
101 variable pointed to by
106 argument is a bitmap of these values:
107 .Bl -item -offset indent
112 call which would block on disk I/O to instead
115 Busy servers may benefit by transferring requests that would
116 block to a separate I/O worker thread.
119 (description missing)
123 sleeps until the network stack no longer references the VM pages
124 of the file, making subsequent modifications to it safe.
125 Please note that this is not a guarantee that the data has actually
129 When using a socket marked for non-blocking I/O,
131 may send fewer bytes than requested.
132 In this case, the number of bytes successfully
133 written is returned in
139 .Sh IMPLEMENTATION NOTES
144 is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided.
146 On some architectures, this system call internally uses a special
149 .Pq Vt "struct sf_buf"
150 to handle sending file data to the client.
151 If the sending socket is
152 blocking, and there are not enough
156 will block and report a state of
158 If the sending socket is non-blocking and there are not enough
160 buffers available, the call will block and wait for the
161 necessary buffers to become available before finishing the call.
165 allocated should be proportional to the number of nmbclusters used to
166 send data to a client via
168 Tune accordingly to avoid blocking!
169 Busy installations that make extensive use of
171 may want to increase these values to be inline with their
172 .Va kern.ipc.nmbclusters
179 buffers available is determined at boot time by either the
184 kernel configuration tunable.
190 .Va kern.ipc.nsfbufsused
192 .Va kern.ipc.nsfbufspeak
195 variables show current and peak
197 buffers usage respectively.
198 These values may also be viewed through
201 If a value of zero is reported for
202 .Va kern.ipc.nsfbufs ,
203 your architecture does not need to use
205 buffers because their task can be efficiently performed
206 by the generic virtual memory structures.
212 The socket is marked for non-blocking I/O and not all data was sent due to
213 the socket buffer being filled.
214 If specified, the number of bytes successfully sent will be returned in
220 is not a valid file descriptor.
225 is not a valid socket descriptor.
227 Completing the entire transfer would have required disk I/O, so
229 Partial data may have been sent.
230 (This error can only occur when
234 An invalid address was specified for an argument.
238 before it could be completed.
239 If specified, the number
240 of bytes successfully sent will be returned in
246 is not a regular file.
251 is not a SOCK_STREAM type socket.
258 An error occurred while reading from
264 points to an unconnected socket.
271 The file system for descriptor
276 The socket peer has closed the connection.
290 .%T A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management
291 .%J The Proceedings of the 2005 USENIX Annual Technical Conference
301 This manual page first appeared in
307 and this manual page were written by
308 .An David G. Lawrence Aq dg@dglawrence.com .