xs_recvmsg: Receive a message part from a socket (zero-copy)

SYNOPSIS

int xs_recvmsg (void *socket, xs_msg_t *msg, int flags);

DESCRIPTION

The xs_recvmsg() function shall receive a message part from the socket referenced by the socket argument and store it in the message referenced by the msg argument. Any content previously stored in msg shall be properly deallocated. If there are no message parts available on the specified socket the xs_recvmsg() function shall block until the request can be satisfied. The flags argument is a combination of the flags defined below:

XS_DONTWAIT

Specifies that the operation should be performed in non-blocking mode. If there are no messages available on the specified socket, the xs_recvmsg() function shall fail with errno set to EAGAIN.

Multi-part messages

A Crossroads message is composed of 1 or more message parts. Each message part is an independent xs_msg_t in its own right. Crossroads ensures atomic delivery of messages; peers shall receive either all message parts of a message or none at all. The total number of message parts is unlimited except by available memory.

An application that processes multipart messages must use the XS_RCVMORE xs_getsockopt(3) option after calling xs_recvmsg() to determine if there are further parts to receive.

RETURN VALUE

The xs_recvmsg() function shall return the number of bytes in the received message if successful. Otherwise it shall return -1 and set errno to one of the values defined below.

ERRORS

EAGAIN

Non-blocking mode was requested and no messages are available at the moment.

ENOTSUP

The xs_recvmsg() operation is not supported by this socket type.

EFSM

The xs_recvmsg() operation cannot be performed on this socket at the moment due to the socket not being in the appropriate state. This error may occur with socket types that switch between several states, such as XS_REP. See the messaging patterns section of xs_socket(3) for more information.

ETERM

The context associated with the specified socket was terminated.

ENOTSOCK

The provided socket was invalid.

EINTR

The operation was interrupted by delivery of a signal before a message was available.

EFAULT

The message passed to the function was invalid.

ETIMEDOUT

Survey has timed out. (Applicable only to XS_SURVEYOR socket.)

EXAMPLE

Receiving a message from a socket.

/* Create an empty message */
xs_msg_t msg;
int rc = xs_msg_init (&msg);
assert (rc == 0);
/* Block until a message is available to be received from socket */
rc = xs_recvmsg (socket, &msg, 0);
assert (rc != -1);
/* Release message */
xs_msg_close (&msg);

Receiving a multi-part message.

int64_t more;
size_t more_size = sizeof more;
do {
    /* Create an empty message to hold the message part */
    xs_msg_t part;
    int rc = xs_msg_init (&part);
    assert (rc == 0);
    /* Block until a message is available to be received from socket */
    rc = xs_recvmsg (socket, &part, 0);
    assert (rc != -1);
    /* Determine if more message parts are to follow */
    rc = xs_getsockopt (socket, XS_RCVMORE, &more, &more_size);
    assert (rc == 0);
    xs_msg_close (&part);
} while (more);

RELATED TO xs_recvmsg…

Applications that do not require zero-copy messaging can use the simpler xs_recv(3) instead of xs_recvmsg().

xs_sendmsg(3) xs_getsockopt(3) xs_socket(7) xs(7)

AUTHORS

This man page was written by Martin Sustrik <\m[blue][email protected]\m[]\s-2\u[1]\d\s+2>, Martin Lucina <\m[blue][email protected]\m[]\s-2\u[2]\d\s+2> and Pieter Hintjens <\m[blue][email protected]\m[]\s-2\u[3]\d\s+2>.