recvmsg - receive a message from a socket
cc [ flag ... ] file ... -lxnet [ library ... ]
#include <sys/socket.h>
ssize_t recvmsg(int socket, struct msghdr *message, int flags);
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.
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.
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.
See attributes(7) for descriptions of the following attributes:
| ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
| Interface Stability |
Standard |
| MT-Level |
MT-Safe |
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)