ssize_t
recv(
int s
, void *buf
, size_t len
, int flags
)
ssize_t
recvfrom(
int s
, void * restrict buf
, size_t len
, int flags
, struct sockaddr * restrict from
, socklen_t * restrict fromlen
)
ssize_t
recvmsg(
int s
, struct msghdr *msg
, int flags
)
)
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 Ap ing
one or more of the values:
MSG_OOB |
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<.blm Pp
: >
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:
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(
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[]; */
};
)
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.
EBADF
]
s
is an invalid descriptor.
ENOTCONN
]
ENOTSOCK
]
s
does not refer to a socket.
EAGAIN
]
EINTR
]
EFAULT
]
EINVAL
]
recvmsg()
will also fail if:
EMSGSIZE
]
msg_iovlen
member of the
msg
structure is less than or equal to 0
or is greater than
{IOV_MAX}
.
)
function call appeared in
4.2BSD.