man pages section 3: Networking 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.

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