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1 .\" $OpenBSD: recv.2,v 1.43 2015/02/16 16:20:15 tedu Exp $
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31 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
32 .\"
33 .Dd October 25, 2016
34 .Dt RECV 2
35 .Os
36 .Sh NAME
37 .Nm recv ,
38 .Nm recvfrom ,
39 .Nm recvmsg
40 .Nd receive a message from a socket
41 .Sh SYNOPSIS
42 .In sys/socket.h
43 .Ft ssize_t
44 .Fn recv "int s" "void *buf" "size_t len" "int flags"
45 .Ft ssize_t
46 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "socklen_t *fromlen"
47 .Ft ssize_t
48 .Fn recvmsg "int s" "struct msghdr *msg" "int flags"
49 .Sh DESCRIPTION
50 .Fn recvfrom
51 and
52 .Fn recvmsg
53 are used to receive messages from a socket,
54 .Fa s ,
55 and may be used to receive
56 data on a socket whether or not it is connection-oriented.
57 .Pp
58 If
59 .Fa from
60 is non-null and the socket is not connection-oriented,
61 the source address of the message is filled in.
62 .Fa fromlen
63 is a value-result parameter, initialized to the size of
64 the buffer associated with
65 .Fa from ,
66 and modified on return to indicate the actual size of the
67 address stored there.
68 .Pp
69 The
70 .Fn recv
71 call is normally used only on a
72 .Em connected
73 socket (see
74 .Xr connect 2 )
75 and is identical to
76 .Fn recvfrom
77 with a null
78 .Fa from
79 parameter.
80 .Pp
81 On successful completion, all three routines return the number of
82 message bytes read.
83 If a message is too long to fit in the supplied
84 buffer, excess bytes may be discarded depending on the type of socket
85 the message is received from (see
86 .Xr socket 2 ) .
87 .Pp
88 If no messages are available at the socket, the
89 receive call waits for a message to arrive, unless
90 the socket is nonblocking (see
91 .Xr fcntl 2 )
92 in which case the value
93 \-1 is returned and the external variable
94 .Va errno
95 set to
96 .Er EAGAIN .
97 The receive calls normally return any data available,
98 up to the requested amount,
99 rather than waiting for receipt of the full amount requested;
100 this behavior is affected by the socket-level options
101 .Dv SO_RCVLOWAT
102 and
103 .Dv SO_RCVTIMEO
104 described in
105 .Xr getsockopt 2 .
106 .Pp
107 The
108 .Xr select 2
109 or
110 .Xr poll 2
111 system calls may be used to determine when more data arrive.
112 .Pp
113 The
114 .Fa flags
115 argument is the bitwise OR of zero or more of the following values:
116 .Pp
117 .Bl -tag -width "MSG_DONTWAITXX" -offset indent -compact
118 .It Dv MSG_OOB
119 process out-of-band data
120 .It Dv MSG_PEEK
121 peek at incoming message
122 .It Dv MSG_WAITALL
123 wait for full request or error
124 .It Dv MSG_DONTWAIT
125 don't block
126 .El
127 .Pp
128 The
129 .Dv MSG_OOB
130 flag requests receipt of out-of-band data
131 that would not be received in the normal data stream.
132 Some protocols place expedited data at the head of the normal
133 data queue, and thus this flag cannot be used with such protocols.
134 The
135 .Dv MSG_PEEK
136 flag causes the receive operation to return data
137 from the beginning of the receive queue without removing that
138 data from the queue.
139 Thus, a subsequent receive call will return the same data.
140 The
141 .Dv MSG_WAITALL
142 flag requests that the operation block until
143 the full request is satisfied.
144 However, the call may still return less data than requested
145 if a signal is caught, an error or disconnect occurs,
146 or the next data to be received is of a different type than that returned.
147 The
148 .Dv MSG_DONTWAIT
149 flag requests the call to return when it would block otherwise.
150 If no data is available,
151 .Va errno
152 is set to
153 .Er EAGAIN .
154 .Pp
155 The
156 .Fn recvmsg
157 call uses a
158 .Fa msghdr
159 structure to minimize the number of directly supplied parameters.
160 This structure has the following form, as defined in
161 .In sys/socket.h :
162 .Bd -literal
163 struct msghdr {
164 void *msg_name; /* optional address */
165 socklen_t msg_namelen; /* size of address */
166 struct iovec *msg_iov; /* scatter/gather array */
167 int msg_iovlen; /* # elements in msg_iov */
168 void *msg_control; /* ancillary data, see below */
169 socklen_t msg_controllen; /* ancillary data buffer len */
170 int msg_flags; /* flags on received message */
171 };
172 .Ed
173 .Pp
174 Here
175 .Fa msg_name
176 and
177 .Fa msg_namelen
178 specify the source address if the socket is unconnected;
179 .Fa msg_name
180 may be given as a null pointer if no names are desired or required.
181 .Fa msg_iov
182 and
183 .Fa msg_iovlen
184 describe scatter gather locations, as discussed in
185 .Xr read 2 .
186 .Fa msg_control ,
187 which has length
188 .Fa msg_controllen ,
189 points to a buffer for other protocol control related messages
190 or other miscellaneous ancillary data.
191 The messages are of the form:
192 .Bd -literal
193 struct cmsghdr {
194 socklen_t cmsg_len; /* data byte count, including hdr */
195 int cmsg_level; /* originating protocol */
196 int cmsg_type; /* protocol-specific type */
197 /* followed by u_char cmsg_data[]; */
198 };
199 .Ed
200 .Pp
201 See
202 .Xr CMSG_DATA 3
203 for how these messages are constructed and decomposed.
204 .Pp
205 Open file descriptors are now passed as ancillary data for
206 .Dv AF_UNIX
207 domain and
208 .Xr socketpair 2
209 sockets, with
210 .Fa cmsg_level
211 set to
212 .Dv SOL_SOCKET
213 and
214 .Fa cmsg_type
215 set to
216 .Dv SCM_RIGHTS .
217 .Pp
218 The
219 .Fa msg_flags
220 field is set on return according to the message received.
221 It will contain zero or more of the following values:
222 .Pp
223 .Bl -tag -width MSG_CTRUNC -offset indent -compact
224 .It Dv MSG_OOB
225 Returned to indicate that expedited or out-of-band data was received.
226 .It Dv MSG_EOR
227 Indicates end-of-record;
228 the data returned completed a record (generally used with sockets of type
229 .Dv SOCK_SEQPACKET ) .
230 .It Dv MSG_TRUNC
231 Indicates that
232 the trailing portion of a datagram was discarded because the datagram
233 was larger than the buffer supplied.
234 .It Dv MSG_CTRUNC
235 Indicates that some
236 control data were discarded due to lack of space in the buffer
237 for ancillary data.
238 .El
239 .Sh RETURN VALUES
240 These calls return the number of bytes received, or \-1 if an error occurred.
241 .Sh ERRORS
242 .Fn recv ,
243 .Fn recvfrom ,
244 and
245 .Fn recvmsg
246 fail if:
247 .Bl -tag -width "[EHOSTUNREACH]"
248 .It Bq Er EBADF
249 The argument
250 .Fa s
251 is an invalid descriptor.
252 .It Bq Er ENOTCONN
253 The socket is associated with a connection-oriented protocol
254 and has not been connected (see
255 .Xr connect 2
256 and
257 .Xr accept 2 ) .
258 .It Bq Er ENOTSOCK
259 The argument
260 .Fa s
261 does not refer to a socket.
262 .It Bq Er EAGAIN
263 The socket is marked non-blocking, and the receive operation
264 would block, or
265 a receive timeout had been set,
266 and the timeout expired before data were received.
267 .It Bq Er EINTR
268 The receive was interrupted by delivery of a signal before
269 any data were available.
270 .It Bq Er EFAULT
271 The receive buffer pointer(s) point outside the process's
272 address space.
273 .It Bq Er ECONNREFUSED
274 The socket is associated with a connection-oriented protocol
275 and the connection was forcefully rejected (see
276 .Xr connect 2 ) .
277 .It Bq Er ECONNRESET
278 The socket is associated with a connection-oriented protocol
279 and the connection was forcefully closed by the peer.
280 .El
281 .Pp
282 In addition,
283 .Fn recv
284 and
285 .Fn recvfrom
286 may return the following error:
287 .Bl -tag -width Er
288 .It Bq Er EINVAL
289 .Fa len
290 was larger than
291 .Dv SSIZE_MAX .
292 .El
293 .Pp
294 And
295 .Fn recvmsg
296 may return one of the following errors:
297 .Bl -tag -width Er
298 .It Bq Er EINVAL
299 The sum of the
300 .Fa iov_len
301 values in the
302 .Fa msg_iov
303 array overflowed an
304 .Em ssize_t .
305 .It Bq Er EMSGSIZE
306 The
307 .Fa msg_iovlen
308 member of
309 .Fa msg
310 was less than 0 or larger than
311 .Dv IOV_MAX .
312 .El
313 .Sh SEE ALSO
314 .Xr connect 2 ,
315 .Xr fcntl 2 ,
316 .Xr getsockopt 2 ,
317 .Xr poll 2 ,
318 .Xr read 2 ,
319 .Xr select 2 ,
320 .Xr socket 2 ,
321 .Xr socketpair 2 ,
322 .Xr CMSG_DATA 3 ,
323 .Xr sockatmark 3
324 .Sh STANDARDS
325 The
326 .Fn recv ,
327 .Fn recvfrom ,
328 and
329 .Fn recvmsg
330 functions conform to
331 .St -p1003.1-2008 .
332 The
333 .Dv MSG_DONTWAIT
334 flag is an extension to that specification.
335 .Sh HISTORY
336 The
337 .Fn recv
338 function call appeared in
339 .Bx 4.2 .