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32 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
41 .Nd receive a message from a socket
43 .Fd #include <sys/types.h>
44 .Fd #include <sys/socket.h>
46 .Fn recv "int s" "void *buf" "size_t len" "int flags"
48 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "int *fromlen"
50 .Fn recvmsg "int s" "struct msghdr *msg" "int flags"
55 are used to receive messages from a socket,
56 and may be used to receive data on a socket whether or not
57 it is connection-oriented.
61 is non-nil, and the socket is not connection-oriented,
62 the source address of the message is filled in.
64 is a value-result parameter, initialized to the size of
65 the buffer associated with
67 and modified on return to indicate the actual size of the
72 call is normally used only on a
81 As it is redundant, it may not be supported in future releases.
83 All three routines return the length of the message on successful
85 If a message is too long to fit in the supplied buffer,
86 excess bytes may be discarded depending on the type of socket
87 the message is received from (see
90 If no messages are available at the socket, the
91 receive call waits for a message to arrive, unless
92 the socket is nonblocking (see
94 in which case the value
95 -1 is returned and the external variable
99 The receive calls normally return any data available,
100 up to the requested amount,
101 rather than waiting for receipt of the full amount requested;
102 this behavior is affected by the socket-level options
111 call may be used to determine when more data arrive.
115 argument to a recv call is formed by
117 one or more of the values:
118 .Bl -column MSG_WAITALL -offset indent
119 .It Dv MSG_OOB Ta process out-of-band data
120 .It Dv MSG_PEEK Ta peek at incoming message
121 .It Dv MSG_WAITALL Ta wait for full request or error
125 flag requests receipt of out-of-band data
126 that would not be received in the normal data stream.
127 Some protocols place expedited data at the head of the normal
128 data queue, and thus this flag cannot be used with such protocols.
129 The MSG_PEEK flag causes the receive operation to return data
130 from the beginning of the receive queue without removing that
132 Thus, a subsequent receive call will return the same data.
133 The MSG_WAITALL flag requests that the operation block until
134 the full request is satisfied.
135 However, the call may still return less data than requested
136 if a signal is caught, an error or disconnect occurs,
137 or the next data to be received is of a different type than that returned.
143 structure to minimize the number of directly supplied parameters.
144 This structure has the following form, as defined in
145 .Ao Pa sys/socket.h Ac :
149 caddr_t msg_name; /* optional address */
150 u_int msg_namelen; /* size of address */
151 struct iovec *msg_iov; /* scatter/gather array */
152 u_int msg_iovlen; /* # elements in msg_iov */
153 caddr_t msg_control; /* ancillary data, see below */
154 u_int msg_controllen; /* ancillary data buffer len */
155 int msg_flags; /* flags on received message */
163 specify the destination address if the socket is unconnected;
165 may be given as a null pointer if no names are desired or required.
169 describe scatter gather locations, as discussed in
174 points to a buffer for other protocol control related messages
175 or other miscellaneous ancillary data.
176 The messages are of the form:
179 u_int cmsg_len; /* data byte count, including hdr */
180 int cmsg_level; /* originating protocol */
181 int cmsg_type; /* protocol-specific type */
183 u_char cmsg_data[]; */
186 As an example, one could use this to learn of changes in the data-stream
187 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
188 a recvmsg with no data buffer provided immediately after an
192 Open file descriptors are now passed as ancillary data for
203 Process credentials can also be passed as ancillary data for
205 domain sockets using a
211 should be a structure of type
214 .Ao Pa sys/socket.h Ac
219 pid_t cmcred_pid; /* PID of sending process */
220 uid_t cmcred_uid; /* real UID of sending process */
221 uid_t cmcred_euid; /* effective UID of sending process */
222 gid_t cmcred_gid; /* real GID of sending process */
223 short cmcred_ngroups; /* number or groups */
224 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
228 The kernel will fill in the credential information of the sending process
229 and deliver it to the receiver.
233 field is set on return according to the message received.
235 indicates end-of-record;
236 the data returned completed a record (generally used with sockets of type
237 .Dv SOCK_SEQPACKET ) .
240 the trailing portion of a datagram was discarded because the datagram
241 was larger than the buffer supplied.
244 control data were discarded due to lack of space in the buffer
247 is returned to indicate that expedited or out-of-band data were received.
250 These calls return the number of bytes received, or -1
251 if an error occurred.
254 .Bl -tag -width ENOTCONNAA
258 is an invalid descriptor.
260 The socket is associated with a connection-oriented protocol
261 and has not been connected (see
268 does not refer to a socket.
270 The socket is marked non-blocking, and the receive operation
272 a receive timeout had been set,
273 and the timeout expired before data were received.
275 The receive was interrupted by delivery of a signal before
276 any data were available.
278 The receive buffer pointer(s) point outside the process's
290 function call appeared in