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


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(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


poll(2), recv(3XNET), recvfrom(3XNET), select(3C), send(3XNET),
sendmsg(3XNET), sendto(3XNET), setsockopt(3XNET), shutdown(3XNET),
socket(3XNET), socket.h(3HEAD), attributes(5), standards(5)


February 27, 2006 RECVMSG(3XNET)