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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
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.
130 The MSG_PEEK flag causes the receive operation to return data
131 from the beginning of the receive queue without removing that
133 Thus, a subsequent receive call will return the same data.
134 The MSG_WAITALL flag requests that the operation block until
135 the full request is satisfied.
136 However, the call may still return less data than requested
137 if a signal is caught, an error or disconnect occurs,
138 or the next data to be received is of a different type than that returned.
144 structure to minimize the number of directly supplied parameters.
145 This structure has the following form, as defined in
146 .Ao Pa sys/socket.h Ac :
150 caddr_t msg_name; /* optional address */
151 u_int msg_namelen; /* size of address */
152 struct iovec *msg_iov; /* scatter/gather array */
153 u_int msg_iovlen; /* # elements in msg_iov */
154 caddr_t msg_control; /* ancillary data, see below */
155 u_int msg_controllen; /* ancillary data buffer len */
156 int msg_flags; /* flags on received message */
164 specify the destination address if the socket is unconnected;
166 may be given as a null pointer if no names are desired or required.
170 describe scatter gather locations, as discussed in
175 points to a buffer for other protocol control related messages
176 or other miscellaneous ancillary data.
177 The messages are of the form:
180 u_int cmsg_len; /* data byte count, including hdr */
181 int cmsg_level; /* originating protocol */
182 int cmsg_type; /* protocol-specific type */
184 u_char cmsg_data[]; */
187 As an example, one could use this to learn of changes in the data-stream
188 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
189 a recvmsg with no data buffer provided immediately after an
193 Open file descriptors are now passed as ancillary data for
204 Process credentials can also be passed as ancillary data for
206 domain sockets using a
212 should be a structure of type
215 .Ao Pa sys/socket.h Ac
220 pid_t cmcred_pid; /* PID of sending process */
221 uid_t cmcred_uid; /* real UID of sending process */
222 uid_t cmcred_euid; /* effective UID of sending process */
223 gid_t cmcred_gid; /* real GID of sending process */
224 short cmcred_ngroups; /* number or groups */
225 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
229 The kernel will fill in the credential information of the sending process
230 and deliver it to the receiver.
234 field is set on return according to the message received.
236 indicates end-of-record;
237 the data returned completed a record (generally used with sockets of type
238 .Dv SOCK_SEQPACKET ) .
241 the trailing portion of a datagram was discarded because the datagram
242 was larger than the buffer supplied.
245 control data were discarded due to lack of space in the buffer
248 is returned to indicate that expedited or out-of-band data were received.
251 These calls return the number of bytes received, or -1
252 if an error occurred.
255 .Bl -tag -width ENOTCONNAA
259 is an invalid descriptor.
261 The socket is associated with a connection-oriented protocol
262 and has not been connected (see
269 does not refer to a socket.
271 The socket is marked non-blocking, and the receive operation
273 a receive timeout had been set,
274 and the timeout expired before data were received.
276 The receive was interrupted by delivery of a signal before
277 any data were available.
279 The receive buffer pointer(s) point outside the process's
291 function call appeared in