If Fa from is non-nil, and the socket is not connection-oriented, the source address of the message is filled in. Fa Fromlen is a value-result parameter, initialized to the size of the buffer associated with Fa from , and modified on return to indicate the actual size of the address stored there.
The Fn recv call is normally used only on a connected socket (see connect(2)) and is identical to Fn recvfrom with a nil Fa from parameter. As it is redundant, it may not be supported in future releases.
All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see socket(2)).
If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1 is returned and the external variable errno set to Er EAGAIN . The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).
The select(2) call may be used to determine when more data arrive.
The Fa flags argument to a recv call is formed by or Ap ing one or more of the values:
The MSG_OOB flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. The MSG_PEEK flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data. The MSG_WAITALL flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned.
The Fn recvmsg call uses a Fa msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in Ao Pa sys/socket.h Ac :
struct msghdr {
caddr_t msg_name; /* optional address */
u_int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
u_int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_control; /* ancillary data, see below */
u_int msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Here Fa msg_name and Fa msg_namelen specify the destination address if the socket is unconnected; Fa msg_name may be given as a null pointer if no names are desired or required. Fa Msg_iov and Fa msg_iovlen describe scatter gather locations, as discussed in read(2). Fa Msg_control , which has length Fa msg_controllen , points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form:
struct cmsghdr {
u_int cmsg_len; /* data byte count, including hdr */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by
u_char cmsg_data[]; */
};
As an example, one could use this to learn of changes in the data-stream
in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
a recvmsg with no data buffer provided immediately after an
Fn accept
call.
Open file descriptors are now passed as ancillary data for AF_UNIX domain sockets, with Fa cmsg_level set to SOL_SOCKET and Fa cmsg_type set to SCM_RIGHTS
The Fa msg_flags field is set on return according to the message received. MSG_EOR indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET ) MSG_TRUNC indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied. MSG_CTRUNC indicates that some control data were discarded due to lack of space in the buffer for ancillary data. MSG_OOB is returned to indicate that expedited or out-of-band data were received.