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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 has one possible value:
110 call which would block on disk I/O to instead
113 Busy servers may benefit by transferring requests that would
114 block to a separate I/O worker thread.
116 When using a socket marked for non-blocking I/O,
118 may send fewer bytes than requested.
119 In this case, the number of bytes successfully
120 written is returned in
126 .Sh IMPLEMENTATION NOTES
131 is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided.
133 On some architectures, this system call internally uses a special
136 .Pq Vt "struct sf_buf"
137 to handle sending file data to the client.
138 If the sending socket is
139 blocking, and there are not enough
143 will block and report a state of
145 If the sending socket is non-blocking and there are not enough
147 buffers available, the call will block and wait for the
148 necessary buffers to become available before finishing the call.
152 allocated should be proportional to the number of nmbclusters used to
153 send data to a client via
155 Tune accordingly to avoid blocking!
156 Busy installations that make extensive use of
158 may want to increase these values to be inline with their
159 .Va kern.ipc.nmbclusters
166 buffers available is determined at boot time by either the
171 kernel configuration tunable.
177 .Va kern.ipc.nsfbufsused
179 .Va kern.ipc.nsfbufspeak
182 variables show current and peak
184 buffers usage respectively.
185 These values may also be viewed through
188 If a value of zero is reported for
189 .Va kern.ipc.nsfbufs ,
190 your architecture does not need to use
192 buffers because their task can be efficiently performed
193 by the generic virtual memory structures.
199 The socket is marked for non-blocking I/O and not all data was sent due to
200 the socket buffer being filled.
201 If specified, the number of bytes successfully sent will be returned in
207 is not a valid file descriptor.
212 is not a valid socket descriptor.
214 Completing the entire transfer would have required disk I/O, so
216 Partial data may have been sent.
217 (This error can only occur when
221 An invalid address was specified for an argument.
225 before it could be completed.
226 If specified, the number
227 of bytes successfully sent will be returned in
233 is not a regular file.
238 is not a SOCK_STREAM type socket.
245 An error occurred while reading from
251 points to an unconnected socket.
258 The file system for descriptor
263 The socket peer has closed the connection.
277 .%T A Portable Kernel Abstraction for Low-Overhead Ephemeral Mapping Management
278 .%J The Proceedings of the 2005 USENIX Annual Technical Conference
288 This manual page first appeared in
294 and this manual page were written by
295 .An David G. Lawrence Aq dg@dglawrence.com .