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28 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
31 .Dd September 12, 2012
38 .Nd receive a message from a socket
45 .Fn recv "int s" "void *buf" "size_t len" "int flags"
47 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen"
49 .Fn recvmsg "int s" "struct msghdr *msg" "int flags"
56 are used to receive messages from a socket,
57 and may be used to receive data on a socket whether or not
58 it is connection-oriented.
63 and the socket is not connection-oriented,
64 the source address of the message is filled in.
68 is a value-result argument, initialized to the size of
69 the buffer associated with
71 and modified on return to indicate the actual size of the
76 function is normally used only on a
83 null pointer passed as its
87 All three routines return the length of the message on successful
89 If a message is too long to fit in the supplied buffer,
90 excess bytes may be discarded depending on the type of socket
91 the message is received from (see
94 If no messages are available at the socket, the
95 receive call waits for a message to arrive, unless
96 the socket is non-blocking (see
98 in which case the value
99 \-1 is returned and the global variable
103 The receive calls normally return any data available,
104 up to the requested amount,
105 rather than waiting for receipt of the full amount requested;
106 this behavior is affected by the socket-level options
115 system call may be used to determine when more data arrives.
121 function is formed by
123 one or more of the values:
124 .Bl -column ".Dv MSG_DONTWAIT" -offset indent
125 .It Dv MSG_OOB Ta process out-of-band data
126 .It Dv MSG_PEEK Ta peek at incoming message
127 .It Dv MSG_WAITALL Ta wait for full request or error
128 .It Dv MSG_DONTWAIT Ta do not block
133 flag requests receipt of out-of-band data
134 that would not be received in the normal data stream.
135 Some protocols place expedited data at the head of the normal
136 data queue, and thus this flag cannot be used with such protocols.
139 flag causes the receive operation to return data
140 from the beginning of the receive queue without removing that
142 Thus, a subsequent receive call will return the same data.
145 flag requests that the operation block until
146 the full request is satisfied.
147 However, the call may still return less data than requested
148 if a signal is caught, an error or disconnect occurs,
149 or the next data to be received is of a different type than that returned.
152 flag requests the call to return when it would block otherwise.
153 If no data is available,
157 This flag is not available in strict
159 or C99 compilation mode.
165 structure to minimize the number of directly supplied arguments.
166 This structure has the following form, as defined in
170 void *msg_name; /* optional address */
171 socklen_t msg_namelen; /* size of address */
172 struct iovec *msg_iov; /* scatter/gather array */
173 int msg_iovlen; /* # elements in msg_iov */
174 void *msg_control; /* ancillary data, see below */
175 socklen_t msg_controllen;/* ancillary data buffer len */
176 int msg_flags; /* flags on received message */
184 specify the destination address if the socket is unconnected;
186 may be given as a null pointer if no names are desired or required.
192 describe scatter gather locations, as discussed in
199 points to a buffer for other protocol control related messages
200 or other miscellaneous ancillary data.
201 The messages are of the form:
204 socklen_t cmsg_len; /* data byte count, including hdr */
205 int cmsg_level; /* originating protocol */
206 int cmsg_type; /* protocol-specific type */
208 u_char cmsg_data[]; */
212 As an example, one could use this to learn of changes in the data-stream
213 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
216 with no data buffer provided immediately after an
220 Open file descriptors are now passed as ancillary data for
231 Process credentials can also be passed as ancillary data for
233 domain sockets using a
239 should be a structure of type
246 pid_t cmcred_pid; /* PID of sending process */
247 uid_t cmcred_uid; /* real UID of sending process */
248 uid_t cmcred_euid; /* effective UID of sending process */
249 gid_t cmcred_gid; /* real GID of sending process */
250 short cmcred_ngroups; /* number or groups */
251 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
255 If a sender supplies ancillary data with enough space for the above struct
258 control message type to the
260 system call, then kernel will fill in the credential information of the
261 sending process and deliver it to the receiver.
262 Since receiver usually has no control over a sender, this method of retrieving
263 credential information isn't reliable.
264 For reliable retrieval of remote side credentials it is advised to use the
266 socket option on the receiving socket.
273 field is set on return according to the message received.
275 indicates end-of-record;
276 the data returned completed a record (generally used with sockets of type
277 .Dv SOCK_SEQPACKET ) .
280 the trailing portion of a datagram was discarded because the datagram
281 was larger than the buffer supplied.
284 control data were discarded due to lack of space in the buffer
287 is returned to indicate that expedited or out-of-band data were received.
289 These calls return the number of bytes received, or -1
290 if an error occurred.
297 is an invalid descriptor.
299 The remote socket end is forcibly closed.
301 The socket is associated with a connection-oriented protocol
302 and has not been connected (see
309 does not refer to a socket.
314 was used to receive rights (file descriptors) that were in flight on the
316 However, the receiving program did not have enough free file
317 descriptor slots to accept them.
318 In this case the descriptors are
319 closed, any pending data can be returned by another call to
322 The socket is marked non-blocking, and the receive operation
324 a receive timeout had been set,
325 and the timeout expired before data were received.
327 The receive was interrupted by delivery of a signal before
328 any data were available.
330 The receive buffer pointer(s) point outside the process's