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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 Do not wait for some kernel resource to become available,
124 The flag does not make the
126 syscall truly non-blocking, since other resources are still allocated
127 in a blocking fashion.
131 sleeps until the network stack no longer references the VM pages
132 of the file, making subsequent modifications to it safe.
133 Please note that this is not a guarantee that the data has actually
137 When using a socket marked for non-blocking I/O,
139 may send fewer bytes than requested.
140 In this case, the number of bytes successfully
141 written is returned in
147 .Sh IMPLEMENTATION NOTES
152 is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided.
154 On some architectures, this system call internally uses a special
157 .Pq Vt "struct sf_buf"
158 to handle sending file data to the client.
159 If the sending socket is
160 blocking, and there are not enough
164 will block and report a state of
166 If the sending socket is non-blocking and there are not enough
168 buffers available, the call will block and wait for the
169 necessary buffers to become available before finishing the call.
173 allocated should be proportional to the number of nmbclusters used to
174 send data to a client via
176 Tune accordingly to avoid blocking!
177 Busy installations that make extensive use of
179 may want to increase these values to be inline with their
180 .Va kern.ipc.nmbclusters
187 buffers available is determined at boot time by either the
192 kernel configuration tunable.
198 .Va kern.ipc.nsfbufsused
200 .Va kern.ipc.nsfbufspeak
203 variables show current and peak
205 buffers usage respectively.
206 These values may also be viewed through
209 If a value of zero is reported for
210 .Va kern.ipc.nsfbufs ,
211 your architecture does not need to use
213 buffers because their task can be efficiently performed
214 by the generic virtual memory structures.
220 The socket is marked for non-blocking I/O and not all data was sent due to
221 the socket buffer being filled.
222 If specified, the number of bytes successfully sent will be returned in
228 is not a valid file descriptor.
233 is not a valid socket descriptor.
235 Completing the entire transfer would have required disk I/O, so
237 Partial data may have been sent.
238 (This error can only occur when
242 An invalid address was specified for an argument.
246 before it could be completed.
247 If specified, the number
248 of bytes successfully sent will be returned in
254 is not a regular file.
259 is not a SOCK_STREAM type socket.
266 An error occurred while reading from
272 points to an unconnected socket.
279 The file system for descriptor
284 The socket peer has closed the connection.
298 .%T A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management
299 .%J The Proceedings of the 2005 USENIX Annual Technical Conference
309 This manual page first appeared in
315 and this manual page were written by
316 .An David G. Lawrence Aq dg@dglawrence.com .