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32 .\" @(#)recv.2 8.3 (Berkeley) 2/21/94
33 .\" $FreeBSD: src/lib/libc/sys/recv.2,v 1.8.2.8 2001/12/14 18:34:01 ru Exp $
34 .\" $DragonFly: src/lib/libc/sys/recv.2,v 1.7 2008/05/02 02:05:04 swildner Exp $
43 .Nd receive a message from a socket
50 .Fn recv "int s" "void *buf" "size_t len" "int flags"
52 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "socklen_t *fromlen"
54 .Fn recvmsg "int s" "struct msghdr *msg" "int flags"
59 are used to receive messages from a socket,
60 and may be used to receive data on a socket whether or not
61 it is connection-oriented.
65 is non-nil, and the socket is not connection-oriented,
66 the source address of the message is filled in.
68 is a value-result parameter, initialized to the size of
69 the buffer associated with
71 and modified on return to indicate the actual size of the
76 call is normally used only on a
85 As it is redundant, it may not be supported in future releases.
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 nonblocking (see
98 in which case the value
99 -1 is returned and the external 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 call may be used to determine when more data arrive.
119 argument to a recv call is formed by
121 one or more of the values:
122 .Bl -column MSG_WAITALL -offset indent
123 .It Dv MSG_OOB Ta process out-of-band data
124 .It Dv MSG_PEEK Ta peek at incoming message
125 .It Dv MSG_WAITALL Ta wait for full request or error
130 flag requests receipt of out-of-band data
131 that would not be received in the normal data stream.
132 Some protocols place expedited data at the head of the normal
133 data queue, and thus this flag cannot be used with such protocols.
134 The MSG_PEEK flag causes the receive operation to return data
135 from the beginning of the receive queue without removing that
137 Thus, a subsequent receive call will return the same data.
138 The MSG_WAITALL 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
153 caddr_t msg_name; /* optional address */
154 u_int msg_namelen; /* size of address */
155 struct iovec *msg_iov; /* scatter/gather array */
156 u_int msg_iovlen; /* # elements in msg_iov */
157 caddr_t msg_control; /* ancillary data, see below */
158 u_int msg_controllen; /* ancillary data buffer len */
159 int msg_flags; /* flags on received message */
167 specify the destination address if the socket is unconnected;
169 may be given as a null pointer if no names are desired or required.
173 describe scatter gather locations, as discussed in
178 points to a buffer for other protocol control related messages
179 or other miscellaneous ancillary data.
180 The messages are of the form:
183 u_int cmsg_len; /* data byte count, including hdr */
184 int cmsg_level; /* originating protocol */
185 int cmsg_type; /* protocol-specific type */
187 u_char cmsg_data[]; */
191 As an example, one could use this to learn of changes in the data-stream
192 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
193 a recvmsg with no data buffer provided immediately after an
197 Open file descriptors are now passed as ancillary data for
208 Process credentials can also be passed as ancillary data for
210 domain sockets using a
216 should be a structure of type
223 pid_t cmcred_pid; /* PID of sending process */
224 uid_t cmcred_uid; /* real UID of sending process */
225 uid_t cmcred_euid; /* effective UID of sending process */
226 gid_t cmcred_gid; /* real GID of sending process */
227 short cmcred_ngroups; /* number or groups */
228 gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
232 The kernel will fill in the credential information of the sending process
233 and deliver it to the receiver.
237 field is set on return according to the message received.
239 indicates end-of-record;
240 the data returned completed a record (generally used with sockets of type
241 .Dv SOCK_SEQPACKET ) .
244 the trailing portion of a datagram was discarded because the datagram
245 was larger than the buffer supplied.
248 control data were discarded due to lack of space in the buffer
251 is returned to indicate that expedited or out-of-band data were received.
253 Upon successful completion the number of bytes which were received is
254 returned. Otherwise -1 is returned and the global variable
256 is set to indicate the error.
263 is an invalid descriptor.
265 The socket is associated with a connection-oriented protocol
266 and has not been connected (see
273 does not refer to a socket.
275 The socket is marked non-blocking, and the receive operation
277 a receive timeout had been set,
278 and the timeout expired before data were received.
280 The receive was interrupted by delivery of a signal before
281 any data were available.
283 The receive buffer pointer(s) point outside the process's
295 function call appeared in