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30 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
39 .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"
54 are used to receive messages from a socket,
55 and may be used to receive data on a socket whether or not
56 it is connection-oriented.
60 is non-nil, and the socket is not connection-oriented,
61 the source address of the message is filled in.
63 is a value-result parameter, initialized to the size of
64 the buffer associated with
66 and modified on return to indicate the actual size of the
71 call is normally used only on a
80 As it is redundant, it may not be supported in future releases.
82 All three routines return the length of the message on successful
84 If a message is too long to fit in the supplied buffer,
85 excess bytes may be discarded depending on the type of socket
86 the message is received from (see
89 If no messages are available at the socket, the
90 receive call waits for a message to arrive, unless
91 the socket is nonblocking (see
93 in which case the value
94 \-1 is returned and the external variable
98 The receive calls normally return any data available,
99 up to the requested amount,
100 rather than waiting for receipt of the full amount requested;
101 this behavior is affected by the socket-level options
112 call may be used to determine when more data arrive.
116 argument to a recv call is formed by
118 one or more of the values:
119 .Bl -column MSG_WAITALL -offset indent
120 .It Dv MSG_OOB Ta process out-of-band data
121 .It Dv MSG_PEEK Ta peek at incoming message
122 .It Dv MSG_WAITALL Ta wait for full request or error
126 flag requests receipt of out-of-band data
127 that would not be received in the normal data stream.
128 Some protocols place expedited data at the head of the normal
129 data queue, and thus this flag cannot be used with such protocols.
132 flag causes the receive operation to return data
133 from the beginning of the receive queue without removing that
135 Thus, a subsequent receive call will return the same data.
138 flag requests that the operation block until
139 the full request is satisfied.
140 However, the call may still return less data than requested
141 if a signal is caught, an error or disconnect occurs,
142 or the next data to be received is of a different type than that returned.
148 structure to minimize the number of directly supplied parameters.
149 This structure has the following form, as defined in
150 .Ao Pa sys/socket.h Ac :
154 void *msg_name; /* optional address */
155 socklen_t msg_namelen; /* size of address */
156 struct iovec *msg_iov; /* scatter/gather array */
157 int msg_iovlen; /* # elements in msg_iov */
158 void *msg_control; /* ancillary data, see below */
159 socklen_t msg_controllen; /* ancillary data buffer len */
160 int msg_flags; /* flags on received message */
168 specify the source address if the socket is unconnected;
170 may be given as a null pointer if no names are desired or required.
171 If the socket is connected,
179 describe scatter gather locations, as discussed in
184 points to a buffer for other protocol control related messages
185 or other miscellaneous ancillary data.
186 The messages are of the form:
189 socklen_t cmsg_len; /* data byte count, including hdr */
190 int cmsg_level; /* originating protocol */
191 int cmsg_type; /* protocol-specific type */
193 u_char cmsg_data[]; */
196 As an example, one could use this to learn of changes in the data-stream
197 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
198 a recvmsg with no data buffer provided immediately after an
202 Open file descriptors are now passed as ancillary data for
215 field is set on return according to the message received.
217 indicates end-of-record;
218 the data returned completed a record (generally used with sockets of type
219 .Dv SOCK_SEQPACKET ) .
222 the trailing portion of a datagram was discarded because the datagram
223 was larger than the buffer supplied.
226 control data were discarded due to lack of space in the buffer
229 is returned to indicate that expedited or out-of-band data were received.
231 These calls return the number of bytes received, or \-1
232 if an error occurred.
239 is an invalid descriptor.
241 The socket is associated with a connection-oriented protocol
242 and has not been connected (see
249 does not refer to a socket.
251 The socket is marked non-blocking, and the receive operation
253 a receive timeout had been set,
254 and the timeout expired before data were received.
256 The receive was interrupted by delivery of a signal before
257 any data were available.
259 The receive buffer pointer(s) point outside the process's
262 The total length of the I/O is more than can be expressed by the ssize_t
274 structure is less than or equal to 0
288 function call appeared in