NAME
recv,
recvfrom,
recvmsg,
recvmmsg —
receive a message from a
socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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);
int
recvmmsg(
int
s,
struct mmsghdr
*mmsg,
unsigned int
vlen,
unsigned int
flags,
struct timespec
*timeout);
DESCRIPTION
recvfrom(),
recvmsg() and
recvmmsg() 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
recvmmsg() call be used to receive multiple messages in
the same call using an array of
mmsghdr elements with
the following form, as defined in
⟨
sys/socket.h⟩:
struct mmsghdr {
struct msghdr msg_hdr; /* the message to be sent */
unsigned int msg_len; /* number of bytes received */
};
The
msg_len member contains the number of bytes received
for each
msg_hdr member. The array has
vlen elements, which is limited to
1024
. If there is an error, a number fewer than
vlen may be returned, and the error may be retrieved
using
getsockopt(2) with
SO_ERROR
. If the flag
MSG_WAITFORONE
is set in
flags
then the
recvmmsg() call will wait for one message, and set
MSG_DONTWAIT
for the rest. If the
timeout parameter is not
NULL
,
then
recvmmsg() will return if that time is exceeded.
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.
recv(),
recvfrom() and
recvmsg() routines return the length of the message on
successful completion.
recvmmsg() returns the number of
messages received. 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
. If no data is available and the remote peer
was shut down, 0 is returned. 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_CMSG_CLOEXEC |
set the close on exec property for passed file
descriptors |
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.
RETURN VALUES
The
recv(),
recvfrom() and
recvmsg() calls return the number of bytes received, or -1
if an error occurred. For connected sockets whose remote peer was shut down, 0
is returned when no more data is available. The
recvmmsg()
calls return the number of messages received, or -1 if an error occurred.
ERRORS
The calls fail if:
-
-
- [
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.
-
-
- [
EBADF
]
- The argument s is an invalid
descriptor.
-
-
- [
EFAULT
]
- The receive buffer pointer(s) point outside the process's
address space.
-
-
- [
EINTR
]
- The receive was interrupted by delivery of a signal before
any data were available.
-
-
- [
EINVAL
]
- The total length of the I/O is more than can be expressed
by the ssize_t return value.
-
-
- [
ENOBUFS
]
- A message was not delivered because it would have
overflowed the buffer.
-
-
- [
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.
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
}.
SEE ALSO
fcntl(2),
getsockopt(2),
poll(2),
read(2),
select(2),
socket(2)
HISTORY
The
recv() function call appeared in
4.2BSD. The
recvmmsg() function call
appeared in Linux 2.6.32 and
NetBSD 7.0.