RECVMSG(3XNET) X/Open Networking Services Library Functions RECVMSG(3XNET)
NAME
recvmsg - receive a message from a socket
SYNOPSIS
cc [
flag ... ]
file ...
-lxnet [
library ... ]
#include <sys/socket.h>
ssize_t recvmsg(
int socket,
struct msghdr *message,
int flags);
DESCRIPTION
The
recvmsg() function receives a message from a connection-mode or
connectionless-mode socket. It is normally used with connectionless-mode
sockets because it permits the application to retrieve the source address
of received data.
The
recvmsg() function receives messages from unconnected or connected
sockets and returns the length of the message.
The
recvmsg() function returns the total length of the message. For
message-based sockets such as
SOCK_DGRAM and
SOCK_SEQPACKET, the entire
message must be read in a single operation. If a message is too long to
fit in the supplied buffers, and
MSG_PEEK is not set in the
flags argument, the excess bytes are discarded, and
MSG_TRUNC is set in the
msg_flags member of the
msghdr structure. For stream-based sockets such
as
SOCK_STREAM, message boundaries are ignored. In this case, data is
returned to the user as soon as it becomes available, and no data is
discarded.
If the
MSG_WAITALL flag is not set, data will be returned only up to the
end of the first message.
If no messages are available at the socket, and
O_NONBLOCK is not set on
the socket's file descriptor,
recvmsg() blocks until a message arrives.
If no messages are available at the socket and
O_NONBLOCK is set on the
socket's file descriptor, the
recvmsg() function fails and sets
errno to
EAGAIN or
EWOULDBLOCK.
In the
msghdr structure, defined in
socket.h(3HEAD), the
msg_name and
msg_namelen members specify the source address if the socket is
unconnected. If the socket is connected, the
msg_name and
msg_namelen members are ignored. The
msg_name member may be a null pointer if no
names are desired or required.
The
msg_control and
msg_controllen members specify a buffer to receive
ancillary data sent along with a message. Ancillary data consists of a
sequence of pairs. Each pair is composed of a
cmsghdr structure followed
by a data array. The
cmsghdr structure, defined in
socket.h(3HEAD),
contains descriptive information which allows an application to correctly
parse data. The data array contains the ancillary data message.
If ancillary data is not transferred,
msg_control is set to
NULL and
msg_controllen is set to
0.
The
msg_iov and
msg_iovlen fields of the
msghdr structure are used to
specify where the received data will be stored.
msg_iov points to an
array of
iovec structures. The
msg_iovlen must be set to the dimension of
this array. In each
iovec structure, the
iov_base field specifies a
storage area and the
iov_len field gives its size in bytes. Each storage
area indicated by
msg_iov is filled with received data in turn until all
of the received data is stored or all of the areas have been filled.
If the
SO_TIMESTAMP option has been enabled through
setsockopt(), then a
struct
timeval is returned following the
cmsghdr, and the
cmsg_len field
of the
cmsghdr indicates the size of the struct
timeval.
On successful completion, the
msg_flags member of the message header is
the bitwise-inclusive OR of all of the following flags that indicate
conditions detected for the received message:
MSG_EOR End of record was received (if supported by the protocol).
MSG_OOB Out-of-band data was received.
MSG_TRUNC Normal data was truncated.
MSG_CTRUNC Control data was truncated.
PARAMETERS
The function takes the following arguments:
socket Specifies the socket file descriptor.
message Points to a
msghdr structure, containing both the buffer to
store the source address and the buffers for the incoming
message. The length and format of the address depend on the
address family of the socket. The
msg_flags member is ignored
on input, but may contain meaningful values on output.
flags Specifies the type of message reception. Values of this
argument are formed by logically OR'ing zero or more of the
following values:
MSG_OOB Requests out-of-band data. The significance and
semantics of out-of-band data are protocol-
specific.
MSG_PEEK Peeks at the incoming message.
MSG_WAITALL Requests that the function block until the full
amount of data requested can be returned. The
function may return a smaller amount of data if
a signal is caught, if the connection is
terminated, if
MSG_PEEK was specified, or if an
error is pending for the socket.
USAGE
The
select(3C) and
poll(2) functions can be used to determine when data
is available to be received.
RETURN VALUES
Upon successful completion,
recvmsg() returns the length of the message
in bytes. If no messages are available to be received and the peer has
performed an orderly shutdown,
recvmsg() returns
0. Otherwise,
-1 is
returned and
errno is set to indicate the error.
ERRORS
In addition to the errors documented below, an asynchronous error
generated by the underlying socket protocol may be returned. For the full
list of errors, please see the corresponding socket protocol manual page.
For example, for a list of TCP errors, please see
tcp(4P).
The
recvmsg() function will fail if:
EAGAIN EWOULDBLOCK The socket's file descriptor is marked
O_NONBLOCK and no
data is waiting to be received; or
MSG_OOB is set and no
out-of-band data is available and either the socket's file
descriptor is marked
O_NONBLOCK or the socket does not
support blocking to await out-of-band data.
EBADF The
socket argument is not a valid open file descriptor.
ECONNRESET A connection was forcibly closed by a peer.
EFAULT The
message parameter, or storage pointed to by the
msg_name,
msg_control or
msg_iov fields of the
message parameter, or storage pointed to by the
iovec structures
pointed to by the
msg_iov field can not be accessed or
written.
EINTR This function was interrupted by a signal before any data
was available.
EINVAL The sum of the
iov_len values overflows an
ssize_t. or the
MSG_OOB flag is set and no out-of-band data is available.
EMSGSIZE The
msg_iovlen member of the
msghdr structure pointed to
by
message is less than or equal to 0, or is greater than
IOV_MAX.
ENOTCONN A receive is attempted on a connection-mode socket that is
not connected.
ENOTSOCK The
socket argument does not refer to a socket.
EOPNOTSUPP The specified flags are not supported for this socket
type.
ETIMEDOUT The connection timed out during connection establishment,
or due to a transmission timeout on active connection.
The
recvmsg() function may fail if:
EIO An
IO error occurred while reading from or writing to the file
system.
ENOBUFS Insufficient resources were available in the system to perform
the operation.
ENOMEM Insufficient memory was available to fulfill the request.
ENOSR There were insufficient STREAMS resources available for the
operation to complete.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+
SEE ALSO
poll(2),
select(3C),
socket.h(3HEAD),
recv(3XNET),
recvfrom(3XNET),
send(3XNET),
sendmsg(3XNET),
sendto(3XNET),
setsockopt(3XNET),
shutdown(3XNET),
socket(3XNET),
tcp(4P),
attributes(7),
standards(7) September 10, 2018
RECVMSG(3XNET)