man zmq_msg_recv (3): receive a message part from a socket

SYNOPSIS

int zmq_msg_recv (zmq_msg_t *msg, void *socket, int flags);

DESCRIPTION

The zmq_msg_recv() function is identical to zmq_recvmsg(3), which shall be deprecated in future versions. zmq_msg_recv() is more consistent with other message manipulation functions.

The zmq_msg_recv() 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 zmq_msg_recv() function shall block until the request can be satisfied. The flags argument is a combination of the flags defined below:

ZMQ_DONTWAIT

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

Multi-part messages

A 0MQ message is composed of 1 or more message parts. Each message part is an independent zmq_msg_t in its own right. 0MQ 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 multi-part messages must use the ZMQ_RCVMORE zmq_getsockopt(3) option after calling zmq_msg_recv() to determine if there are further parts to receive.

RETURN VALUE

The zmq_msg_recv() function shall return number of bytes in the 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 zmq_msg_recv() operation is not supported by this socket type.

EFSM

: The zmq_msg_recv() 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 ZMQ_REP. See the messaging patterns section of zmq_socket(3) for more information.

ETERM

: The 0MQ 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.

EXAMPLE

Receiving a message from a socket.

/* Create an empty 0MQ message */
zmq_msg_t msg;
int rc = zmq_msg_init (&msg);
assert (rc == 0);
/* Block until a message is available to be received from socket */
rc = zmq_msg_recv (&msg, socket, 0);
assert (rc != -1);
/* Release message */
zmq_msg_close (&msg);

Receiving a multi-part message.

int64_t more;
size_t more_size = sizeof more;
do {
    /* Create an empty 0MQ message to hold the message part */
    zmq_msg_t part;
    int rc = zmq_msg_init (&part);
    assert (rc == 0);
    /* Block until a message is available to be received from socket */
    rc = zmq_msg_recv (&part, socket, 0);
    assert (rc != -1);
    /* Determine if more message parts are to follow */
    rc = zmq_getsockopt (socket, ZMQ_RCVMORE, &more, &more_size);
    assert (rc == 0);
    zmq_msg_close (&part);
} while (more);

AUTHORS

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