recv, recvfrom, recvmsg - receive a message from a socket
Standard C Library (libc, -lc)
#include <sys/socket.h>
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);
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 connectionoriented.
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 {
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 msg_name and msg_namelen specify the destination address if the
socket is unconnected; msg_name may be given as a null pointer if no
names are desired or required. 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 {
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 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 outof-band
data were received.
These calls return the number of bytes received, or -1 if an error
occurred.
The calls fail if:
[EBADF] The argument s is an invalid descriptor.
[ENOTCONN] The socket is associated with a connection-oriented
protocol and has not been connected (see connect(2)
and accept(2)).
[ENOTSOCK] The argument s does not refer to a socket.
[EAGAIN] The socket is marked non-blocking, and the receive
operation would block, or a receive timeout had been
set, and the timeout expired before data were
received.
[EINTR] The receive was interrupted by delivery of a signal
before any data were available.
[EFAULT] The receive buffer pointer(s) point outside the process's
address space.
[EINVAL] The total length of the I/O is more than can be
expressed by the ssize_t return value.
recvmsg() will also fail if:
[EMSGSIZE] The msg_iovlen member of the msg structure is less
than or equal to 0 or is greater than {IOV_MAX}.
fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2)
The recv() function call appeared in 4.2BSD.
BSD October 22, 2001 BSD
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