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32 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
42 .Nd receive a message from a socket
44 .Fd #include <sys/types.h>
45 .Fd #include <sys/socket.h>
47 .Fn recv "int s" "void *buf" "size_t len" "int flags"
49 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "int *fromlen"
51 .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.
62 is non-nil, and the socket is not connection-oriented,
63 the source address of the message is filled in.
65 is a value-result parameter, initialized to the size of
66 the buffer associated with
68 and modified on return to indicate the actual size of the
73 call is normally used only on a
82 As it is redundant, it may not be supported in future releases.
84 All three routines return the length of the message on successful
86 If a message is too long to fit in the supplied buffer,
87 excess bytes may be discarded depending on the type of socket
88 the message is received from (see
91 If no messages are available at the socket, the
92 receive call waits for a message to arrive, unless
93 the socket is nonblocking (see
95 in which case the value
96 -1 is returned and the external variable
100 The receive calls normally return any data available,
101 up to the requested amount,
102 rather than waiting for receipt of the full amount requested;
103 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
127 flag requests receipt of out-of-band data
128 that would not be received in the normal data stream.
129 Some protocols place expedited data at the head of the normal
130 data queue, and thus this flag cannot be used with such protocols.
131 The MSG_PEEK flag causes the receive operation to return data
132 from the beginning of the receive queue without removing that
134 Thus, a subsequent receive call will return the same data.
135 The MSG_WAITALL flag requests that the operation block until
136 the full request is satisfied.
137 However, the call may still return less data than requested
138 if a signal is caught, an error or disconnect occurs,
139 or the next data to be received is of a different type than that returned.
145 structure to minimize the number of directly supplied parameters.
146 This structure has the following form, as defined in
147 .Ao Pa sys/socket.h Ac :
151 caddr_t msg_name; /* optional address */
152 u_int msg_namelen; /* size of address */
153 struct iovec *msg_iov; /* scatter/gather array */
154 u_int msg_iovlen; /* # elements in msg_iov */
155 caddr_t msg_control; /* ancillary data, see below */
156 u_int msg_controllen; /* ancillary data buffer len */
157 int msg_flags; /* flags on received message */
165 specify the destination address if the socket is unconnected;
167 may be given as a null pointer if no names are desired or required.
171 describe scatter gather locations, as discussed in
176 points to a buffer for other protocol control related messages
177 or other miscellaneous ancillary data.
178 The messages are of the form:
181 u_int cmsg_len; /* data byte count, including hdr */
182 int cmsg_level; /* originating protocol */
183 int cmsg_type; /* protocol-specific type */
185 u_char cmsg_data[]; */
189 As an example, one could use this to learn of changes in the data-stream
190 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
191 a recvmsg with no data buffer provided immediately after an
195 Open file descriptors are now passed as ancillary data for
206 Process credentials can also be passed as ancillary data for
208 domain sockets using a
214 should be a structure of type
217 .Ao Pa sys/socket.h Ac
222 pid_t cmcred_pid; /* PID of sending process */
223 uid_t cmcred_uid; /* real UID of sending process */
224 uid_t cmcred_euid; /* effective UID of sending process */
225 gid_t cmcred_gid; /* real GID of sending process */
226 short cmcred_ngroups; /* number or groups */
227 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
231 The kernel will fill in the credential information of the sending process
232 and deliver it to the receiver.
236 field is set on return according to the message received.
238 indicates end-of-record;
239 the data returned completed a record (generally used with sockets of type
240 .Dv SOCK_SEQPACKET ) .
243 the trailing portion of a datagram was discarded because the datagram
244 was larger than the buffer supplied.
247 control data were discarded due to lack of space in the buffer
250 is returned to indicate that expedited or out-of-band data were received.
253 These calls return the number of bytes received, or -1
254 if an error occurred.
257 .Bl -tag -width ENOTCONNAA
261 is an invalid descriptor.
263 The socket is associated with a connection-oriented protocol
264 and has not been connected (see
271 does not refer to a socket.
273 The socket is marked non-blocking, and the receive operation
275 a receive timeout had been set,
276 and the timeout expired before data were received.
278 The receive was interrupted by delivery of a signal before
279 any data were available.
281 The receive buffer pointer(s) point outside the process's
293 function call appeared in