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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 *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_CMSG_CLOEXEC" -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
129 .It Dv MSG_CMSG_CLOEXEC Ta set received fds close-on-exec
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
171 void *msg_name; /* optional address */
172 socklen_t msg_namelen; /* size of address */
173 struct iovec *msg_iov; /* scatter/gather array */
174 int msg_iovlen; /* # elements in msg_iov */
175 void *msg_control; /* ancillary data, see below */
176 socklen_t msg_controllen;/* ancillary data buffer len */
177 int msg_flags; /* flags on received message */
185 specify the destination address if the socket is unconnected;
187 may be given as a null pointer if no names are desired or required.
193 describe scatter gather locations, as discussed in
200 points to a buffer for other protocol control related messages
201 or other miscellaneous ancillary data.
202 The messages are of the form:
205 socklen_t cmsg_len; /* data byte count, including hdr */
206 int cmsg_level; /* originating protocol */
207 int cmsg_type; /* protocol-specific type */
209 u_char cmsg_data[]; */
213 As an example, one could use this to learn of changes in the data-stream
214 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
217 with no data buffer provided immediately after an
221 Open file descriptors are now passed as ancillary data for
231 The close-on-exec flag on received descriptors is set according to the
236 Process credentials can also be passed as ancillary data for
238 domain sockets using a
244 should be a structure of type
251 pid_t cmcred_pid; /* PID of sending process */
252 uid_t cmcred_uid; /* real UID of sending process */
253 uid_t cmcred_euid; /* effective UID of sending process */
254 gid_t cmcred_gid; /* real GID of sending process */
255 short cmcred_ngroups; /* number or groups */
256 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
260 If a sender supplies ancillary data with enough space for the above struct
263 control message type to the
265 system call, then kernel will fill in the credential information of the
266 sending process and deliver it to the receiver.
267 Since receiver usually has no control over a sender, this method of retrieving
268 credential information isn't reliable.
269 For reliable retrieval of remote side credentials it is advised to use the
271 socket option on the receiving socket.
278 field is set on return according to the message received.
280 indicates end-of-record;
281 the data returned completed a record (generally used with sockets of type
282 .Dv SOCK_SEQPACKET ) .
285 the trailing portion of a datagram was discarded because the datagram
286 was larger than the buffer supplied.
289 control data were discarded due to lack of space in the buffer
292 is returned to indicate that expedited or out-of-band data were received.
294 These calls return the number of bytes received, or -1
295 if an error occurred.
302 is an invalid descriptor.
304 The remote socket end is forcibly closed.
306 The socket is associated with a connection-oriented protocol
307 and has not been connected (see
314 does not refer to a socket.
319 was used to receive rights (file descriptors) that were in flight on the
321 However, the receiving program did not have enough free file
322 descriptor slots to accept them.
323 In this case the descriptors are
324 closed, any pending data can be returned by another call to
327 The socket is marked non-blocking, and the receive operation
329 a receive timeout had been set,
330 and the timeout expired before data were received.
332 The receive was interrupted by delivery of a signal before
333 any data were available.
335 The receive buffer pointer(s) point outside the process's