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28 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
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 * restrict 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
86 As it is redundant, it may not be supported in future releases.
88 All three routines return the length of the message on successful
90 If a message is too long to fit in the supplied buffer,
91 excess bytes may be discarded depending on the type of socket
92 the message is received from (see
95 If no messages are available at the socket, the
96 receive call waits for a message to arrive, unless
97 the socket is non-blocking (see
99 in which case the value
100 \-1 is returned and the global variable
104 The receive calls normally return any data available,
105 up to the requested amount,
106 rather than waiting for receipt of the full amount requested;
107 this behavior is affected by the socket-level options
116 system call may be used to determine when more data arrives.
122 function is formed by
124 one or more of the values:
125 .Bl -column ".Dv MSG_DONTWAIT" -offset indent
126 .It Dv MSG_OOB Ta process out-of-band data
127 .It Dv MSG_PEEK Ta peek at incoming message
128 .It Dv MSG_WAITALL Ta wait for full request or error
129 .It Dv MSG_DONTWAIT Ta do not block
134 flag requests receipt of out-of-band data
135 that would not be received in the normal data stream.
136 Some protocols place expedited data at the head of the normal
137 data queue, and thus this flag cannot be used with such protocols.
140 flag causes the receive operation to return data
141 from the beginning of the receive queue without removing that
143 Thus, a subsequent receive call will return the same data.
146 flag requests that the operation block until
147 the full request is satisfied.
148 However, the call may still return less data than requested
149 if a signal is caught, an error or disconnect occurs,
150 or the next data to be received is of a different type than that returned.
153 flag requests the call to return when it would block otherwise.
154 If no data is available,
158 This flag is not available in strict
160 or C99 compilation mode.
166 structure to minimize the number of directly supplied arguments.
167 This structure has the following form, as defined in
172 void *msg_name; /* optional address */
173 socklen_t msg_namelen; /* size of address */
174 struct iovec *msg_iov; /* scatter/gather array */
175 int msg_iovlen; /* # elements in msg_iov */
176 void *msg_control; /* ancillary data, see below */
177 socklen_t msg_controllen;/* ancillary data buffer len */
178 int msg_flags; /* flags on received message */
186 specify the destination address if the socket is unconnected;
188 may be given as a null pointer if no names are desired or required.
194 describe scatter gather locations, as discussed in
201 points to a buffer for other protocol control related messages
202 or other miscellaneous ancillary data.
203 The messages are of the form:
206 socklen_t cmsg_len; /* data byte count, including hdr */
207 int cmsg_level; /* originating protocol */
208 int cmsg_type; /* protocol-specific type */
210 u_char cmsg_data[]; */
214 As an example, one could use this to learn of changes in the data-stream
215 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
218 with no data buffer provided immediately after an
222 Open file descriptors are now passed as ancillary data for
233 Process credentials can also be passed as ancillary data for
235 domain sockets using a
241 should be a structure of type
249 pid_t cmcred_pid; /* PID of sending process */
250 uid_t cmcred_uid; /* real UID of sending process */
251 uid_t cmcred_euid; /* effective UID of sending process */
252 gid_t cmcred_gid; /* real GID of sending process */
253 short cmcred_ngroups; /* number or groups */
254 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
258 The kernel will fill in the credential information of the sending process
259 and deliver it to the receiver.
263 field is set on return according to the message received.
265 indicates end-of-record;
266 the data returned completed a record (generally used with sockets of type
267 .Dv SOCK_SEQPACKET ) .
270 the trailing portion of a datagram was discarded because the datagram
271 was larger than the buffer supplied.
274 control data were discarded due to lack of space in the buffer
277 is returned to indicate that expedited or out-of-band data were received.
279 These calls return the number of bytes received, or -1
280 if an error occurred.
287 is an invalid descriptor.
289 The remote socket end is forcibly closed.
291 The socket is associated with a connection-oriented protocol
292 and has not been connected (see
299 does not refer to a socket.
304 was used to receive rights (file descriptors) that were in flight on the
306 However, the receiving program did not have enough free file
307 descriptor slots to accept them.
308 In this case the descriptors are
309 closed, any pending data can be returned by another call to
312 The socket is marked non-blocking, and the receive operation
314 a receive timeout had been set,
315 and the timeout expired before data were received.
317 The receive was interrupted by delivery of a signal before
318 any data were available.
320 The receive buffer pointer(s) point outside the process's