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28 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
38 .Nd receive message(s) from a socket
44 .Fn recv "int s" "void *buf" "size_t len" "int flags"
46 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen"
48 .Fn recvmsg "int s" "struct msghdr *msg" "int flags"
50 .Fn recvmmsg "int s" "struct mmsghdr * restrict msgvec" "size_t vlen" "int flags" "const struct timespec * restrict timeout"
58 are used to receive messages from a socket,
59 and may be used to receive data on a socket whether or not
60 it is connection-oriented.
65 and the socket is not connection-oriented,
66 the source address of the message is filled in.
70 is a value-result argument, initialized to the size of
71 the buffer associated with
73 and modified on return to indicate the actual size of the
78 function is normally used only on a
85 null pointer passed as its
91 function is used to receive multiple
93 Their number is supplied by
95 The messages are placed in the buffers described by
97 vector, after reception.
98 The size of each received message is placed in the
100 field of each element of the vector.
103 is NULL the call blocks until the data is available for each
104 supplied message buffer.
105 Otherwise it waits for data for the specified amount of time.
106 If the timeout expired and there is no data received,
107 a value 0 is returned.
110 system call is used to implement the timeout mechanism,
111 before first receive is performed.
118 return the length of the message on successful
121 returns the number of received messages.
122 If a message is too long to fit in the supplied buffer,
123 excess bytes may be discarded depending on the type of socket
124 the message is received from (see
127 If no messages are available at the socket, the
128 receive call waits for a message to arrive, unless
129 the socket is non-blocking (see
131 in which case the value
132 \-1 is returned and the global variable
136 The receive calls except
138 normally return any data available,
139 up to the requested amount,
140 rather than waiting for receipt of the full amount requested;
141 this behavior is affected by the socket-level options
149 function implements this behaviour for each message in the vector.
153 system call may be used to determine when more data arrives.
159 function is formed by
161 one or more of the values:
162 .Bl -column ".Dv MSG_CMSG_CLOEXEC" -offset indent
163 .It Dv MSG_OOB Ta process out-of-band data
164 .It Dv MSG_PEEK Ta peek at incoming message
165 .It Dv MSG_TRUNC Ta return real packet or datagram length
166 .It Dv MSG_WAITALL Ta wait for full request or error
167 .It Dv MSG_DONTWAIT Ta do not block
168 .It Dv MSG_CMSG_CLOEXEC Ta set received fds close-on-exec
169 .It Dv MSG_WAITFORONE Ta do not block after receiving the first message
177 flag requests receipt of out-of-band data
178 that would not be received in the normal data stream.
179 Some protocols place expedited data at the head of the normal
180 data queue, and thus this flag cannot be used with such protocols.
183 flag causes the receive operation to return data
184 from the beginning of the receive queue without removing that
186 Thus, a subsequent receive call will return the same data.
189 flag causes the receive operation to return the full length of the packet
190 or datagram even if larger than provided buffer. The flag is supported
191 on SOCK_DGRAM sockets for
200 flag requests that the operation block until
201 the full request is satisfied.
202 However, the call may still return less data than requested
203 if a signal is caught, an error or disconnect occurs,
204 or the next data to be received is of a different type than that returned.
207 flag requests the call to return when it would block otherwise.
208 If no data is available,
212 This flag is not available in
219 flag sets MSG_DONTWAIT after the first message has been received.
220 This flag is only relevant for
227 structure to minimize the number of directly supplied arguments.
228 This structure has the following form, as defined in
232 void *msg_name; /* optional address */
233 socklen_t msg_namelen; /* size of address */
234 struct iovec *msg_iov; /* scatter/gather array */
235 int msg_iovlen; /* # elements in msg_iov */
236 void *msg_control; /* ancillary data, see below */
237 socklen_t msg_controllen;/* ancillary data buffer len */
238 int msg_flags; /* flags on received message */
246 specify the source address if the socket is unconnected;
248 may be given as a null pointer if no names are desired or required.
254 describe scatter gather locations, as discussed in
261 points to a buffer for other protocol control related messages
262 or other miscellaneous ancillary data.
263 The messages are of the form:
266 socklen_t cmsg_len; /* data byte count, including hdr */
267 int cmsg_level; /* originating protocol */
268 int cmsg_type; /* protocol-specific type */
270 u_char cmsg_data[]; */
274 As an example, the SO_TIMESTAMP socket option returns a reception
275 timestamp for UDP packets.
279 domain sockets, ancillary data can be used to pass file descriptors and
287 field is set on return according to the message received.
289 indicates end-of-record;
290 the data returned completed a record (generally used with sockets of type
291 .Dv SOCK_SEQPACKET ) .
294 the trailing portion of a datagram was discarded because the datagram
295 was larger than the buffer supplied.
298 control data were discarded due to lack of space in the buffer
301 is returned to indicate that expedited or out-of-band data were received.
307 structure, defined as follows in the
312 struct msghdr msg_hdr; /* message header */
313 ssize_t msg_len; /* message length */
317 On data reception the
319 field is updated to the length of the received message.
323 return the number of bytes received.
325 returns the number of messages received.
326 A value of -1 is returned if an error occurred.
333 is an invalid descriptor.
335 The remote socket end is forcibly closed.
337 The socket is associated with a connection-oriented protocol
338 and has not been connected (see
345 does not refer to a socket.
350 was used to receive rights (file descriptors) that were in flight on the
352 However, the receiving program did not have enough free file
353 descriptor slots to accept them.
354 In this case the descriptors are closed, with pending data either discarded
355 in the case of the unreliable datagram protocol or preserved in the case of a
357 The pending data can be retrieved with another call to
364 structure pointed to by
366 is less than or equal to 0, or is greater than
369 The socket is marked non-blocking and the receive operation
371 a receive timeout had been set
372 and the timeout expired before data were received.
374 The receive was interrupted by delivery of a signal before
375 any data were available.
377 The receive buffer pointer(s) point outside the process's
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