Tribblix: manual page: recvmsg.3xnet

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