recvfrom() and
recvmsg() are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.
If
from is non-nil, and the socket is not connection-oriented, the source address of the message is filled in.
fromlen is a value-result parameter, initialized to the size of the buffer associated with
from, and modified on return to indicate the actual size of the address stored there.
The
recv() call is normally used only on a
connected socket (see
connect(2)) and is identical to
recvfrom() with a nil
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
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) or
poll(2) call may be used to determine when more data arrive.
The
flags argument to a recv call is formed by
or'ing one or more of the values:
MSG_OOB
process out-of-band data
MSG_PEEK
peek at incoming message
MSG_WAITALL
wait for full request or error
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
recvmsg() call uses a
msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in <
sys/socket.h>:
struct msghdr {
void *msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data, see below */
socklen_t msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Here
msg_name and
msg_namelen specify the source address if the socket is unconnected;
msg_name may be given as a null pointer if no names are desired or required. If the socket is connected,
msg_name and
msg_namelen are ignored.
msg_iov and
msg_iovlen describe scatter gather locations, as discussed in
read(2).
msg_control, which has length
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 {
socklen_t 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
accept() call.
Open file descriptors are now passed as ancillary data for
AF_LOCAL domain sockets, with
cmsg_level set to
SOL_SOCKET and
cmsg_type set to
SCM_RIGHTS.
The
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.