recv, recvfrom, recvmsg - receive a message from a socket
#include <sys/types.h>
#include <sys/socket.h>
ssize_t
recv(int s, void *buf, size_t len, int flags);
ssize_t
recvfrom(int s, void *buf, size_t len, int flags, struct
sockaddr *from,
socklen_t *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-null 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 null from
parameter.
As it is redundant, it may not be supported in future releases.
On successful completion, all three routines return the number of message
bytes read. 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) system calls may be used to determine when more
data arrive.
The flags argument to a recv call is formed by ORing 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
MSG_DONTWAIT don't block
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 MSG_DONTWAIT flag requests the call to return
when it would
block otherwise. If no data is available, errno is set to
EAGAIN. This
flag is not available in strict ANSI or C99 compilation
mode.
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 */
unsigned 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. 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_UNIX domain
and socketpair(2) 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.
It will contain zero or more of the following values:
MSG_OOB Returned to indicate that expedited or
out-of-band data
was 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_BCAST Indicates that the packet was received as
broadcast.
MSG_MCAST Indicates that the packet was received as
multicast.
These calls return the number of bytes received, or -1 if an
error occurred.
recv(), recvfrom(), and recvmsg() fail if:
[EBADF] The argument s is an invalid descriptor.
[ENOTCONN] The socket is associated with a connectionoriented 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.
[EHOSTUNREACH] A socket operation was attempted to an unreachable host.
[EHOSTDOWN] A socket operation failed because the destination host
was down.
[ENETDOWN] A socket operation encountered a dead network.
In addition, recv() and recvfrom() may return the following
error:
[EINVAL] len was larger than SSIZE_MAX.
Also, recv() may return the following error:
[ECONNREFUSED] The socket is associated with a connectionoriented protocol
and the connection was forcefully rejected (see
connect(2)).
And recvmsg() may return one of the following errors:
[EINVAL] The sum of the iov_len values in the msg_iov
array overflowed
an ssize_t.
[EMSGSIZE] The msg_iovlen member of msg was less than 0
or larger than
IOV_MAX.
connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2),
socket(2), socketpair(2)
The recv() function call appeared in 4.2BSD.
OpenBSD 3.6 February 15, 1999
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