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10 .TH RECVMSG 3XNET "Feb 27, 2006"
11 .SH NAME
12 recvmsg \- receive a message from a socket
13 .SH SYNOPSIS
14 .LP
15 .nf
16 \fBcc\fR [ \fIflag\fR ... ] \fIfile\fR ... \fB-lxnet\fR [ \fIlibrary\fR ... ]
17 #include <sys/socket.h>
19 \fBssize_t\fR \fBrecvmsg\fR(\fBint\fR \fIsocket\fR, \fBstruct msghdr *\fR\fImessage\fR, \fBint\fR \fIflags\fR);
20 .fi
22 .SH DESCRIPTION
23 .sp
24 .LP
25 The \fBrecvmsg()\fR function receives a message from a connection-mode or
26 connectionless-mode socket. It is normally used with connectionless-mode
27 sockets because it permits the application to retrieve the source address of
28 received data.
29 .sp
30 .LP
31 The \fBrecvmsg()\fR function receives messages from unconnected or connected
32 sockets and returns the length of the message.
33 .sp
34 .LP
35 The \fBrecvmsg()\fR function returns the total length of the message. For
36 message-based sockets such as \fBSOCK_DGRAM\fR and \fBSOCK_SEQPACKET\fR, the
37 entire message must be read in a single operation. If a message is too long to
38 fit in the supplied buffers, and \fBMSG_PEEK\fR is not set in the \fIflags\fR
39 argument, the excess bytes are discarded, and \fBMSG_TRUNC\fR is set in the
40 \fBmsg_flags\fR member of the \fBmsghdr\fR structure. For stream-based sockets
41 such as \fBSOCK_STREAM\fR, message boundaries are ignored. In this case, data
42 is returned to the user as soon as it becomes available, and no data is
43 discarded.
44 .sp
45 .LP
46 If the \fBMSG_WAITALL\fR flag is not set, data will be returned only up to the
47 end of the first message.
48 .sp
49 .LP
50 If no messages are available at the socket, and \fBO_NONBLOCK\fR is not set on
51 the socket's file descriptor, \fBrecvmsg()\fR blocks until a message arrives.
52 If no messages are available at the socket and \fBO_NONBLOCK\fR is set on the
53 socket's file descriptor, the \fBrecvmsg()\fR function fails and sets
54 \fBerrno\fR to \fBEAGAIN\fR or \fBEWOULDBLOCK\fR.
55 .sp
56 .LP
57 In the \fBmsghdr\fR structure, defined in \fBsocket.h\fR(3HEAD), the
58 \fBmsg_name\fR and \fBmsg_namelen\fR members specify the source address if the
59 socket is unconnected. If the socket is connected, the \fBmsg_name\fR and
60 \fBmsg_namelen\fR members are ignored. The \fBmsg_name\fR member may be a null
61 pointer if no names are desired or required.
62 .sp
63 .LP
64 The \fBmsg_control\fR and \fBmsg_controllen\fR members specify a buffer to
65 receive ancillary data sent along with a message. Ancillary data consists of a
66 sequence of pairs. Each pair is composed of a \fBcmsghdr\fR structure followed
67 by a data array. The \fBcmsghdr\fR structure, defined in \fBsocket.h\fR(3HEAD),
68 contains descriptive information which allows an application to correctly parse
69 data. The data array contains the ancillary data message.
70 .sp
71 .LP
72 If ancillary data is not transferred, \fBmsg_control\fR is set to \fBNULL\fR
73 and \fBmsg_controllen\fR is set to \fB0\fR.
74 .sp
75 .LP
76 The \fBmsg_iov\fR and \fBmsg_iovlen\fR fields of the \fBmsghdr\fR structure are
77 used to specify where the received data will be stored. \fBmsg_iov\fR points to
78 an array of \fBiovec\fR structures. The \fBmsg_iovlen\fR must be set to the
79 dimension of this array. In each \fBiovec\fR structure, the \fBiov_base\fR
80 field specifies a storage area and the \fBiov_len\fR field gives its size in
81 bytes. Each storage area indicated by \fBmsg_iov\fR is filled with received
82 data in turn until all of the received data is stored or all of the areas have
83 been filled.
84 .sp
85 .LP
86 If the \fBSO_TIMESTAMP\fR option has been enabled through \fBsetsockopt()\fR,
87 then a struct \fBtimeval\fR is returned following the \fBcmsghdr\fR, and the
88 \fBcmsg_len\fR field of the \fBcmsghdr\fR indicates the size of the struct
89 \fBtimeval\fR.
90 .sp
91 .LP
92 On successful completion, the \fBmsg_flags\fR member of the message header is
93 the bitwise-inclusive OR of all of the following flags that indicate conditions
94 detected for the received message:
95 .sp
96 .ne 2
97 .na
98 \fB\fBMSG_EOR\fR\fR
99 .ad
100 .RS 14n
101 End of record was received (if supported by the protocol).
102 .RE
104 .sp
105 .ne 2
106 .na
107 \fB\fBMSG_OOB\fR\fR
108 .ad
109 .RS 14n
110 Out-of-band data was received.
111 .RE
113 .sp
114 .ne 2
115 .na
116 \fB\fBMSG_TRUNC\fR\fR
117 .ad
118 .RS 14n
119 Normal data was truncated.
120 .RE
122 .sp
123 .ne 2
124 .na
125 \fB\fBMSG_CTRUNC\fR\fR
126 .ad
127 .RS 14n
128 Control data was truncated.
129 .RE
131 .SH PARAMETERS
132 .sp
133 .LP
134 The function takes the following arguments:
135 .sp
136 .ne 2
137 .na
138 \fB\fIsocket\fR\fR
139 .ad
140 .RS 11n
141 Specifies the socket file descriptor.
142 .RE
144 .sp
145 .ne 2
146 .na
147 \fB\fImessage\fR\fR
148 .ad
149 .RS 11n
150 Points to a \fBmsghdr\fR structure, containing both the buffer to store the
151 source address and the buffers for the incoming message. The length and format
152 of the address depend on the address family of the socket. The \fBmsg_flags\fR
153 member is ignored on input, but may contain meaningful values on output.
154 .RE
156 .sp
157 .ne 2
158 .na
159 \fB\fIflags\fR\fR
160 .ad
161 .RS 11n
162 Specifies the type of message reception. Values of this argument are formed by
163 logically OR'ing zero or more of the following values:
164 .sp
165 .ne 2
166 .na
167 \fB\fBMSG_OOB\fR\fR
168 .ad
169 .RS 15n
170 Requests out-of-band data. The significance and semantics of out-of-band data
171 are protocol-specific.
172 .RE
174 .sp
175 .ne 2
176 .na
177 \fB\fBMSG_PEEK\fR\fR
178 .ad
179 .RS 15n
180 Peeks at the incoming message.
181 .RE
183 .sp
184 .ne 2
185 .na
186 \fB\fBMSG_WAITALL\fR\fR
187 .ad
188 .RS 15n
189 Requests that the function block until the full amount of data requested can be
190 returned. The function may return a smaller amount of data if a signal is
191 caught, if the connection is terminated, if \fBMSG_PEEK\fR was specified, or if
192 an error is pending for the socket.
193 .RE
195 .RE
197 .SH USAGE
198 .sp
199 .LP
200 The \fBselect\fR(3C) and \fBpoll\fR(2) functions can be used to determine when
201 data is available to be received.
202 .SH RETURN VALUES
203 .sp
204 .LP
205 Upon successful completion, \fBrecvmsg()\fR returns the length of the message
206 in bytes. If no messages are available to be received and the peer has
207 performed an orderly shutdown, \fBrecvmsg()\fR returns \fB0\fR. Otherwise,
208 \fB\(mi1\fR is returned and \fBerrno\fR is set to indicate the error.
209 .SH ERRORS
210 .sp
211 .LP
212 The \fBrecvmsg()\fR function will fail if:
213 .sp
214 .ne 2
215 .na
216 \fB\fBEAGAIN\fR\fR
217 .ad
218 .br
219 .na
220 \fB\fBEWOULDBLOCK\fR\fR
221 .ad
222 .RS 15n
223 The socket's file descriptor is marked \fBO_NONBLOCK\fR and no data is waiting
224 to be received; or \fBMSG_OOB\fR is set and no out-of-band data is available
225 and either the socket's file descriptor is marked \fBO_NONBLOCK\fR or the
226 socket does not support blocking to await out-of-band data.
227 .RE
229 .sp
230 .ne 2
231 .na
232 \fB\fBEBADF\fR\fR
233 .ad
234 .RS 15n
235 The \fIsocket\fR argument is not a valid open file descriptor.
236 .RE
238 .sp
239 .ne 2
240 .na
241 \fB\fBECONNRESET\fR\fR
242 .ad
243 .RS 15n
244 A connection was forcibly closed by a peer.
245 .RE
247 .sp
248 .ne 2
249 .na
250 \fB\fBEFAULT\fR\fR
251 .ad
252 .RS 15n
253 The \fImessage\fR parameter, or storage pointed to by the \fImsg_name\fR,
254 \fImsg_control\fR or \fImsg_iov\fR fields of the \fImessage\fR parameter, or
255 storage pointed to by the \fBiovec\fR structures pointed to by the
256 \fImsg_iov\fR field can not be accessed or written.
257 .RE
259 .sp
260 .ne 2
261 .na
262 \fB\fBEINTR\fR\fR
263 .ad
264 .RS 15n
265 This function was interrupted by a signal before any data was available.
266 .RE
268 .sp
269 .ne 2
270 .na
271 \fB\fBEINVAL\fR\fR
272 .ad
273 .RS 15n
274 The sum of the \fBiov_len\fR values overflows an \fBssize_t\fR. or the MSG_OOB
275 flag is set and no out-of-band data is available.
276 .RE
278 .sp
279 .ne 2
280 .na
281 \fB\fBEMSGSIZE\fR\fR
282 .ad
283 .RS 15n
284 The \fBmsg_iovlen\fR member of the \fBmsghdr\fR structure pointed to by
285 \fImessage\fR is less than or equal to 0, or is greater than \fBIOV_MAX\fR.
286 .RE
288 .sp
289 .ne 2
290 .na
291 \fB\fBENOTCONN\fR\fR
292 .ad
293 .RS 15n
294 A receive is attempted on a connection-mode socket that is not connected.
295 .RE
297 .sp
298 .ne 2
299 .na
300 \fB\fBENOTSOCK\fR\fR
301 .ad
302 .RS 15n
303 The \fIsocket\fR argument does not refer to a socket.
304 .RE
306 .sp
307 .ne 2
308 .na
309 \fB\fBEOPNOTSUPP\fR\fR
310 .ad
311 .RS 15n
312 The specified flags are not supported for this socket type.
313 .RE
315 .sp
316 .ne 2
317 .na
318 \fB\fBETIMEDOUT\fR\fR
319 .ad
320 .RS 15n
321 The connection timed out during connection establishment, or due to a
322 transmission timeout on active connection.
323 .RE
325 .sp
326 .LP
327 The \fBrecvmsg()\fR function may fail if:
328 .sp
329 .ne 2
330 .na
331 \fB\fBEIO\fR\fR
332 .ad
333 .RS 11n
334 An \fBIO\fR error occurred while reading from or writing to the file system.
335 .RE
337 .sp
338 .ne 2
339 .na
340 \fB\fBENOBUFS\fR\fR
341 .ad
342 .RS 11n
343 Insufficient resources were available in the system to perform the operation.
344 .RE
346 .sp
347 .ne 2
348 .na
349 \fB\fBENOMEM\fR\fR
350 .ad
351 .RS 11n
352 Insufficient memory was available to fulfill the request.
353 .RE
355 .sp
356 .ne 2
357 .na
358 \fB\fBENOSR\fR\fR
359 .ad
360 .RS 11n
361 There were insufficient STREAMS resources available for the operation to
362 complete.
363 .RE
365 .SH ATTRIBUTES
366 .sp
367 .LP
368 See \fBattributes\fR(5) for descriptions of the following attributes:
369 .sp
371 .sp
372 .TS
373 box;
374 c | c
375 l | l .
376 ATTRIBUTE TYPE ATTRIBUTE VALUE
377 _
378 Interface Stability Standard
379 _
380 MT-Level MT-Safe
381 .TE
383 .SH SEE ALSO
384 .sp
385 .LP
386 \fBpoll\fR(2), \fBrecv\fR(3XNET), \fBrecvfrom\fR(3XNET), \fBselect\fR(3C),
387 \fBsend\fR(3XNET), \fBsendmsg\fR(3XNET), \fBsendto\fR(3XNET),
388 \fBsetsockopt\fR(3XNET), \fBshutdown\fR(3XNET), \fBsocket\fR(3XNET),
389 \fBsocket.h\fR(3HEAD), \fBattributes\fR(5), \fBstandards\fR(5)